Post date: Dec 25, 2014 12:03:03 AM
In the early-to-mid 1970s I worked as a roofing engineer. Most of our customers were big buildings with flat roofs. So I learned a bit about built-up asphalt roofing. We put on a few rubber roofs and some metal roofs, but most were primarily made from the left-overs of petroleum refining, known as asphalt. These roofs had fairly short lives - relative to the buildings they protected - because parts of the asphalt were soluble in water. When they leached out the roof became brittle and less water tight. Today almost no one installs built-up roofing.
Its direct descendent is so-called modified bitumen roofing. This is still basically a reinforced asphalt product. Because it comes in rolls and is stiff, it must be heated to get it to lay flat. Thus it is called a 'torch-down' roof. It is quite popular because it is cheap. But considering the number of roof-installation fires that I am directly aware of, I would not touch this method with a ten foot pole.
Today there are now many ways to waterproof a flat or low-slope roof, making selection of a roof system more difficult. Single-ply flexible membranes are a big improvement over built-up asphalt products. They are much faster to install and look better, among other advantages.
The same kind of rubber roof (EPDM) that we put on forty years ago is still being used today. The rubber itself holds up very well, but the joints between sheets are joined with special tape. The seams and joints are most vulnerable to poor quality application, and they tend to be the place that fails. EPDM is basically black, so it absorbs a lot of the sun's heat in the summer. It can be painted white, but needs to be repainted every decade or so. Properly applied EPDM can be expected to last 30 years or more.
PVC (Polyvinyl Chloride) is the same stuff made into water pipe, and is naturally rigid. It is naturally white, so reflects much of the summer heat, reducing the load on air conditioning systems. But the sun bakes out the plasticizers (the chemicals that make plastics soft and flexible), leaving the PVC brittle. When a hailstorm comes along, or someone walks on the roof, dropping a heavy tool, the early PVC roofs tended to shatter, a catastrophic failure. So they made the PVC layer thicker, to slow the migration of the plasticizers and extend the flexible life of the membrane.
Companies then bonded a fabric scrim to reinforce the plastic, which helped for a while, but they still became brittle.
Finally DuPont discovered another polymer (KEE) that could be combined with PVC to form a product that does not get brittle with age. Different roofing manufacturers bond this to polyester felt, woven, or knitted fabrics to make a strong, durable, long-lasting product. KEE membrane can be thinner because it does not need a reservoir of plasticizers, as does PVC, to remain flexible over 30 years. The fabric makes it strong, and the polymer protects the fabric and makes it waterproof.
One distinct advantage of PVC and KEE is that joints can be welded using hot air. That makes the joints just as strong as the rest of the membrane - a distinct advantage. That means not only the seams between 10' wide rolls, but also around the edges and all perforations (for vent pipes, etc.). As a matter of fact, most systems, including roofing, fail at the joints (intersections, seams, boundaries, connections, edges, etc.).
I started wanting EPDM, but installers are hard to find, and black roofs are not in style with conservationists. So now I'm seriously thinking of KEE-modified PCV. I still have a few months to decide. I have found one installer with an excellent reputation. Single-ply roofs are smooth and enable collection of clean water for storage.
FiberTite sent me a very nice marketing kit. It has three samples of roof construction with foam insulation topped by FiberTite, PVC, and TPO (poly olefin), along with a nail set tool. The idea is to demonstrate how tough each of the products is - by trying to puncture the membrane with the nail set. Impressive. The kit also shows how their product is built up during their manufacturing process from polyester fabric, impregnation, backing, and face. For a mechanic and chemist like me, this marketing kit was a home run. FiberTite also has a very complete Foreman's Installation Reference, in both English and Spanish, that is easy to understand and filled with illustrations on how to design and create a good roof.