Advantages and disadvantages of Various Roof Systems
There are many different roof systems available in today’s market, each with various strengths and weaknesses. Below I will describe each system briefly, and then list the pros and cons.
Thermoplastic and PVC- Single ply, can be mechanically attached, fully adhered or ballasted.
Pros- Superior performance in ponding water situations. Very lightweight roofs, in the neighborhood of 30lbs. per 100 sq. ft. Comes in white if a cool roof is desired, or in several different colors for aesthetic concerns. Another plus with these roofs is that large custom made field sheets can be ordered from the factory, minimizing seams sealed by the crew on the jobsite. Resistant to ozone, algae and field seams are 3-4 times stronger than EPDM filed seams. There is also a tremendous amount of flexibility in the material. And since seams can be welded with hot air, there are no noxious fumes to deal with. Both 15 and 20 year warranties available.
Cons- Being single ply, they are vulnerable to discarded cigarettes, sharp objects dropped on the membrane, and broken glass from bottles thrown on roof (this happens a LOT) I generally discourage customers from installing these types of systems if the roof has a lot of foot traffic or is easily accessible to unauthorized personnel. In the past, material shrinkage was at times a severe problem on Thermoplastic and PVC roofs, but changes in composition and manufacturing has ameliorated this issue. These roofs still shrink over time, though not to the point of failure as in the past.
Bottom line- Great roofs, especially on roofs that pond water heavily, as long as the roof is generally accessible only to authorized personnel.
Asphalt Modified Bitumen- This roof is what a lot of people call a rubber roof, when it is in fact composed mainly of asphalt. It comes in 3 ft. wide rolls and is usually torched down with an open flame, though they can be applied with hot air welders in certain conditions as well.
Pros- In my opinion, the biggest strength of these roofs is how well they stand up to outside stresses. They handle foot traffic better than any other membrane, and are difficult to puncture, especially if they have a granulated surface. These roofs are so tough in fact, that most insurance companies won’t even recognize hail damage as they do on other roofs (I have personally seen modified roofs absolutely pummeled with hail and come out completely undamaged)
Cons- The most glaring disadvantage with these systems is that there is a seam every three feet, which makes for a LOT of seams. If you have a diligent crew with a competent foreman who will ensure that every seam on the roof is triple checked, you’ll probably be fine with this system. If not, you can be pretty sure you will end up with leaks fairly soon, well before the warranty expires in my experience. And large ponding areas of water on these systems will degrade the material prematurely.
Bottom line- With a good, experienced crew this can be a good roof on a building that does not pond water excessively. And if you have a building with a lot of rooftop foot traffic, this is probably the only way to go.
Foam roofs- This system is applied with a large spray unit, spraying two liquid components (isocyanate and resin) When the liquids are mixed together, they expand 20-30 times and will adhere to concrete, wood, steel and most existing roof systems, but according to the manufacturers, NOT to modified bitumen.
Pros- When complete, the roof is monolithic with no seams, fully adhered and has insulating properties.
Cons- Full disclosure, I have limited experience with these roofs here in NE Illinois. We don’t install them, but I have been on 9-10 buildings with this system, and all of them leaked badly, in a couple of cases within months of being installed. Leaks are very difficult to track down and difficult to fix even when found, as repair materials don’t adhere to this surface very well.
Bottom line- Based on my admittedly limited experience with this system, I would not advise my clients to go this route when a new roof is needed.
EPDM Rubber- Rubber roofs can be fully adhered, mechanically fastened or weighed down with ballast (Riverbed gravel or pavers) They are versatile, appropriate for many installations and have a long track record.
Pros- Rubber roofs are relatively inexpensive, easy to install and repair. They are available in large sheets to minimize the number of field seams, and there is quite bit of flexibility in the membrane.
Cons- Field seams on these roofs are not as strong as Thermoplastic roofs. Flashings and roof edges tend to wear out more quickly than the remainder of the roof. Shrinkage over long periods can be dramatic in some applications.
Bottom line- Can be a good investment for a building owner, but all of the circumstances of a given installation must be carefully weighed. And good workmanship on the install is vital.
Spray applied roofing- These are not technically re-roofs, they are considered coatings. Some companies refer to them as “re-plys”, as you are essentially adding a ply to the existing roof surface. There are numerous manufacturers offering various products with different compositions and specification, too numerous to go into fully here. If an existing roof is experiencing leaks but has not advanced to the point of outright failure, them may be good candidates for this solution.
Pros- Can be extremely cost effective, especially on roofs that have a lot of projections and rooftop HVAC units. The material is pure white, which in some cases (especially on metal roofs) can dramatically reduce AC cooling costs. As this is considered to be roof repairs rather than a new roof, there are tax advantages as well.
Cons- The existing roof must still be in somewhat decent condition. If applied to a roof that has already failed, your money will basically be wasted. Also, since this material is sprayed on, any cars within 100 ft or so must be moved during the application process, as this material will be extremely difficult if not impossible to remove from a car’s finish. Some manufacturers do not warranty this material if there is ANY ponding water on the roof. Cannot be applied to a roof with an existing gravel ballast.
Bottom line- This solution may save a building owner a lot of money, if the existing roof is in good enough condition to accept the coating. Excellent solution for metal roofs in particular.
As you can see, there are many different solutions to your flat roofing needs. It is vital however to choose a roofer who will recommend the best system for your particular roof and circumstances, and not the best solution for his wallet. Be sure you understand why a roofer is recommending a particular system for your project, and which system will be the best investment for you.