When you need a new roof, the whole process can feel overwhelming. There are so many types of roofing to choose from that you may not know what you need. It also doesn’t help that there’s a lot of technical information out there that may be difficult for a layperson to understand.
The first thing to know is that there are two different kinds of roofing in general: low sloped roofs and steep roofs. The materials and methods used for each type differ significantly. Steep slope roofing is usually made from tile, slate or shingles, while low slope roofing uses membranes, sheets or coatings.
To make things a little easier, we’re just going to discuss the two most common roofing options for low sloped roofs: single ply roofing and multi-ply roofing. Let’s break it down by looking at what those terms actually mean, installation methods, and a comparison of the major characteristics to think about for your next roofing project.
Table of Contents
Single-Ply Roofing
Single-ply roofing is a broad term used to describe rubber or plastic roofing made up of one layer, called a membrane. The plastic materials used to create a membrane are either thermoplastic or thermoset. Thermoplastics, like PVC (polyvinyl chloride) and TPO (thermoplastic olefin), can be heated multiple times to become soft and malleable, but will harden when cool. Thermosets, like EPDM (ethylene propylene diene terpolymer), start as a viscous liquid made from synthetic rubber that will not soften again after hardening.
Installation is simply a matter of chemically gluing, ballasting or mechanically fastening the membrane sheet onto the cover board. The cover board is a thin layer that is installed on top of the insulation.
Multi-Ply Roofing
Multi-ply roofing is a century-old system that uses multiple layers or membranes. Similar to single-ply roofing, multi-ply is an umbrella term that encompasses several different styles. Tar and gravel, also known as built up roofing, consists of alternating layers of asphalt (bitumen) and fabrics, upon which a final layer of gravel is applied. Modified bitumen roofing uses asphalt mixed with rubber and other chemicals to create multi-layered plies with a higher degree of durability. Lastly, a cold applied coating is liquid rubber that layers on top of another roofing system.
The installation of multi-ply systems is somewhat more complicated. Each layer is attached to the preceding layer in an alternation of material type. There may be up to five alternated layers in total. The final layer is usually rock, stone or gravel.
Single-Ply vs. Multi-Ply Comparisons
Cost
In this department, single-ply wins at $3.50 to $6.50 per square foot of TPO or PVC and $3.50 to $7.50 for EPDM. For multi-ply, the cost is $5.50 to 8.50 per square foot, depending on the material. This price difference may not be drastic when comparing a single-ply material at $5 with multi-ply at the lowest $5.50 rate. However, the difference between the lower-end single-ply and higher-end multi-ply can be a matter of thousands of dollars.
Lifespan
Single-ply roofing can usually last up to 30 years, whereas multi-ply roofing lasts about 40 years. The lifespan of any roof will largely depend on whether it was properly installed and maintained. Both roofing styles are considered extremely durable and require minimal maintenance.
Fire Resistance
Single-ply roofing, especially TPO and EPDM, can often be modified to contain flame retardants. Asphalt, used in built up and modified bitumen roofing, is much more likely to catch fire, particularly during the installation process. Cold applied roofing, however, does not pose the same threat during installation.
Water Resistance
Multi-ply systems are the better option for preventing water leakage. They possess a smooth, solid surface free of joints and seams, preventing water from slipping through minuscule cracks. Single-ply roofing does have joints and seams, which are more likely to be susceptible to leakage. On the other hand, as long as those weak points are adequately sealed during installation, single-ply will be equally water-resistant.
UV Protection
Both single-ply and multi-ply roofing perform well against UV rays, but they each have their drawbacks as well. After a long time, UV radiation can wear down the adhesives used to seal the joints and seams in single-ply roofs. Consequently, those areas may lose their resistance to water, as discussed previously. Multi-ply roofing does reflect UV radiation, but the topmost membranes will still break down over time. Unless you have that protective gravel layer on top, multi-ply is no more or less resistant to UV exposure.
Energy Efficiency
Asphalt-based multi-ply systems require a considerable amount of energy to install and create pollution as well. They also tend to be dark-colored, which is more absorbent than reflective of heat from the sun. The cold applied method does not require so much energy to install and does not send toxic fumes into the air. The trade-off is that multi-ply systems as a whole tend to provide more insulation due to the additional layers. A single-ply system using TPO is considered one of the most preferred materials for energy efficiency due to its light color and high reflectivity. EPDM, a dark-colored material, is more likely to absorb heat.
Keep in mind that heat is not always a harmful side effect, and coolness is not always preferable. If you live in a region with more cold days than hot days during the year, then a hotter roof may relieve the building’s internal heating systems. Likewise, regions with more hot days than cool days should focus on reflective, light-colored materials.
There are a variety of factors to consider when you’re thinking about getting a new roof installed. You can’t go wrong when both single-ply roofing and multi-ply roofing are structurally sound, reliable and popular systems. Depending on your building’s particular needs, however, one system will surely offer the precise characteristics that will best protect you.