By: Kevin Duffy
In last month’s newsletter, we explored the pros and cons of single ply roof systems. In this second installment of Sullivan Engineering’s series on the pros and cons of various roof systems, we will explore modified bitumen membranes.
There are 2 basic types of modified bitumen membranes, Styrene butadiene styrene (SBS) and atactic polypropylene (APP).The main difference between the 2 systems can be boiled down to the fact that the SBS is considered “rubber” meaning it is elastic, while the APP is more rigid and considered “plastic”. The main disadvantage to the APP is that due to it’s “plastic” nature, it is more susceptible to damage in colder climates because it becomes brittle.
Due to the fact that both systems exhibit mostly the same advantages and disadvantages, they will be lumped together for the purposes of this article. The modified bitumen roof systems can be installed over any structural roof deck, however the approach can vary. For wood decks, using a torch is dangerous and illegal in some municipalities, therefore, cold applied adhesives are generally used. Over metal and concrete decks the membranes are usually torched down, but could require a fire watch, again depending on local building codes. Recently, some manufacturers have introduced a self-adhered base sheet that makes installation quicker and more cost effective. The modified bitumen membranes utilize 2 or 3 layers, which introduces redundancy into the system. This means that if the cap sheet (top layer) is damaged, the base sheet will not allow any water past it, preventing a leak. The flexibility of the SBS modified bitumen roof system makes it an ideal fit for climates that have a large temperature swing between summer and winter. The redundancy of the modified bitumen systems makes them ideal for installation under various types of overburdens, such as pavers or roof top gardens, as they are more durable and will last longer than the typical single ply roof system.
One of the drawbacks of the modified bitumen roof systems is the price of installation. Due to the redundancy mentioned earlier, the cost increases since you are essentially installing 2-3 “single” plies on your roof. Another drawback to the modified bitumen systems is that there is no set of composition standards, meaning different manufacturers can make their products completely different from one another. Resulting in a wide range of products on the market and some could be lower cost, but could lead to premature failures due to the blend of chemicals in the sheet. It is recommended to that a reputable contractor and/or engineer/architect be retained when dealing with modified bitumen roof products, as these professionals will help you select the right modified bitumen manufacturer and sheet, ensuring the roof remains waterproof for many years to come.
In addition to helping choose the right product, a reputable contractor and engineer/architect can ensure that the new roof system is covered by a warranty. Sullivan Engineering recommends always getting a 20 year no dollar limit warranty from the manufacturer. This warranty ensures that the full cost of the roof will be covered and it will not be pro-rated. For more on warranties, see Adam McManus’ post on Exclusions in Low Slope Roof Warranties.