Title: SURFACE COVERING The invention is directed to a surface covering, to a method for preparing said surface covering, and to a method for applying said surface covering. Watertight surface coverings are used in construction for waterproofing surfaces, such as roofs, facades, etc. Conventional materials used for such surface coverings are based, for instance, on bitumen, polyvinyl chloride, polychloroprene, or ethylene propylene diene monomer (EPDM). Another material suggested in the art to be advantageous for flashing material or roofing material is polyvinyl butyral. For example, WO-A-2016/108686 discloses a lead alternative flashing material comprising a perforated sheet of metal attached to a thermoplastic layer that may comprise polyvinyl butyral. WO-A-2020/122727 discloses a synthetic flashing material comprising 75 wt.% or more of polymer, such as polyvinyl butyral, and less than 10 wt.% of metal. WO-A-2021/064634 discloses a construction material comprising polyvinyl butyral and an ammonium polyphosphate fire retardant. WO-A-2017/064260 discloses a decorative multi-layer surface covering comprising polyvinyl butyral. WO-A-2020/156740 discloses cellulose ester compositions for surface coverings. Polyvinyl butyral is an industrially important polymer widely used in laminated safety glass and surface treatments because it exhibits a very high adhesion to glass. The primary use of polyvinyl butyral is in safety glass laminates, particularly in automotive and architectural glass. A layer of polyvinyl butyral is compressed between two sheets of glass under heat and pressure. The resulting glass sandwich looks like any other normal sheet of glass. Although laminated glass can break, the pieces of glass remain intact, adhering to the protective polyvinyl butyral layer. Laminated glass has numerous benefits as a result of its safety properties (no injuries caused by flying shards of glass) and its sound insulation properties. Recycling safety glass and extracting the polyvinyl butyral therefore provides an easily accessible and widely available source of polyvinyl butyral. Like polyvinyl chloride, polyvinyl butyral is a promising material for use in surface coverings. However both materials behave fundamentally different, because polyvinyl chloride is a hydrophobic material, and as a consequence impermeable to moisture. Additionally, polyvinyl chloride typically suffers from leaking plasticiser, which causes the polyvinyl chloride surface covering to become brittle over time. Polyvinyl butyral on the other hand can take up and release relatively large amounts of moisture. Although the material is not permeable to water in liquid form, it does allow permeation of vapour. Polyvinyl butyral further has limited capacity for oil uptake. While polyvinyl butyral is a promising material for use in surface coverings, the compounding of polyvinyl butyral with other components is challenging, in particular when the polyvinyl butyral used is recycled polyvinyl butyral that originates from various recycle sources. Processability problems arise when blending and extruding the polyvinyl butyral with other components. Addition of such other components, however, may be required for depending on the application of the product. Polyvinyl butyral is, for instance, typically heavily plasticised in order to improve mechanical and rheological properties. Furthermore, when used as surface covering measures have to be taken in order to prevent the material from combusting. When exposed to fire, polymers (being mainly made up of carbon and hydrogen) generally burn rapidly, releasing a lot of heat and smoke, and potentially causing great damage. The use of flame retardants has therefore become essential from the viewpoint of safety and environment. Flame retardants stop or inhibit the polymer combustion process, acting physically or chemically, by interfering with heating, pyrolysis, ignition, and/or thermal degradation. Many flame retardants, however, are still based on petroleum resources. Flame retardants from bio-based resources would be a desirable alternative. The invention aims at providing a surface covering comprising polyvinyl butyral that has excellent processability. Further objective of the invention is to provide a polyvinyl butyral based surface covering that can be easily compounded with other components. Another objective of the invention is to provide a polyvinyl butyral based surface covering having limited amount of plasticiser. Yet a further objective of the invention is to provide a polyvinyl butyral based surface covering comprising a bio-based flame retardant. Yet another objective of the invention is to provide a surface covering that can be easily glued together with good adhesion strength. The inventors found that one or more of these objectives can be met, at least in part, by a surface covering comprising a specific combination of polyvinyl butyral and a phosphorus containing oil that simultaneously acts as plasticiser and flame retardant. Accordingly, in one aspect the invention is directed to a surface covering comprising - 60-90 % by total weight of the surface covering of polyvinyl butyral, and - 1-15 % by total weight of the surface covering of one or more organophosphorus oils and/or one or more organohalogen oils. The inventors found that the use of an organophosphorus oil and/or an organohalogen oil surprisingly improves the processability during compounding of the polyvinyl butyral based composition. The organophosphorus oil and/or the organohalogen oil further act both as plasticiser, so the currently used plasticiser content can be lowered, and it can act as flame retardant. Furthermore the oil helps to disperse other flame retardants (inorganic) and therefore increases the effectiveness of the flame retardant action. The surface covering of the invention as a result requires overall less amount of plasticiser which makes that the resulting product can be better glued to another surface covering comprising polyvinyl butyral, preferably another surface covering of the invention. As will be described herein, the oil may advantageously be based on vegetable oil. Accordingly, the organophosphorus oil may be a phosphorus containing vegetable oil based derivative and/or the organohalogen oil may be a halogen containing vegetable oil based derivative. Such vegetable oils are domestically abundant, cost-effective, and non-toxic. In addition, they possess functional moieties such as double bonds, hydroxyl groups and ester groups which can readily undergo derivatisation reactions such as epoxidation, esterification, urethanation, alcoholysis, etc. which can then be easily modified e.g. by phosphorylation to introduce phosphorus and/or by halogenation to introduce halogen. The surface covering of the invention is particularly suitable for covering the surface of a roof, a basement, a façade, a wall, a floor or a pool. In such applications, the surface covering serves as a waterproof membrane. The term “polyvinyl butyral” as used herein is meant to refer to a polyacetal prepared by reacting poly(vinyl alcohol) with butyraldehyde, after obtaining poly(vinyl alcohol) by means of transesterification of from poly(vinyl acetate) usually with methanol and base catalysis. Polyvinyl butyral is commercially available from Monsanto Company as Butvar ^® . The conditions of the reaction between polyvinyl alcohol and butyraldehyde, and their relative concentrations are closely controlled to form polymers containing predetermined proportions of hydroxyl, acetate, and butyral groups. Although chemically not entirely correct, the term poly(vinyl butyrate) is in the art often interchangeably used with the term poly(vinyl butyral). As used in this application, the term “poly(vinyl butyrate)” is meant to refer to a polymer that comprises butyral groups. Since not all hydroxyl groups of the poly(vinyl alcohol) react with aldehyde, polyvinyl butyral invariably contains a certain percentage of hydroxyl groups. Moreover, a small group of the acetal groups always remain in the polymer chain from the upstream transesterification during which poly(vinyl acetate) is converted to poly(vinyl alcohol). Therefore, the final product has the character of a terpolymer of vinyl butyral, vinyl alcohol, and vinyl acetate. The vinyl alcohol unit is polar and hydrophilic and the vinyl butyral unit is hydrophobic. The chemical structure of polyvinyl butyral is shown below. The vinyl alcohol content in commercial polyvinyl butyral can typically be in the range of 5-30 % by total weight of the polyvinyl butyral, such as from 10-25 %, whereas the vinyl acetate content may be in the range of 0-2.5 % by total weight of the polyvinyl butyral, such as 0-2 %. The polyvinyl butyral may be virgin polyvinyl butyral, but it is also possible to use polyvinyl butyral that originates from a recycle stream. Such polyvinyl butyral is referred herein as recycled polyvinyl butyral. The recycled polyvinyl butyral may originate from various sources, including laminated safety glass (such as from automobile windshields or from construction), photovoltaic modules, flexible packaging, etc. Typically, the polyvinyl butyral has a weight average molecular weight M _w of 70000 g/mol or more, as determined by size exclusion chromatography using low angle laser light scattering, such as 100000 g/mol to 250000 g/mol, or 120000 g/mol to 200000 g/mol. Suitably, the polyvinyl butyral has a melt flow index of 1-10 g/10 min, as determined by ISO 1133 at a load of 2.16 kg and at a temperature of 190 °C, preferably 2-7 g/10 min, such as 3-6 g/10 min, or 4-5 g/10 min. The surface covering of the invention comprises polyvinyl butyral in an amount of 60-90 % by total weight of the surface covering. Preferably, the surface covering comprises polyvinyl butyral in an amount of 62-88 % by total weight of the surface covering, such as 65-85 %, 68-82 %, or 70-80 %. Suitably, the surface covering can comprise an amount of polymer material of 60 % or more based on total weight of surface covering, such as 65 % or more, or 75 % or more. The term “organophosphorus oil” as used herein is meant to refer to any nonpolar chemical substance that comprises at least one phosphorus atom, is a viscous liquid at 20 °C, and is both hydrophobic and lipophilic. The organophosphorus oil may suitably be a phosphorus containing vegetable-oil based derivative, i.e. a vegetable oil that has been modified to comprise one or more phosphorus atoms. The term “organohalogen oil” as used herein is meant to refer to any nonpolar chemical substance that comprises at least one halogen atom, is a viscous liquid at 20 °C, and is both hydrophobic and lipophilic. The organohalogen oil may suitably be a halogen containing vegetable-oil based derivative, i.e. a vegetable oil that has been modified to comprise one or more halogen atoms. Preferably, the surface covering of the invention at least comprises one or more organophosphorus oils, such as one or more phosphorus containing vegetable oil-based derivatives. The organophosphorus oil and/or organohalogen oil may be based on one or more vegetable oils. Examples thereof include castor oil, linseed oil, sunflower oil, corn oil, cotton oil, coconut oil, algae oil, jatropha oil, soybean oil, cashew nut oil, rapeseed oil, walnut oil, safflower oil, grapeseed oil, canola oil, sesame oil, peanut oil, olive oil, palm oil, camelina oil and the like. These vegetable oils may be treated in a variety of conventional ways that are commonly known to the person skilled in the art in order to produce derivatives that contain phosphorus and/or halogen. One possible way of obtaining a phosphorus containing vegetable oil-based derivative is by epoxidation. By way of example, castor oil can be esterified at hydroxyl groups and then epoxidised at unsaturation producing epoxidised castor oil polyol ester, which can subsequently be treated with phosphorus oxychloride forming chlorophosphate ester of castor oil. Another example is to epoxidise castor oil at the double bond and then modifying the inserted oxirane ring with diethyl phosphate in the presence of triphenylphosphine, thereby producing a castor oil phosphate ester. Yet a further example is to epoxidise soybean oil followed by epoxide ring opening reaction with phosphoric acid, and treatment of the formed phosphorylated polyols with polymeric diphenyl methane diisocyanate to produce phosphorylated polyol polyurethanes. In another example, castor oil is epoxidised, and phosphaphenanthrene groups are inserted on epoxidised castor oil by oxirane ring opening reaction. The hydroxyl groups of castor oil and hydroxyl groups formed during oxirane ring opening can then be esterified yielding a castor oil polyester with phosphaphenanthrene groups Another approach to obtain a phosphorus containing vegetable oil-based derivative is by glycerolysis. For example, castor oil reacted with glycerol in the presence of sodium methoxide and triethanolamine thereby forming monoglyceride and diglyceride of castor oil. These can be further epoxidised at double bonds, and the epoxy ring opening reaction with diethyl phosphate to form a phosphorus containing polyol derivative of castor oil. According to another example, glycerolysed products of castor oil, monoglyceride and diglyceride, are epoxidised and phosphaphenanthrene groups are inserted in castor oil mono- and diglycerides by epoxide ring opening reaction. The hydroxyl groups of castor oil and those formed by epoxide ring opening are then further esterified to produce phosphaphenanthrene-containing castor oil polyols. Many other possible synthesis routes exist and are readily available to the person skilled in the art. The phosphorus may be introduced into the vegetable oil by means of functional groups including phosphaphenanthrene groups, diphenyl phosphine oxide groups, diethyl phosphate groups, chlorinated phosphate groups, phosphonate groups, phosphine groups, phosphine oxide groups, phosphinite groups, phosphonite groups, phosphinate groups, phosphate groups, any combinations thereof. The phosphorus containing vegetable oil-based derivative can comprise one or more phosphorus containing groups selected from the group consisting of phosphaphenanthrene groups, diphenyl phosphine oxide groups, diethyl phosphate groups, chlorinated phosphate groups, and phosphonate groups. Apart from using vegetable oils, it is also possible to use other organophosphorus oils, such as tris(dichloropropyl) phosphate (available under the trademark FYROL ^® FR-2 from ICL Industrial Products), neopentyl glycol bis(diphenyl phosphate), resorcinol bis(diphenyl phosphate) (available under the trademark FYROLFLEX ^® RDP from ICL Industrial Products), bisphenol A bis(diphenyl phosphate) (available under the trademark FYROLFLEX ^® BDP from ICL Industrial Products), butylated triphenyl phosphate (available under the trademark PHOSFLEX ^® 71B from ICL Industrial Products), and isopropylated triphenyl phosphate (available under the trademark PHOSFLEX ^® 31L from ICL Industrial Products). Preferably, the surface covering of the invention comprises resorcinol bis(diphenyl phosphate) and/or bisphenol A bis(diphenyl phosphate) as organophosphorus oil. According to the invention, the surface covering may, alternatively or in addition, include one or more organohalogen oils, preferably organobromine oils and/or organochlorine oils. Suitably organohalogen oils, for example, include halogenated vegetable oils, such as halogenated castor oil, halogenated soybean oil, and halogenated ricinoleic esters. Such compounds can, for example, be synthesised by reacting epoxidized vegetable oil with hydrochloric or hydrobromic acid in a solvent. Other synthesis routes include, e.g. providing hydroxylated natural oil, and contacting the hydroxylated natural oil with halogenated reactive material such as halogenated anhydride or organic acid and alkyl esters or halogenated anhydrides or organic esters to halogenise the natural oil. The total amount of organophosphorus oil and/or organohalogen oil in the surface covering is in the range of 1-15 % by total weight of the surface covering. Preferably the total amount of organophosphorus oil and/or organohalogen oil in the surface covering is in the range of 2-14 %, 2-13 %, 3-12 %, 4-12 %, 4-11 %, 3-11 %, 3-10 %, or 5-10 %. Such amounts allow for excellent flame retardancy properties, while at the same time providing desirable processability. In a preferred embodiment, the surface covering of the invention comprises one or more organophosphorus oils containing diphenyl phosphate and/or triphenyl phosphate groups, such as neopentyl glycol bis(diphenyl phosphate), resorcinol bis(diphenyl phosphate), bisphenol A bis(diphenyl phosphate), butylated triphenyl phosphate, isopropylated triphenyl phosphate, triphenyl phosphate, isodecyl diphenyl phosphate, and cresyl diphenyl phosphate. Such organophosphorus oils may preferably be present in an amount of 1-15 % by total weight of the surface covering, such as 2-14 %, 2-13 %, 3-12 %, 4-12 %, 4-11 %, 3-11 %, 3-10 %, or 5-10 %. As the organophosphorus oil and/or organohalogen oil has flame retarding properties, the presence of additional flame retardants that are not organophosphorus oil or organohalogen oil may be limited. Additional flame retardants that may be present in the surface covering of the invention include e.g. one or more selected from the group consisting of ammonium polyphosphate, aluminium trihydrate, magnesium hydroxide, zinc stannate, antimony trioxide, zirconium phosphate, melamine cyanurate, melamine phosphate, melamine pyrophosphate, melamine polyphosphate, 2,4,6-triamino-1,3,5-triazine, 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide, mixtures of piperazine pyrophosphate and melamine pyrophosphate, piperazine pyrophosphate, a phosphonitrilic chloride, a phosphorus ester amide, a phosphoric acid amide, a phosphonic acid amide, a phosphinic acid amide, piperizine polyphosphate, phenyl-bis(dodecyl) phosphate, phenyl-bis(neopentyl) phosphate, phenyl ethylene hydrogen phosphate, phenyl-bis(3,5,5'-trimethylhexyl phosphate), ethyldiphenyl phosphate, 2-ethylhexyl di(p-tolyl)phosphate, diphenyl hydrogen phosphate, bis(2-ethylhexyl) p-tolylphosphate, tritolyl phosphate, bis(2-ethylhexyl)-phenyl phosphate, tri(nonylphenyl) phosphate, phenylmethyl hydrogen phosphate, di(dodecyl) p-tolyl phosphate, tricresyl phosphate, triphenyl phosphate, dibutylphenyl phosphate, 2-chloroethyldiphenyl phosphate, diphenyl hydrogen phosphate, resorcinol diphenyl phosphate, bisphenol A polyphosphate, bisphenol A diphenyl phosphate, bisphenol A diphosphate, (2,6-dimethylphenol)-1,3-phenylene bisphosphate, magnesium hydroxide, aluminium trihydrate (Al ₂ O ₃ ·H ₂ O), aluminium hydroxide Al(OH) ₃ , huntite, hydromagnesite, antimony trioxide, potassium hydroxide, and other flame retardants, such as aluminium anhydrate, molybdenum disulphide, clay, diatomite, kaolinite, montmorillonite, hydrotalcite, talc, silica (e.g. precipitated silica and silicates, fumed silica, etc.), white carbon, celite, asbestos, ground minerals, lithopone, and any combination thereof. Such additional flame retardants that are not organophosphorus oil or organohalogen oil (in particular the flame retardants as mentioned in the list above) may be present in the surface covering in amounts of 15 % or less by total weight of the surface covering, such as 10 % or less, 5 % or less, 1 % or less, 0.5 % or less, or 0.1 % or less. In a preferred embodiment, the surface covering comprises, next to the organophosphorus and/or organohalogen oil, as additional flame retardants one or more selected from ammonium polyphosphate and aluminium trihydrate, preferably both. In view of costs, the relatively expensive ammonium polyphosphate is used in limited amounts. The weight ratio between aluminium trihydrate and ammonium polyphosphate can accordingly be in the range of from 20 : 1 to 3 : 1, preferably from 15 : 1 to 5 : 1, more preferably from 12 : 1 to 8 : 1. Preferably, the surface covering of the invention comprises 1-5 % by total weight of the surface covering of ammonium polyphosphate (such as 2-4 %) and 10-40 % by total weight of the surface covering of aluminium trihydrate (such as 15-35 %). In a special embodiment, the surface covering further comprises a crosslinker selected from the group consisting of pentaerythritol, stearyl alcohol and mannitol, preferably pentaerythritol. Upon heating, such as above 250 °C, these compounds cause cross-linking of the polyvinyl butyral material, thereby forming a thick carbon barrier. As the organophosphorus oil and/or the organohalogen oil has plasticising properties, the presence of additional plasticisers that are not organophosphorus oil or organohalogen oil may be limited. Additional plasticisers that may be present in the surface covering of the invention include e.g. one or more selected from the group consisting of adipates, di-adipates, maleates, glycols, and citrates. Particular plasticisers that are not based on vegetable oil include, aromatic diesters, such as diisononyl phthalate, diisodecyl phthalate, linear dinonyl phthalate (L9P), dioctyl terephthalate, dibutyl phthalate, dioctyl phthalate, benzylbutylphthalate and dihexyl phthalate, aliphatic diesters, such as diisononyl adipate and diisodecyl adipate, aromatic sulphonamides, such as N-n-butylbenzenesulphonamide, aromatic phosphate esters, such as tricresyl phosphate and trixylyl phosphate, alkyl phosphate esters, such as tributyl phosphate and tri-iso-octyl phosphate, dialkylether aromatic esters, such as dibutoxyethyl phthalate, dialkylether diesters, tricarboxylic esters, polymeric polyester plasticisers, polyglycol diesters, alkyl alkylether diesters, such as dibutoxyethyl glutarate, di-(2-butoxyethyl) adipate, di-(butylethoxyethyl)glutarate and di-(butoxyethoxyethyl)adipate, aromatic trimesters, such as trioctyl trimellitate and triisooctyl trimellitate, epoxodised esters, chlorinated hydrocarbons or paraffins, aromatic oils, alkylether monoesters, naphthenic oils, alkylmonoesters, glyceride oils, paraffinic oils, silicone oils, citrate plasticisers, such as tributyl citrate, and sebacate plasticisers, such as dibutyl sebacate. It is preferred that the amount of such additional plasticisers that are not organophosphorus oil or organohalogen oil in the surface covering of the invention is low. Such plasticisers not being organophosphorus oil or organohalogen oil (in particular plasticisers as mentioned in the list above) may, for instance, be present in amounts of 5 % or less by total weight of the surface covering, such as 4 % or less, 1 % or less, 0.5 % or less, or 0.1 % or less. The presence of the organophosphorus oil and/or organohalogen oil improves the processability of the polyvinyl butyral surprisingly well. This may be illustrated by the change in melt flow index of the compound composition used for preparing the surface covering of the invention as compared to the melt flow index of the polyvinyl butyral alone. The melt flow index of the compound composition used for preparing the surface covering is suitably 10-50 g/10 min, as determined by ISO 1133 at a load of 2.16 kg and at a temperature of 190 °C, preferably 12-40 g/10 min, such as 14-35 g/10 min, 15-30 g/10 min, or 15-25 g/10 min. In certain embodiments, the surface covering may comprise one or more colouring agent, in particular a white pigment. This may, for example, be advantageous in applications where the surface covering is exposed to ultraviolet radiation, such as by sunlight. Preferred colouring agents, for example, include titanium dioxide, and barium sulphate. In such applications, the colouring agent may serve to increase reflection of ultraviolet radiation. As a further advantage, the increased reflection has the effect that the surface covering will heat up less. Additionally, the white pigment may serve as a blocker of ultraviolet radiation and slow down any degradation of organic material, such as the polyvinyl butyral and/or phosphorus containing vegetable oil-based derivative. The presence of a colouring agent, such as a white pigment, is particularly advantageous in case the surface covering is a roof covering, or a façade covering. The amount of colouring agent in the surface covering may be in the range of 10 % or less by total weight of the surface covering, such as in the range of 0.1-10 %, 0.1-9 %, 0.2-8 %, 0.3-7 %, 0.5-5 %, or 1-3 %. The reflectance of the surface covering may also be expressed as in terms of Solar Reflectance Index (SRI). The SRI is a measure of the solar reflectance and emissivity of materials that can be used as an indicator of how hot materials are likely to become when solar radiation is incident on the surface thereof. The lower the SRI, the hotter a material is likely to become in the sunshine. SRI can be measured in accordance with ASTM E 1980-01. Preferably, the surface covering of the invention has a SRI of 20 or higher, such as 30 or higher, 40 or higher, or even 50 or higher. The surface covering of the invention may further comprise one or more additives. Such additives may suitably be selected from the group consisting of ultraviolet blocking agents, blowing agents, antioxidants, processing aids, pigments, dyes, fillers, antibacterial agents, release agents, heat stabilisers, light stabilisers, compatibilisers, inorganic material additives, surfactants, coupling agents, impact-reinforcing agents, lubricants, weather-resistant agents, adhesion aids, adhesives, and any combination thereof. Examples of suitable ultraviolet blocking agents include titanium dioxide, carbon black, and combinations thereof. Examples of suitable blowing agents include azodicarbonamide, expandable microspheres, p-p'-oxy-bis(benzenesulphonylhydrazide), p-toluene sulphonyl semicarbizide, sodium bicarbonate, citric acid, and any combination thereof. If present, the amount of the blowing agents can be 5 % or less by total weight of the surface covering, preferably in the range of 0.01-4 %, more preferably in the range of 0.05-3 %. Examples of suitable processing aids include metal salts of carboxylic acids, such as zinc stearate or calcium stearate, fatty acids, such as stearic acid, oleic acid or erucic acid, fatty amides, such as stearamide, oleamide, erucamide or N,N'-ethylene bis-stearamide, polyethylene wax, oxidised polyethylene wax, polymers of ethylene oxide, copolymers of ethylene oxide and propylene oxide, vegetable waxes, petroleum waxes, non-ionic surfactants, fluoropolymers, such as polytetrafluoroethylene and the like, and polysiloxanes. The amount of the processing aids can be 5 % or less by total weight of the surface covering, preferably in the range of 0.05-5 %, more preferably in the range of 0.1-3 %. Examples of suitable pigments include carbon black, titanium dioxide, barium sulphate, and any combination thereof. The amount of the pigments can be 10 % or less by total weight of the surface covering, preferably in the range of 0.5-10 %, more preferably in the range of 1-5 %. Examples of suitable dyes include organic dyes, such as coumarins, lanthanide complexes, hydrocarbon and substituted hydrocarbon dyes, polycyclic aromatic hydrocarbons, scintillation dyes (preferably oxazoles and oxadiazoles), aryl- or heteroaryl-substituted poly(C _2-8 olefins), carbocyanine dyes, phthalocyanine dyes and pigments, oxazine dyes, carbostyryl dyes, porphyrin dyes, acridine dyes, anthraquinone dyes, arylmethane dyes, azo dyes, diazonium dyes, nitro dyes, quinone imine dyes, tetrazolium dyes, thiazole dyes, perylene dyes, perinone dyes, bis-benzoxazolylthiophene, xanthene dyes, and any combination thereof. The amount of the dyes can be 5 % or less by total weight of the surface covering, such as 0.1-5 %, 0.2-4 %, 0.3-3 %, or 0.5-2 %. Examples of suitable fillers include carbon black, wollastonite, solid microspheres, hollow microspheres, kaolin, clay-based minerals, bauxite, calcium carbonate, feldspar, barium sulphate, titanium dioxide, talc, pyrophyllite, quartz, natural silica, such as crystalline silica and microcrystalline silica, synthetic silicates, such as calcium silicate, zirconium silicate and aluminium silicate, including mullite, sillimanite, cyanite, andalusite and synthetic alkali metal aluminosilicates, microcrystalline novaculite, diatomaceous silica, perlite, synthetic silica, such as fumed silica and precipitated silica, antimony oxide, bentonite, mica, vermiculite, zeolite, and combinations of metals with various salts, such as calcium, magnesium, zinc, barium, aluminium combined with oxide, sulphate, borate, phosphate, carbonate, hydroxide and the like, and any combination thereof. It is preferred that the amount of fillers (such as carbon black) is 10 % or less by total weight of the surface covering, such as 1-8 %, or 2-5 %. Examples of suitable antioxidants include hindered phenols, such as tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate )] methane, bis[(β-(3,5-di-tert-butyl-4-hydroxybenzyl)-methylcarboxyeth yl)] sulphide, 4,4'-thio-bis(2-methyl-6-tert-butylphenol), 4,4'-thio-bis(2-tert-butyl-5-methylphenol), 2,2'-thio-bis(4-methyl-6-tert-butylphenol), thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate, phosphites and phosphonites, such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenyl-phosphonite, thio compounds, such as dilaurylthiodipropionate, dimyristylthiodipropionate and distearylthiodipropionate, various siloxanes, polymerised 2,2,4-trimethyl-1,2-dihydroquinoline, N,N'-bis(1,4-dimethylpentyl-p-phenylenediamine), alkylated diphenylamines, 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, diphenyl-p-phenylenediamine, mixed di-aryl-p-phenylenediamines and other hindered amine antidegradants or stabilisers, and or any combination thereof. Both primary and secondary antioxidants may be applied. Primary antioxidants function as radical catchers and particularly remove peroxyl radicals (ROO•) and, to a lesser extent, alkoxy radicals (RO•), hydroxyl radicals (HO•), and alkyl radicals (R•). Oxidation begins with the formation of alkyl radicals, which react rapidly with molecular oxygen and thus form peroxyl radicals. Secondary antioxidants particularly remove organic hydroperoxides (ROOH) formed by the effect of primary oxidants. Hydroperoxides are less reactive than radicals, but undergo hemolytic bonding and break new radicals. The amount of the antioxidants can be 5 % or less by total weight of the surface covering, preferably in the range of 0.1-5 %, more preferably in the range of 0.2-3 %. Examples of the weather-resistance agents include benzophenone-type weather resistance agents, amine-type weather resistance agents, and combinations thereof. The total amount of additives is preferably 10 % or less by total weight of the surface covering, such as 0.1-10 %, 0.2-8 %, 0.3-5 %, or 0.5-3 %. In certain applications, the surface covering can comprise a support carrier. Such a support carrier may further improve the mechanical properties of the surface covering. Suitable support carriers may be selected from the group consisting of glass fibre, carbon fibre, basalt fibre, natural fibres (e.g. flax fibre, and hemp fibre), aramid fibre, polyester fibre, polyethylene fibre (including e.g. Dyneema ^® fibre), and any combination thereof. More preferably said support carrier comprises glass fibre. The support may have the form of a laminate. In other embodiments, the surface covering is self-supporting. In such embodiments, the surface covering may have a fibrous material content of 1 % or less based on total weight of the surface covering, such as 0.5 % or less, or 0.1 % or less. Preferably, the surface covering is free from fibrous material in these embodiments. Apart from polyvinyl butyral, the surface covering may comprise one or more additional polymers. These can, for instance, be selected from the group consisting of thermoplastic polyurethanes, polyisobutenes, polychloroprene, polycaprolactones, polyesters, thermoplastic polyolefins, polyolefins, silicones, siloxanes, thermoplastic elastomers, including vulcanised thermoplastic elastomers and styrenic block copolymers, polyvinyl chlorides, styrene butadiene copolymers including poly(styrene-butadiene-styrene), poly(styrene-ethylene/butadiene-styrene) and poly(styrene-ethylene/propylene-styrene), copolymers of ethylene and vinylacetate. Preferably, the additional polymers can comprise one or more selected from the group consisting of polyisobutenes, polycaprolactones, polyesters, silicones, thermoplastic elastomers, such as vulcanised thermoplastic elastomers and styrenic block copolymers, thermoplastic polyolefins, and thermoplastic polyurethanes. In a preferred embodiment the surface covering of the invention comprises - polyvinyl butyral, preferably in an amount of 50-90 % by total weight of the surface covering, such as 60-90 %, 60-80 %, or 60-70 %; - one or more organophosphorus oils and/or one or more organohalogen oils, preferably in an amount of 1-15 % by total weight of the surface covering, such as 2-12 % or 3-10 %; - ammonium polyphosphate, preferably in an amount of 1-5 % by total weight of the surface covering, such as 2-4 %; and - aluminium trihydrate, preferably in an amount of 10-40 % by total weight of the surface covering, such as 15-35 %, or 20-30 %. Even more preferred is a surface covering comprising - polyvinyl butyral, preferably in an amount of 50-90 % by total weight of the surface covering, such as 60-90 %, 60-80 %, or 60-70 %; - organophosphorus oil comprising one or more biphenyl phosphate and/or triphenyl phosphate groups, preferably in an amount of 1-15 % by total weight of the surface covering, such as 2-12 % or 3-10 %; - ammonium polyphosphate, preferably in an amount of 1-5 % by total weight of the surface covering, such as 2-4 %; and - aluminium trihydrate, preferably in an amount of 10-40 % by total weight of the surface covering, such as 15-35 %, or 20-30 %. Different components of the surface covering described herein may be present in a single layer (i.e. two or more selected from polyvinyl butyral, one or more organophosphorus oils and/or one or more organohalogen oils, additional flame retardant, crosslinker, additional plasticiser, colouring agent, ultraviolet blocking agents, blowing agents, antioxidants, processing aids, pigments, dyes, fillers, antibacterial agents, release agents, heat stabilisers, light stabilisers, compatibilisers, inorganic material additives, surfactants, coupling agents, impact-reinforcing agents, lubricants, weather-resistant agents, adhesion aids, adhesives). In particular, it is preferred that the polyvinyl butyral and the one or more organophosphorus oils and/or one or more organohalogen oils are present in the same layer of the surface covering. More preferably, the surface covering is a single layer surface covering. The surface covering of the invention may suitably have a thickness in the range of 0.3-5 mm, such as 0.5-4 mm, 0.8-3 mm, or 1-2 mm. The surface covering may optionally comprise at least one release sheet for storage. The presence of such release sheet may provide protection to the synthetic flashing material when stored, such as on a roll. The release sheet may suitably be removed from the surface covering by peeling off before, during and/or after applying the surface covering to a surface, such as the surface of a construction. The release sheet may be present on the top and/or bottom of the surface covering. The release sheet may comprise one or more materials selected from the group consisting of polytetrafluoroethylene, silicones, waxed paper, parchment paper, plastic sheets, such as bubble wrap, and other release sheets known in the art. The surface covering of the invention can reach UL 94 V-2 flammability rating at a thickness of 1.5 mm, preferably UL 94 V-1, more preferably UL 94 V-0. The ratings refer to Underwriters-Laboratories standard UL 94. In the UL 94 test, a specimen is exposed vertically to a flame for 10 seconds. The specimen is ignited at the bottom and burns up. If the specimen self-extinguishes within 30 seconds, another 10 seconds application is made. Flaming droplets are allowed to fall on cotton located below the sample. If the average burning time is less than 5 seconds (per application of flame) and the droplets do not ignite the cotton, the material is classified as UL 94 V-0. If the average of burning time is less than 25 seconds and the droplets do not ignite the cotton, the material is classified as UL 94 V-1. If the average burning time is less than 25 seconds but the droplets ignite the cotton, the material is classified as UL 94 V-2. Another way of determining flame retardancy is by performing a limiting oxygen index (LOI) measurement according to ISO 4589. Preferably, the surface covering of the invention can reach a LOI value of 21.0 % or more, more preferably 22.0 % or more. The inventors surprisingly found that the presence of the organophosphorus oil and/or organohalogen oil (preferably those comprising one or more diphenyl phosphate and/or triphenyl phosphate groups) in the surface covering of the invention yields remarkably good fire retardancy. In addition, the inventors surprisingly found that a surface covering of the invention including the organophosphorus oil and/or organohalogen oil has improved rigidness, as compared to a surface covering lacking said component. In a further aspect, the invention is directed to a method for preparing a surface covering as described herein, comprising a) providing a feed stream of recycled polyvinyl butyral, b) optionally, pre-treating the feed stream of recycled polyvinyl butyral to remove one or more components therefrom, preferably said one or more components comprise volatile plasticisers, c) compounding the feed stream of recycled polyvinyl butyral from step a) or the pre-treated feed stream of recycled polyvinyl butyral from step b) with one or more organophosphorus oils and/or one or more organohalogen oils, and d) forming the compounded material into a layer to produce the surface covering. Suitably, the feed stream of recycled polyvinyl butyral in step a) has a melt flow index of 1-10 g/10 min, as determined by ISO 1133 at a load of 2.16 kg and at a temperature of 190 °C, preferably 2-7 g/10 min, such as 3-6 g/10 min, or 4-5 g/10 min. The optional pre-treatment in step b) is highly advantageous since recycled polyvinyl butyral typically originates from various sources with varying plasticiser content. The optional pre-treatment in step b) allows for removing undesirable components (such as relatively volatile components including plasticisers) from the recycled polyvinyl butyral, resulting in a polymer product having more constant properties, such as melt flow index. Preferably, said pre-treating step b) comprises feeding the feed stream of recycled polyvinyl butyral to an extruder, melting said feed stream in the extruder to produce polymer melt and passing said polymer melt through one or more degassing zones connected to a vacuum pump, and measuring the melt flow index of the polymer product, wherein the vacuum pump is operated at a pressure that is controlled by the measured melt flow index of the polymer product. Such pre-treatment is described in more detail in WO-A-2022/098237. In step c), the feed stream of recycled polyvinyl butyral from step a) or the pre-treated feed stream of recycled polyvinyl butyral from step b) is compounded together with one or more organophosphorus oils and/or organohalogen oils and, optionally, any other components that are used in the surface covering. The melt flow index of the compound composition with the organophosphorus oil and/or organohalogen oil in step c) that is used for forming the surface covering in step d) is suitably 10-50 g/10 min, as determined by ISO 1133 at a load of 2.16 kg and at a temperature of 190 °C, preferably 12-40 g/10 min, such as 14-35 g/10 min, 15-30 g/10 min, or 15-25 g/10 min. Then, the compounded material may be formed into a layer to produce the surface covering. Preferably, step c) and/or step d) of the method for preparing the surface covering of the invention are performed at a temperature of 190 °C or less, more preferably at a temperature of 180 °C, such as at a temperature of 175 °C or less. At temperatures above 190 °C, both flame retardancy properties and mechanical properties are reduced, due to degradation of components. For practical processability, the surface covering is then preferably supplied on a roll for storage and transport, optionally using a release sheet as described herein. Such a roll may have a width of typically 0.5-5 m, such as 1-4 m, or 1.5-3 m. The lengths of the roll may be 5-200 m, such as 10-150 m, 12-100 m or 15-80 m. In yet a further aspect, the invention is directed to a method for applying a surface covering as described herein onto a surface, comprising i) applying strips of surface covering on a surface, whereby adjacent strips of surface covering have overlapping regions; and ii) bonding the overlapping regions of the strips of surface covering using an alcohol solvent. The surface covering can thus readily be applied on a surface, thereby allowing for a waterproof seam. The alcohol solvent preferably comprises one or more selected from the group of methanol, ethanol, iso-propyl alcohol, n-propyl alcohol, tert-butyl alcohol, n-butanol, and iso-butyl alcohol. More preferably, the alcohol solvent comprises or is iso-propyl alcohol. Preferably, the alcohol solvent is provided with a colour indicator. Some suitable but non-limiting examples thereof include methylene blue, methyl viologen, isosulfan blue and indigo carmine. Preferably, the alcohol solvent comprises methylene blue. These colour indicators allow visualising which parts have treated with the alcohol solvent. As such, the operator is able to check whether he has not missed any spots. When the surface covering is applied on a surface that is exposed to UV radiation, such as on a roof, then any colour indicator exposed to UV radiation will degrade. However, colour indicator that is present between the overlapping parts of the strips of surface covering is not exposed to the UV radiation and will accordingly not degrade (or to a much less extent). Upon recycling the surface covering, the strips of surface covering are separated and collected. In the overlapping regions, the colour indicator will be uncovered, which can advantageously serve as an identifier for the origin of the strips of surface covering. In a special embodiment, small pieces of surface covering are fixed on the surface, such as by screwing, at regular intervals. The strips of surface covering can thereafter be bonded to these fixed small pieces using alcohol solvent. This allows easy recovery of the strips of surface covering at the end of its service life. New strips of surface covering can then be applied on the remaining fixed small pieces. All references cited herein are hereby completely incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. Preferred embodiments of this invention are described herein. Variation of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject-matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. The invention will now be further illustrated by means of the following examples. Examples Different extrusion compound examples were prepared as shown in Table 1 below, wherein PVB is polyvinyl butyral, ATH is aluminium trihydrate, APP is ammonium polyphosphate, and RDP is resorcinol bis(diphenyl phosphate). The samples were injection moulding into tensile test bars for limiting oxygen index (LOI) measurements according to ISO 4589 on a FTT LOI apparatus. In a controlled oxygen percentage (O ₂ /N ₂ mixture) atmosphere a specimen was ignited by a flame and the fire behaviour and burning time was recorded. When the sample extinguished the oxygen percentage was increased and when the sample did not extinguish the oxygen percentage was decreased. The lowest oxygen percentage at which the specimen extinguished was the recorded LOI value. The results are shown in Table 1 below. Table 1 Examples D and E, which include the resorcinol bis(diphenyl phosphate) showed a surprising increase in limiting oxygen index, as compared to Examples A and C, which have about the same total amount of fire retardant as Example D and E, respectively. Additionally, tensile test specimens type 1A according to ISO 527-2 were cut from a surface covering and analysed using dynamic mechanical analysis (DMA). The storage modulus and loss modulus of the samples was determined at room temperature. The sample was clamped in a single cantilever clamp and oscillated at different frequencies (1 to 100 Hz) and an amplitude of 0.1 % strain. The storage modulus represents the strength of the material and the loss modulus represents the ability to absorb and dissipate energy. The results of three different samples are shown in Figure 1. Samples 1 and 3 are samples with 70 wt.% PVB, 27.27 wt.% of ATH and 2.73 wt.% of APP. Sample 2 is a sample with 70.9 wt.% PVB, 18.2 wt.% of ATH, 1.82 wt.% of APP, and 9.09 wt.% of RDP oil. Both samples without the resorcinol bis(diphenyl phosphate) oil (Samples 1 and 3) have comparable strength values. The sample with resorcinol bis(diphenyl phosphate) oil (Sample 2) is surprisingly more rigid than the other samples.