By: Kevin Duffy
Anyone that has ever owned, managed, maintained or even lived in a private home or apartment building knows how important the roof is. If the roof leaks, it may cause damage to the interior finishes, decrease the value of the property, and, possibly lead to lawsuits filed by disgruntled tenants. Depending on the type of building, its location and the slope of the roof, there are many options for a new roof system. In the first of a series of article outlining the pros and cons of roofing systems, Sullivan Engineering will outline the benefits and drawbacks of several popular low-slope roofing solutions.
The first step in determining the best roof system for a low slope roof, is to establish whether it is actually a low slope roof. Depending on which agency you consult, the definitions can vary. According to the National Roofing Contractors Association (NRCA), a low slope roof is a roof system that is installed at a slope of 3:12 (14 degrees) or less. According to the American Society of Testing and Materials (ASTM), the cut off of a slope for low slope roof is 2:12 (9.5 degrees). Sullivan Engineering, like many roof consultants, agrees with the NRCA’s standard of 3:12.
After a roof is determined to be low slope, there are several different types of roof system options. The common systems that will be explored in this article consist of single ply, sheet membranes and liquid applied systems. Although there are other types of systems on the market, they collectively make up a very small percentage of newly installed roofs. Each category can be broken down into subcategories, making the right decision for roof systems even more difficult.
The first category of roof systems commonly used in application, is single ply. As the name implies, these roof systems consists of a single waterproofing between the structure and the elements. Ply systems are less labor intensive because they come in large rolls that can be mechanically fastened resulting in fewer seams to be sealed, the most popular single ply system is ethylene propylene diene monomer, more commonly referred to as EPDM. This roof system has many advantages and is relatively inexpensive to install. It is environmentally friendly because many manufacturers make the product from recycled rubber. EPDM is less labor intensive because it comes in large rolls that can be mechanically fastened resulting in fewer seams to be sealed. Because it has been on the market for well over 40 years, it is familiar to both designers and installers and has a proven track record. An additional advantage to this roof system is that it’s easily repaired. The supplies are readily available at any roofing supply store, and, although we recommend hiring a competent professional, a handy person can install it
The main con of all single ply roof systems, including EPDM, is its lack of redundancy. If it is subject to heavy foot traffic, it can become worn and punctured. Undetectable microscopic holes can allow water infiltration. Flashings are also a concern with EPDM, as they are the most commonly overlooked maintenance items. The predominant flashing method for tightly grouped details is the pitch pocket. Pitch pockets are boxes made of sheet metal and filled with pourable sealer. The pitch pocket is designed to be easily flashed by the EPDM, however, the pourable sealer shrinks over time and must be replaced approximately every 5 years. In recent years, certain manufacturers have developed alternate flashing designs requiring less regular maintenance; however the pitch pocket is still the most commonly used method of flashing.
Another single ply system that has become popular in recent years is polyvinyl chloride, or PVC. Its white color helps to reflect the solar rays that drive up building temperatures and energy costs in the summer. PVC provides a “cool” roof alternative to the black EPDM. PVC is known for its heat weldable seams. Unlike EPDM which “glues” its seams together, the PVC seam is heated up with a hot air welder, allowing it to melt together and effectively become 1 sheet. Additionally, the PVC membranes are fire retardant. There are certain cons to this system also. Due to the plasticizers used to make the PVC flexible enough to be installed as a roof system, it is not recommended for installation over asphalt based roof systems (in re-roof projects) or certain types of insulation, such as polystyrene. These materials cause the plasticizers to leach out of the membrane, making it more brittle and susceptible to cracking. Even when the PVC membrane is installed over the proper materials; over time, it will lose the plasticizers, causing the membrane to shrink. This will pull the seams, lift the corners and cause cracks, all of which can allow water infiltration. Additionally, PVC roofs, especially older systems, are more susceptible to cold temperatures. The NRCA recommends no foot traffic on the membrane when the outside temperature is below 50 degrees.
The third popular type of single ply roof is Thermoplastic Polyolefin, or TPO. This is the newest system on the market, becoming commercially available in the early 1990’s. Of the single ply materials discussed, TPO has the worst reputation. The main benefit of TPO is that it is a cost effective solution for a new roof system. Other benefits include its white color, which in certain areas could be considered a “cool” roof, and, that unlike PVC, TPO has no plasticizers that can leach out and cause the membrane to become brittle. The TPO roof system, however, has significant drawbacks. Installers need to be extremely fastidious. The heat welding of the seams requires that the temperature be within a very precise range. If the Contractor fails to calibrate the machine correctly, improperly welded seams may come loose during the first freeze/thaw cycle and lead to water infiltration. Also, the seams are difficult to re-weld after they have been exposed to the sun. This makes repairs very difficult. Additionally, because the membrane has no plasticizers, it is fairly stiff and does not relax easily. This can cause wrinkles in the membrane.