A flat roof is a type of covering for a building. In contrast to the more sloped form of roof, a flat roof is horizontal or nearly horizontal.
The slope of a roof is properly known as its pitch: Roofs which are nearly flat are known as low-pitch roofs (as the angle of slope increases, so the pitch is said to increase).
Any sheet of material used to cover a flat or low-pitched roof is usually known as a membrane and the primary purpose of these membranes is to waterproof the roof area. Materials that cover flat roofs typically allow the water to run off from a slight inclination or camber into a gutter system. Water from some flat roofs such as on garden sheds sometimes flows freely off the edge of a roof, though gutter systems are of advantage in keeping both walls and foundations dry. Gutters on smaller roofs often lead water directly onto the ground, or better, into a specially made soakaway. Gutters on larger roofs usually lead water into the rainwater drainage system of any built up area. Occasionally flat roofs though are designed to collect water in a pool, usually for aesthetic purposes, or for rainwater buffering.
Flat roofs exist all over the world and each area has its own tradition or preference for materials used. In warmer climes where rainfall is less and freezing is unlikely to occur, many flat roofs are simply built of masonry or concrete and this is good at keeping out the heat of the sun and cheap and easy to build where timber is not readily available. In areas where the roof could become saturated by rain and leak, or where water soaked into the brickwork could freeze to ice and thus lead to 'blowing' (breaking up of the mortar/brickwork/concrete by the expansion of ice as it forms) these roofs are not suitable.
Traditionally most flat roofs in the western world make use of use of tar or more usually tar paper applied over roof decking to keep a building watertight. The tar or tarpaper is in turn covered in gravel to keep the sun's heat, UV rays and weather off it and helps protect it from cracking or blistering and degradation. Roof decking is usually of plywood, chipboard or OSB boards (OSB = Oriented Strand Board, also known as Sterling board) of around 18mm thickness. The tar coating is applied in one or more coats as a hot liquid, heated in a tar kettle, or as sheets of tar impregnated 'paper' glued down and sealed together at joints by hot tar - once it has cooled, the gravel is spread evenly over it.
A main reason for failure of these traditional roofs is ignorance or lack of maintenance where people or events cause the gravel to be moved or removed from the tar or tarpaper waterproofing, thus exposing it to weather and sun whereupon cracking and blistering occurs over time and eventually water gets in.
Tarpaper is usually a 'paper' or fibre material soaked or impregnated in tar. As gravel cannot protect tarpaper surfaces where they rise vertically from the roof such as on parapet walls or upstands, tarpaper variants are produced with fine gravel applied to the hot tar during the process of manufacture such that a permanent layer of gravel is stuck to it in order to give it ongoing protection.
Tarpapers vary in quality as does the workmanship and training of those applying it to produce more or less successful results. In some micro climates or shaded areas these rather 'basic' tarpaper roofs can last well in relation to the cost of materials purchase and cost of laying them, however the cost of modern membranes such as EPDM has come down over recent years to make them more and more affordable. There are now more firms supplying modern alternatives and there is generally a growing awareness of their performance and availability.
If a leak does occur on a flat roof then damage often goes unnoticed for long periods of time during which water penetrates and soaks the decking and any insulation and/or structure beneath it. This can lead to expensive damage from the rot which often develops and if left can weaken the roof structure. There are also various health risks to people and animals breathing the mould spores: the severity of this health risk remains a debated point.
One problem with maintaining flat roofs is that if water does penetrate the barrier covering (be it traditional or a modern membrane) it can travel a long way before causing visible damage or leaking into a building where it can be seen. Thus it is not easy to see where it is leaking in order to repair it. Once underlying roof decking is soaked it often sags causing a further collection or puddling of water in that area and the problem worsens.
Another common reason for failure of flat roofs is lack of maintenance of drains whereby gravel, leaves and debris block water outlets (be they spiggots, drains, downpipes or gutters), thus water cannot drain away and builds up. This causes a pressure head of water (the deeper the water the greater the pressure) which can 'push' more water into the smallest hole or crack. In colder climates, puddling water can freeze into ice which breaks up the roof surface as it expands whilst it freezes.
An important consideration in tarred flat roof quality is differentiating what many call simply 'tar' into rather different products: tar or pitch (which is derived from wood resins), coal tar, asphalt and bitumen. Some of these products appear to have been interchanged in their use and are sometimes used inappropriately, as each has different characteristics, for example their ability or not soak into wood/boards, their anti-funghal properties and their reaction to exposure to sun and weather and varying temperatures.
Modern flat roofs can use single large factory made sheets (eg EPDM synthetic rubber) and these are usually of excellent quality, though unfortunately one-piece membranes are seldom seen used in practice. Rather, strips of EPDM, PVC, tar paper etc. which are widely available in rolls (typically of 900 mm or 1200mm width) are bonded together in either hot or cold seeming processes on the roof during the fitting process where labour skill and training play a large part in determining the quality of roof protection attained. Reasons for not using excellent one-piece membranes include practicality and cost-saving:- on all but the smallest of roofs it can be difficult to lift a huge and heavy membrane to the roof (a crane or lift is required) and if there is any wind at all it can be difficult to control and bond the membrane smoothly and properly to the roof.
Detailing of these various systems also plays a part in success or failure: In some systems ready-made details (such as internal and external corners, through-roof pipe flashings, cable or rooflight flashings etc.) are available from the membrane manufacturer and can be expediently and well bonded to the main sheet, whereas with materials such as tar papers this is usually not the case - a fitter has to make these shapes up on-site or on the roof and success depends largely on their levels of skill, enthusiasm and training - results can vary hugely.
Metals are also used for flat roofs, these can be made from lead (welded or folded-seamed), tin (folded, soldered or folded-seamed) or copper. These are often expensive options and vulnerable to being stolen and sold as scrap metal.
Flat roofs tend to be sensitive to human traffic. Anything which produces a crack or puncture in the waterproofing membrane can quite readily lead to leaks. Flat roofs can fail, for example; when subsequent work is carried out on the roof, when new through-roof service pipes/cables are installed or when plant such as Air Conditioning Units are installed. A good roofer should be called to make sure the roof is left properly watertight before it is left. In trafficked areas, proper advisory/warning signs should be put up and walkways of rubber matting, wooden or plastic duck-boarding etc. should be installed to protect the roof membrane. On some membranes even stone or concrete paving can be fitted. For one-off works, old carpet or smooth wooden planks for workers to walk or stand on will usually provide reasonable protection.
Traditionally the smelly, hot, physically demanding and sometimes dangerous work of tarring flat roofs has often meant that uneducated fitters of doubtful reputation have done work to a poor standard: This together with a lack of regular inspection and maintenance has meant that flat roofs have a poor reputation and there is an unwillingness to retain or to build them which is unfortunate given the potential usefulness of flat areas, the more so with the excellent performance of modern membranes, many of which come with long warranties and which provide an excellent roof covering.
Modernist architecture often viewed the flat roof as a living area. Le Corbusier's theoretical works, particularly Vers une Architecture, and the influential Villa Savoye and Unité d'Habitation prominently feature rooftop terraces. That said, Villa Savoye's roof commenced leaking almost immediately after the Savoye family moved in. Le Corbusier only narrowly avoided a lawsuit from the family due to the fact they had to flee the country as France succumbed to the German Army in WWII.
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Grass or turf roofs have been around since the Viking times if not far earlier and make for a decorative and durable roof covering. In recent years 'green roofs' have been made by depositing topsoil or other growth media on flat roofs and seeding them (or allowing them to self-seed as nature takes its course). Maintenance in the form of simple visible inspection and removal of larger rooting plants has allowed these roofs to be rather successful in that they provide an excellent covering and UV light barrier for the roof waterproofing membrane. With some systems the manufacturer requires that a root barrier membrane be laid above the waterproofing membrane. If well planned and fitted, the mass of the soil or growth medium can provide a good heat buffer for the building - storing the heat of the sun and releasing it into the building at night and thus keeping inside temperatures more even. Sudden cold spells are also buffered from the building.
One predicted problem with large green roofs is that fire may be able to spread rapidly across areas of dry grasses and plants when they are dried, for instance, in summer by hot weather: Various countries are stipulating fire barrier areas made of, for example, wide strips of (partly decorative) gravel.
Green roofs are generally pleasing to the eye and can provide a habitat for all manner of flora and fauna.
As research goes on and practical knowledge of the behaviour of various green roof designs is gained, so various particular types of plants are used. Sedum is emerging as a favourite as it's easily transported and requires little maintenance as it is a succulent plant which remains close to the ground throughout its growth, has mild roots which don't damage the waterproofing membrane and changes colour in various seasons in greens, browns and purples to give a pleasing effect to the eye.
Water run-off and flash floods have become a problem especially in areas where there is a large amount of paving such as in inner cities: When rain falls (instead of draining into the ground over a large area as previously) a rainwater system's pipes take water run-off from huge areas of paving, road surfaces and roof areas - as areas become more and more built up these systems cope less and less well until even a rain-shower can produce backing up of water from pipes which can't remove the large water volume and flooding occurs. By buffering rainfall, such as by fitting green roofs, floods can be reduced or avoided: the rain is absorbed into the soil/roof medium and runs off the roof bit by bit as the roof becomes soaked.
Brown field sites are construction sites where an existing building or man made structure have been before, as opposed to green field sites where man expands his building activities into natural areas. An interesting idea is brown-roofs: Here the idea is to use building waste to provide a new growth medium on a flat roof (e.g. rubble/bricks produced as a by-product when an existing structure is demolished). Advantages are that there is no need to transport rubble away from the site, and that various flora and fauna species which thrive on rubble landscapes can prosper - there are numerous examples of these. Unfortunately, in the UK at least, the idea is not so easy to put into practice, as bureaucracy dictates that rubble be analysed and often small amounts of what may be 'technically' termed pollution which may have lain 'happily' for many years forbids that it be reused on the roof. If the architect of the building-to-be has specified a brown roof then sometimes rubble is specially brought in on trucks, which to some extent defeats the object of nature conservancy, landfill waste reduction, transport pollution reduction and transport volume reduction. These brown roofs do provide a successful wildlife habitat when planned and implemented correctly. They are either specially seeded with chosen plant types or allowed to self-seed and make an attractive and visually pleasing roof.
Asphalt is an aliphatic compound and in almost all cases a byproduct of the oil industry. Some asphalt is manufactured from oil as the intended purpose, and this is limited to high quality asphalt produced for longer lasting asphalt built-up roofs. Asphalt ages through photo-oxidation accelerated by heat. As it ages, the asphalts melt point rises and there is a loss of plasticizers. As mass is lost, the asphalt shrinks and forms a surface similar to alligator skin. Asphalt breaks down slowly in water, and the more exposure the more rapid the degradation. Asphalt also dissolves readily when exposed to oils and some solvents.
There are four types of roofing asphalt. Each type is created by heating and blowing with oxygen. The longer the process the higher the melt-point of the asphalt. Therefore, Type I asphalt has characteristics closest to coal tar and can only be used on dead level surfaces. Type II, is considered flat and can be applied to surfaces up to 1/4 in 12 slopes. Type III, is considered to be "steep" asphalt but is limited to slopes up to 2 in 12, and Type IV is "special steep". The drawback is, the longer it is processed, the shorter the life. Dead level roofs where Type I asphalt was used as the flood and gravel adhesive has performed nearly as well as Coal Tar. Asphalt roofs are also sustainable by restoring the lifecycle by making repairs and recoating with compatible products. The process can be repeated as necessary at a significant cost savings with very little impact on the environment.
Asphalt BUR is made up of multiple layers of reinforcing plies and asphalt forming a redundancy of water proofing layers. The reflectivity of built up roofs depend on the surfacing material used. Gravel is the most common and they are referred to as asphalt and gravel roofs. Asphalt degradation is a growing concern. UV-rays oxidize the surface of the asphalt and produces a chalk-like residue. As plasticizers leach out of the asphalt, asphalt built up roofs becomes brittle. Cracking and alligatoring inevitably follows, allowing water to penetrate the system causing blisters, cracks and leaks. Compared to other systems, installation of asphalt roofs is energy-intensive (hot processes typically use LP gas as the heat source), and contributes to atmospheric air pollution (toxic, and green-house gases are lost from the asphalt during installation).
EPDM
Ethylene Propylene Diene Monomer is a synthetic rubber most commonly used in single-ply roofing because it is readily available and relatively simple to apply. EPDM as a roofing membrane has advanced significantly over recent years. Problems previously associated with it included moisture gain under the membrane by vapour drive (occurring on roofs with air conditioned space beneath), and that EPDM did not like to adhere to itself and seam problems occurred. Simply adding a vapour barrier will help to resolve vapour drive.
Seaming has become simple with the addition of Factory Applied Tape, resulting in a faster installation. The addition of these tapes has reduced labour by as much as 75%. Rolls of EPDM are available with Factory Applied Tape pre-applied to one edge. This is an uncured EPDM tape. The other edge is marked to indicate the appropriate ovelap. The Factory Applied Tape is laid into the primed overlap and rolled with a little pressure. The resulting seam is stronger, and neater. Any details are taken care of with the appropriate tape. The process involves applying primer with a brush, allowing it to flash off to touch dry (this takes moments), then applying the tape and rolling to ensure it is properly bonded.
It is a low cost membrane, but when properly applied in appropriate places, its current warranted life-span has reached 30 years and its expected life-span has reached 50 years and this continues to rise with every year that passes.
Typically, there are three installation methods. Ballasted at 1,000 lbs/sq or 10 lbs/sq.ft. with large round stones. Mechanically attached is another method and is suitable in some applications where wind velocities are not usually high. Fully adhered is the most expensive installation method but proves to give the longest performance of the three methods.
The new generation of EPDM, with a FleeceBack, has been combined with a polyester fleece backing and fabricated with a patented hot melt adhesive technology which provides consistent bond strength between the fleece backing and the membrane. This has resulted in largely eliminating shrinkage of the product, whilst still allowing it to stretch up to 300% and move with the building through the seasons. The fleece improves puncture and tear resistance considerably and .045 mil EPDM with a fleece backing is 180% stronger than .060 mil bare EPDM. Fleecebacked EPDM has a tear strength of 39.9N/mm compared to 13.1N/mm of that without the fleece reinforcement, more than 3X the strength. Bare .045 EPDM rubber is a 1.14mm (.045) thick membrane with no fleece backing and holds a L4 puncture rating (the highest rating)
Turbo Seal
Turbo Seal [1] is a self healing gel like membrane that never cures. Made of 45% recycled tire rubber, it goes on top of existing tar (asphalt) roofs then capped with a sheet membrane.
CSPE
Chlorosulfonated Polyethylene is a synthetic rubber roof flashing material. It is more popularly known as Hypalon. This product is usually reinforced and depending upon manufacturer, seams can be heat welded or adhered with a solvent based adhesive. (No longer available in the US as a full roof membrane.)
Modified Bitumen
A bitumen is a term applied to both coal tar pitch and asphalt products. Modified Bitumens were developed in Europe in the 1970s when Europeans became concerned with the lower performance standards of roofing asphalt. Modifiers were added to replace the plasticizers that had been removed by advanced methods in the distillation process. The two most common modifiers are APP (attactic polypropylene) from Italy and SBS (styrene-butadiene-styrene)from France. The United States started developing modified bitumen compounds in the late 1970s and early 1980s. APP was added to asphalt to enhance aging characteristics and was applied to polyester, fiberglass, or polyester and fiberglass membranes to form a sheet good, cut in manageable lengths for handling. Usually applied by heating up the underside of the roll with a torch provided a significant fire hazard and was outlawed in some municipalities when buildings caught fire and some burnt to the ground. Another problem developed when a lack of standards allowed some manufacturers to produce goods with amounts of APP insufficient to enhance the aging characteristics. SBS is used as a modifier for enhancing substandard asphalt and provides a degree of flexibility much like rubber. It also is applied to a myriad of carriers and produced as a sheet-good in rolls that can be easily handled. SEBS - (styrene ethylene butadiene styrene) is a relatively new formulation increasing flexibility of the sheet and longevity.
Cold applied liquid membranes
An increasingly popular choice for new roofs and roof refurbishment. This type of roof membrane is generally referred to as Liquid Roofing and involves the application of a cold liquid roof coating. No open flames or other heat sources are needed and the glassfibre reinforced systems provide seamless waterproofing around roof protrusions and details. Systems are based on flexible thermoset resin systems such as polyester and polyurethane. In the United Kingdom, liquid coatings are the fastest growing sector of the flat roof refurbishment market. Between 2005 and 2009 the UK's leading manufacturers have reported a 70% increase in the roof area covered by the coating systems supplied.[1]
PVC (vinyl) membrane roofing
Polyvinyl Chloride (PVC) membrane roofing is also known as vinyl roofing. Vinyl is derived from two simple ingredients: fossil fuel and salt. Petroleum or natural gas is processed to make ethylene, and salt is subjected to electrolysis to separate out the natural element chlorine. Ethylene and chlorine are combined to produce ethylene dichloride (EDC), which is further processed into a gas called vinyl chloride monomer (VCM). In the next step, known as polymerization, the VCM molecule forms chains, converting the gas into a fine, white powder – vinyl resin – which becomes the basis for the final process, compounding. In compounding, vinyl resin may be blended with additives such as stabilizers for durability, plasticizers for flexibility and pigments for color.
Thermoplastic PVC roofing is extremely strong, as its heat-welded seams form a permanent, watertight bond that is stronger than the membrane itself. PVC resin is modified with plasticizers and UV stabilizers, and reinforced with fiberglass non-woven mats or polyester woven scrims, for use as a flexible roofing membrane. PVC is, however, subject to plasticizer migration. (a process by which the plasticizers migrate out of the sheet causing it to become brittle.) Thus a thicker membrane has a larger reservoir of plasticizer to maintain flexibility over its lifespan. PVC is often blended with other polymers to add to the performance capabilities of the original PVC formulation, such as KEE - Keytone Ethylene Ester. Such blends are referred to as either a CPA - Copolymer Alloy, or a TPA - Tripolymer Alloy.
Vinyl roofs are inherently fire resistant due to their chemical composition and have a broader range of fire ratings over common substrates.
PVC has been sold for commercial roofing use for more than 40 years. Vinyl roofing membranes' long life cycle – and the associated lower energy consumption to both produce the raw material and process it into useful products – is a significant factor in their sustainability as a building product.
Vinyl roofs provide an energy-efficient roofing option due to their inherently light coloring. While the surface of a black roof can experience a temperature increase of as much as 90 degrees under the heat of the full sun, a white reflective roof typically increases only 10-25 degrees Fahrenheit.
Vinyl membranes can also be used in waterproofing applications for roofing. This is a common technique used in association with green, or planted, roofs.
It worthy of note that many Green Building organizations recommend not using PVC roofing due to significant environmental hazards from the toxicity of the manufacturing process as well as the noxious compounds released in a fire such as hydrochloric acid fumes and byproducts including dioxin, a potent carcinogen.
KEE
Keytone Ethylene Ester is much like PVC membrane roofing in many of its physical characteristics and appearance. However, some of its chemical characteristics are an improvement over the traditional PVC. For example, KEE has little or no plasticizer migration over time far increasing its lifespan over that of traditional PVC. This is the reason for several CPA and TPA blends by various manufacturers. KEE is more popularly known as Evaloy.
TPO
Thermoplastic Polyolefin single-ply roofing. This roofing material can be fully adhered, mechanically fastened, or ballasted. TPO seam strengths are reported to be three to four times higher than EPDM roofing systems. This is a popular choice for "Green" building. It is available in white, grey, and black.[2] Using white roof material helps reduce the "heat island effect" and solar heat gain in the building. However, TPO has changed formulations over the years and each manufacturer has its own mix of "polyolefins" (plastics.) This means that TPO remains largely unproven in real world applications as its current formulation exists today.
Curon
Cold-curing glass-reinforced polymer composite.
Coal-Tar Pitch Built Up Roof
Coal Tar Pitch is a known carcinogen, forbidden by code in many areas, and even where permitted it should be avoided where possible, due to health concerns. In patching existing coal tar roofs, worker and building occupant exposure should be avoided, or minimized to the maximum extent possible. Coal Tar fumes are hazardous and provisions must be made during application to prevent fumes from getting into the building. Workers should wear protective equipment and clothing, and commonly get higher compensation (Pitch Pay) for exposure to the health risks.
Coal Tar is an aromatic hydrocarbon and a by-product from the coking process of the coal industry. It is historically in abundance where coal is used in steel manufacturing. It ages very slowly through volatilization and is an excellent waterproofing and oil resistant product. Roofs are manufactured by heating the coal tar and applying between layers of coal tar saturated felts. It has limitations to application on dead level or flat roofs with slopes less than 1/4 in 12. It has a tendency to soften in warm temperatures and "heal" itself. It is always ballasted with gravel to provide a walking surface. Coal Tar provides an extremely long life cycle that is sustainable and renewable. It takes energy to manufacture and to construct a roof with it but its proven longevity with periodic maintenance provides service for many years, with ages from 50 to 70 years not uncommon, with some now performing for over a century.
Coal tar pitch is often confused with asphalt, and asphalt with coal tar pitch. Although they are both black and both are melted in a kettle when used in roofing, that is where the similarity stops.
A flat roof is the most cost-efficient roof shape as all room space can be used fully (below and above the roof) and as this roof allows easy revision/placement of solar panels[3] They also provide space for outdoor recreational use such as roof gardens. Applying a tough waterproofing membrane forms the ideal substrate for green roof planting schemes.
In general, a flat roof lasts longer if it is properly maintained. The life expectancy of a flat roof can be proportional to the maintenance done on it. Some assessors use 10 years as an average life cycle, although this is dependent on the type of flat roof system in place. Some old tar and gravel roofers quietly acknowledge that unless a roof has been neglected for too long and there are many problems in many areas, a BUR (a built up roof of tar, paper and gravel) will last 20 – 30 years. There are BUR systems in place dating to the early 1900s.
Modern cold applied liquid membranes have been durability rated by the British Board of Agreement (BBA) for 30 years. BBA approval is an important benchmark in determining the suitability of a particular fibreglass roofing system. If standard fibreglass polyester resin is used such as the same resin used in boat repairs, then there will be problems with the roof being too inflexible and not able to accommodate expansion and contraction of the building. A fit-for-purpose flexible/elastomeric resin system used as a waterproofing membrane will last for many years with just occasional inspection needed. The fact that such membranes do not require stone chippings to deflect heat means there is no risk of stones blocking drains. Liquid applied membranes are also naturally resistant to moss and lichen.
General flat roof maintenance includes getting rid of ponding water, typically within 48 hours. This is accomplished by adding roof drains or scuppers for a pond at an edge or automatic siphons for ponds in the center of roofs. An automatic siphon can be created with an inverted ring shaped sprinkler, a garden hose, a wet/dry vacuum, a check valve installed in the vacuum, and a digital timer. The timer runs two or three times a day for a minute or two to start water in the hose. The timer then turns off the vacuum, but the weight of water in the hose continues the siphon and soon opens the check valve in the vacuum. The best time to address the issue of ponding water is during the design phase of a new roofing project when sufficient falls can be designed-in to take standing water away. The quicker you get the water off the roof, the less chance there is for a roof leak to occur.
All roofs should be inspected semi-annually and after major storms. During the roof inspection particular attention should be paid to the flashings around all of the roof top penetrations. The sharp bends at such places can open up and need to be sealed with plastic cement, mesh and a small mason's trowel. Additionally, repairs to lap seams in base flashings should be made. 90% of all roof leaks and failure occur at the flashings. Another important maintenance item, often neglected, is to simply keep the roof drains free of debris. A clogged roof drain will cause water to pond, leading to increased "dead load" weight on building that may not be engineered to accommodate that weight. Additionally, ponding water on a roof can freeze. Often, water finds its way into a flashing seam and freezes, weakening the seam.
For bitumen based roof coverings maintenance also includes keeping the tar paper covered with gravel, an older method, currently being replaced with bituminous roofing membranes and the like, which must be 'glued' in place so wind and waves do not move it causing scouring and more bare spots. The glue can be any exterior grade glue like driveway coating.
Maintenance also includes fixing blisters (delaminations) or creases that may not yet be leaking but will leak over time. They may need experienced help as they require scraping away the gravel on a cool morning when the tar is brittle, cutting open, and covering with plastic cement or mastic and mesh. Any moisture trapped in a blister has to be dried before being repaired.
Roof coatings can be used to fix leaks and extend the life of all types of flat roofs by preventing degradation by the sun (ultra-violet radiation). A thickness of 30 dry mils is usually preferred and once it is fully cured, you will have a seamless, watertight membrane.
Infrared thermography is being used to take pictures of roofs at night to find trouble spots. When the roof is cooling, wet spots not visible to the naked eye, continue to emit heat. The infrared cameras read the heat that is trapped in sections of wet insulation.
Roofing systems that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to release a large percentage of absorbed, or non-reflected solar energy) are called cool roofs. Cool roofs fall into one of three categories: inherently cool, green planted roofs or coated with a cool material.
Cool roofs of all types offer various benefits. Cool roofs offer both immediate and long-term savings in building energy costs. Inherently cool roofs, coated roofs and planted or green roofs can: