Roofing Basics
Flat roof
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.
Types of Flat Roofs
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 by-product 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).
Benefits of Flat Roofs
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.