A flat roof is a type of covering of a building. In contrast to the sloped form of a roof, a flat roof is horizontal or nearly horizontal. Materials that cover flat roofs should allow the water to run off freely from a very slight inclination.
Traditionally flat roofs would use a tar and gravel based surface which, as long as there was no pooling of water, was sufficient to prevent penetration. However, these surfaces would tend to fail in colder climates, where ice dams and the like could block the flow of water. Similarly, they tend to be sensitive to sagging of the roof reversing the subtle grading of the surface.
Modern flat roofs tend to use a continuous membrane covering which can better resist pools of standing water. These membranes are applied as a continuous sheet where possible, though sealants and adhesives are available to allow for bonding multiple sheets and dealing with structures penetrating the roof surface. Far more expensive flat roof options include sealed metal roofs using copper or tin. These are soldered interlocking systems of metal panels.
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.
Flat roofs tend to be sensitive to human traffic. Anything which produces a crack or puncture in the surface can quite readily lead to leaks. In other words, this sort of roof has a major weakness to failure from subsequent work done on the roof - such as upgrading building HVAC systems and so forth. It is thus not generally advisable to use a flat roof as a living area unless steps are taken to protect the roofing membrane from those using the area, for example, by building a wooden deck over the surface or using paving stones or similar materials to protect the roof membrane. It is not advisable in general to have living areas directly under such a roof either, due to the high likelihood of eventual leakage.
One of the more interesting (re)emerging methods of protecting the roofing membrane is to use a layer of topsoil and grasses. Care should be taken not to plant anything the roots of which will penetrate the membrane surface. The green roof interestingly enough, tends to trap moisture on the roof, but keeps it up in the soil and plants, rather than having it pool down on the membrane surface.
Types of Flat Roofs Edit
Polyurethane Foam Roofing is the most energy efficient, waterproof roofing material available for flat roofs. Coated with white, elastomeric coating, sprayed on polyurethane roofing can last 30-50 years if re-coats are applied at 5 years, 25 years, and every 20 years afterwards. These roofs provide a thermal barrier that has no air infiltration.
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 the most common type of flat roof at this time. It 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).
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, such as a DPM will resolve vapour drive.
Seaming has become simple with the addition of Factory Applied Tape, resulting in fast installation & perfect joins every time. 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, neater and highly unlikely to fail. 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’s watertight. It’s a low cost membrane, but when properly applied in appropriate places, it's current 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, 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.
Turbo Seal 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.
Chlorosulfonated Polyethylene is a synthetic rubber roof. 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.
A bitumen is a term applied to both coal tar pitch and asphalt products. Modified Bitumens were developed in Europe in the 1970's 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 1970's and early 1980's. 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.
Cold applied liquid membranes
An increasingly popular choice for new roofs and roof refurbishment. 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.
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.
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.
Thermoplastic Polyolefin single-ply roofing. This roofing material can be fully adhered, mechanically fastened, or balasted. 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.  Using white roof material helps reduce the "heat island effect" and solar heat gain in the building.
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.
Benefits of Flat RoofsEdit
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  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.
Maintenance and Assessment of Flat Roofs Edit
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.
Cool Roofs Edit
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.
- Inherently cool roofs: Roof membranes made of white or light colored material are inherently reflective and achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80% or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning – a strain on both operating costs and the electric power grid.
- Green planted roofs: A green roof is a roof that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. A green roof typically consists of many layers, including an insulation layer; a waterproof membrane, often vinyl; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. Green roofs are classified as either intensive or extensive, depending on the depth of planting medium and amount of maintenance required. Traditional roof gardens, which are labor intensive and require a reasonable depth of soil to grow large plants are considered intensive, while extensive green roofs are nearly self-sustaining and require less maintenance.
- Coated roofs: One way to make an existing or new roof reflective is by applying a specifically designed white roof coatings (not simply white paint) on the roof's surface. The coating must be Energy Star rated. Reflectivity and emissivity ratings for all reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website
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:
- Reduce building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day
- Enhance the life expectancy of both the roof membrane and the building’s cooling equipment.
- Improve thermal efficiency of the roof insulation; this is because as temperature increases, the thermal conductivity of the roof’s insulation also increases.
- Reduce the demand for electric power by as much as 10 percent on hot days.
- Reduce resulting air pollution and greenhouse gas emissions.
- Provide energy savings, even in northern climates on sunny (not necessarily “hot”) days.
- ↑ Ching, Francis D. K. (1995). A Visual Dictionary of Architecture. Van Nostrand Reinhold Company, ISBN 0-442-02462-2. p. 208.
- ↑ http://www.roofhelp.com/choices/tpo/
- ↑ "Understanding How a Flat Roof Works". Free Articles. http://www.articlesbase.com/home-improvement-articles/understanding-how-a-flat-roof-works-376740.html.
- ↑ Konopacki and H. Akbari (June 2001). "Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin". Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division. http://www.vinylroofs.org/downloads/sustainability/LBNL_study2.pdf.
- ↑ "Comprehensive Cool Roof Guide from the Vinyl Roofing Division of the Chemical Fabrics and Film Association". http://vinylroofs.org/cool-roofs/cool-roofs-explained.html.
- Comprehensive Cool Roof Guide from the Vinyl Roofing Division of the Chemical Fabrics and Film Association
- Cool Roofs
- Cool Colors Project
- Heat Island
- Flat roof innovations
- Reference source for roofs; nrca is the national roofing association for america
- TPO Membranes - Technical Specifications
- The Vinyl Roofing Division of the Chemical Fabrics and Film Association