Roofs that are flat or have minimal slope to them are commonly seen to be problematic, poorly designed and prone to leakage. From a more informed point of view, a properly designed and built low pitch roof can be an easily constructed and cost effective option. Leakage can be problematic, but it usually is due more to the roof covering technologies most often seen used on flat roofs.
A low slope or flat roof has both advantages and disadvantages. Among the benefits, a flat roof will require less material to build, since it encompasses less surface area, also because it weighs less and does not require as much structural support.
A flat roof is also easier to work on, and can even be used for the new green roof concepts being touted as environmentally beneficial. Disadvantages include the fact that rain and snow collect on the roof, causing leaks and undue weight strain on the support structure, which is why choosing the right roof material becomes important.
Materials for flat roofs fall into one of two general categories: old fashioned, and modern. The older materials are found on many older buildings and although are rarely used on new commercial building nowadays, they can still be found on new residential roofs; some of the technology is more than a hundred years old (not necessarily a bad thing, mind you). Modern materials are synthetics that have been introduced in the past 30 years or so to solve a number of problems with flat roofing construction.
Build up Roof, also referred to as Tar & Gravel or Torch-down, is a layered ply roof of 3-5 rolled asphalt plys, each soaked with tar applied with either an open flame torch, or preheated then applied with a mop.
Gravel ballast is used as cover on top of the asphalt plies. The gravel holds the material under it down in high winds, and protects against foot traffic and degradation from ultraviolet light.
There is also a type of build up roof (BUR) using felt soaked in coal-tar pitch rather than asphalt. Asphalt BUR is the most prevalent flat roof material type at present. Disadvantages include the fact that installation is energy-intensive owing to the need for heating up the tar, and contributes toxic and green-house gases to the atmosphere. Asphalt BUR retains heat and can become very hot in the summer months, lowering the efficiency of air conditioning equipment and raising energy costs.
Modified Bitumen is another older roof material used on flat roofs. Developed in the 1970’s in Europe, they are a rolled roof single-ply of polyester, fiberglass, or polyester impregnated with rubber and elastomers.
Hot torch heating of the adhesive is done as the material is unrolled, although safer peel-and-stick systems have been developed recently. Modified bitumen roofing is stronger and more flexible than asphalt therefore lasts longer, but has most of the same disadvantages.
The use of open flame torches during installation and repair of these materials poses a fire hazard. Ironically, the gravel used on tar and gravel roofs is an excellent fire retardant, though on the downside, it can make finding the inevitable leaks quite difficult. These older materials, although still useful, are inefficient, have a shorter service life, are more troublesome to maintain than the newer materials, and can wind up costing you more in the long run.
Newer materials take the form of large single ply membranes bonded to the roof sheathing with adhesives or mechanical fasteners. Usually some type of moisture barrier is installed between the membrane and the sheathing.
They have the advantage of having far better attachment at seams; seams between adjacent strips of roofing material are the most common sources of leaks on flat roofs. As such, they last up to twice as long as older roof materials. These newer materials, referred to as single-ply membranes are safer and faster to install, making them the top choice for material on new and replacement flat roofs.
The other advantage of these new materials is they constitute what is called a Cool Roof, meaning they absorb much less heat during the summer. Because they are white in color, rather than black like build up asphalt or bitumen roofing, they reflect rather than absorb heat, saving energy during times of warm weather.
Ethylene Propylene Diene Monomer, (EPDM) is a black synthetic rubber membrane, usually non-reinforced. It is usually glued down to the sheathing and glued at seams and flashings. No special equipment is required for installation, making it the least expensive single ply membrane material.
An EPDM roof will have a service life of ten to fifteen years, but tend to fail at the seams when they get old. Warranties on EPDM usually do not cover the seams or ponding of water, which is another cause of leaks in them, but only cover the material itself from failures. EPDM has now been largely replaced by TPO and PVC in new building construction.
Thermo Plastic Olefin (TPO) roofing uses a two play flexible thermosetting resin system. The bottom ply is TPO and the thicker of the two. The top ply is comprised of pigments, UV stabilizers and other compounds which increase durability, fire resistance etc.; in between the two plies there is reinforcement webbing. The seams in TPO are hot air welded, which is an improvement over EPDM, but introduces the requirement for expensive hot air welding equipment for installation.
Thermoplastic olefin roofs first became widely used in the early ‘90s and have gone through 3 generations of formulations. The systems are in a state of evolving development by manufacturers; various problems, such as seam failure, cracking, hardening, and reparability are still being worked out. TPO roof materials have a service life of 7- 15 years or longer, depending on type and manufacturer, but as these materials are still relatively new, they have not yet had a chance to establish a reliable record.
Polyvinyl Chloride (PVC) is another type of two ply seamless thermoplastic membrane. It is made by various manufacturers, including GAF, IB, Firestone, Carlise, and JohnsManville. Like TPO, they feature hot air welded seams, but PVC membrane roofs have been around since the early ‘80s and have a better track record than TPO, although they are more expensive.
Hot air welding the sheets to each other at the seams and to flashings physically fuses the two together, creating a seamless bond, much like what happens when you put a vulcanized patch on an inner tube. The physical bond is superior to chemical adhesive bonds, since it does not degrade or delaminate over time. The service life of a PVC flat roof is 20 to 30 years.
Although flat roofs are more prone to leaks than sloping roofs, proper design and materials will go a long way to making them last and providing your home the protection from the elements it needs.