In Part II of this series, we compared a low-slope roofing system to an open-faced sandwich and dove into the first two layers – the roof deck (the bread) and the insulation and/or cover board (the meat). No sandwich is truly complete without some delicious cheese, though. In our roof sandwich, that is the roof covering.
In this post we’ll review the four low-slope roof covering types – built-up roofing (BUR), modified bitumen (mod bit), single-ply membranes (EPDM, TPO and PVC) and fluid-applied systems (roof coatings) – and the characteristics that give each its distinctive flavor.
Built-up Roofing
First used in the 1800s, BUR is the oldest low-slope roofing option in use today. Multiple alternating layers of hot asphalt and fiberglass reinforcing fabric are installed above the insulation layer. A top layer – either gravel embedded in the final layer of asphalt, or a mineral surface cap sheet laid on top of the asphalt – creates a continuous sealed surface that protects the underlying layers from damage. The redundancy of BUR systems provides a strong, long-lasting, low-maintenance roof.
BUR systems are not as common as they once were, being overtaken by newer options that are easier to install (with no kettles of hot asphalt and no asphalt smell, too), are more environmentally friendly, deliver similar longevity, and often cost less.
Modified Bitumen
Arriving in the United States in the 1970s, mod bit systems are an updated take on BUR systems. They are still asphalt-based, but the introduction of rubberized modifiers to the sheet goods increased their flexibility and made installation faster and simpler. A base sheet is attached directly to the roof deck or insulation layer. The base sheet is then covered by a cap sheet made of polymer-modified asphalt reinforced with fabric layers.
Mod bit roofing systems are low-maintenance, budget-friendly and able to take abuse from sun, heat, cold, snow, rain, hail and foot traffic. And with the membrane and attachment options available, they are a highly versatile solution.
There are two types of mod bit cap sheets – SBS (styrene-butadiene-styrene) and APP (atactic polypropylene). In both cases, the asphalt is modified with a polymer and reinforced with one or more layers of fabric. The polymer used is different, however, making each better suited to certain environments.
- SBS membranes have greater elongation and remain flexible in cold temperatures, helping prevent cracks that can result in leaks. This makes them the right choice for buildings in northern climates.
- APP membranes have a higher softening point, giving them superior weatherability, puncture resistance and ability to withstand foot traffic scuffing at elevated temperatures. They are the better option for buildings in southern climates.
Mod bit cap sheets can be self-adhering or torch-applied.
- Self-adhering cap sheets go on quick, clean and torch-free. Simply position the sheet, let the membrane relax, peel away the release liner, press the membrane into place and roll the membrane to ensure adhesion. The adhesive is susceptible to cold temperatures, however, so in many parts of the country, self-adhering mod bit membranes are best thought of as a three-season solution that can be used roughly six months of the year.
- Torch-applied cap sheets are installed using propane-fueled torches – generally a hand-held roofing torch for flashing and a rolling torch wagon for the field sheet. The welder melts the asphalt surfacing compound, turning it into a liquid that, as it cools, bonds with the substrate. Torch-applied membranes often cost less than self-adhering systems, and because the membranes are adhered using traditional torch roofing techniques, seams can be completed in conjunction with the field sheet, speeding up installation. Any torch-applied roofing installation comes with an inherent risk of fire, however, and appropriate precautions must always be taken to protect people and property.
Single-Ply Membranes
Single-ply systems are the dominant low-slope roofing choice today, having built a decades-long track record of strength, durability, weatherability, easy installation and energy efficiency. In these systems, a flexible membrane layer is attached to the substrate to create a protective waterproof surface.
EPDM (ethylene propylene diene monomer) membranes were used in irrigation ditches in the United States beginning in the 1950s and on roofs starting in the 1960s. PVC (polyvinyl chloride) roofing membranes were developed in Europe in the 1960s and made their way to the United States in the 1970s. The timing of these developments was perfect, as tight supplies and rising prices for asphalt had contractors and building owners seeking more readily available, lower-cost alternatives to asphaltic roofing products.
TPO (thermoplastic polyolefin) arrived in the United States from Europe in the 1990s. While the “baby” of the single-ply family, it has grown to become the most-used and fastest-growing single-ply membrane.
Single-ply systems offer several benefits:
- Low installation costs make them a budget-friendly option.
- For installers, the capital investment needed to get into the single-ply business is small.
- A wide variety of options are available, allowing contractors and specifiers to create customized solutions for virtually any low-slope roof.
- Single-ply membranes are lightweight and can often be used to recover existing roofing systems, delaying the need for a more costly and disruptive tear-off and keeping old roofing material out of landfills.
- White membranes can significantly enhance a building’s energy efficiency. They reflect most of the sun’s rays away from the rooftop and emit much of the energy that is absorbed back into the atmosphere. This lowers temperatures on the rooftop and inside the building, reducing cooling costs and maximizing the efficiency of HVAC equipment while keeping building occupants comfortable.
Membranes are available in several thicknesses to accommodate various project requirements. Thickness is measured in “mils,” with each mil equivalent to 0.001 inch. Thicker membranes offer greater resistance to hail and other extreme weather conditions, foot traffic, pollution and abrasion and are also a good option when the building owner simply wants an extra-long lifespan.
Membranes can be mechanically attached with fasteners and plates, fully adhered using adhesive, or ballasted, in which a layer of gravel or pavers is placed on top of the membrane to hold it in place.
EPDM
- EPDM is a thermoset membrane, meaning it takes on a different state when heated and maintains its shape and form after cooling. Key characteristics include:
Membranes can be reinforced or non-reinforced. Reinforced membranes are more resistant to fatigue, tears and punctures and are ideally suited to mechanically attached systems. Non-reinforced membranes are the most economical single-ply membrane and are appropriate for use in fully adhered and ballasted systems. - Seams are sealed using tape and primer, making installation quick and easy and eliminating the need for heat welding equipment. While not as strong as the heat-welded seams in TPO and PVC systems, EPDM seaming technology has made great strides over the years, allowing manufacturers to confidently offer no dollar limit (NDL) system warranties of up to 30 years.
- Self-adhering EPDM provides quick and easy “peel-and-stick” installation.
- Most EPDM membranes are black. Their ability to absorb more heat from the sun’s rays can make them the right choice for buildings in cooler climates, where managing heating costs may be a higher priority than reducing cooling costs.
- White-on-black EPDM is a “cool roof” option for buildings in warmer, sunnier climates or other circumstances in which reducing air-conditioning costs is important.
TPO and PVC
- TPO and PVC are thermoplastic membranes. Even after curing, they can be melted, softened and reformed with heat. Sheets can, therefore, be fused together by heating them with a hot air welder and allowing the material to cool. Key characteristics include:
- TPO and PVC systems are most often mechanically fastened. Self-adhering TPO membranes are also available, making installation fast and easy.
- TPO and PVC membranes are always reinforced with polyester fabric, giving them added durability and resistance to tears and punctures.
- Seams are heat welded, making them exceptionally strong; when completed correctly, the seams are stronger than the membranes themselves. Typically, field seams are completed with automatic welders and details are hand-welded.
- Most TPO and PVC membranes are white, making them sustainable, “cool roof” choices.
TPO systems typically cost more than EPDM systems but less than PVC systems.
Self-adhering TPO membranes provide fast, simple installation. TPO membranes are also available with a temporary film on the top surface that protects the membrane from dirt and scuffs during installation, eliminating the need to clean the roof when the job is done.
PVC is highly resistant to greases, animal fats, jet fuels, acids and other chemicals, making it the best solution for restaurants, food processing plants, airports, manufacturing facilities and other roofs that will regularly be exposed to these substances. It is also exceptionally fire-resistant – self-extinguishing, in fact, once the source of the flame has been removed.
Fluid-applied Systems
Roof coatings are the fastest-growing segment of the low-slope roofing industry … and for several good reasons.
- Roof coatings can return an aging low-slope roof to near-new performance, delaying for years (even decades) the need for a far more costly and disruptive replacement.
In many cases, the roof can later be recoated, extending its life even further. - Coatings are a sustainable choice, reducing the amount of old roofing material that ends up in landfills and giving building owners a “cool roof” choice that can reduce their energy use and costs.
- Fluid-applied systems are easy to apply using a spray rig, rollers and brushes.
- Coatings are a low-barrier way for contractors to expand the menu of low-slope solutions they offer or even take their first steps into commercial roofing. There is no expensive, complicated equipment to purchase and master. Crews need minimal training.
Acrylic and silicone coatings are most common, with silicone leading the pack by a wide margin. So, which to choose? Factors to consider include:
- Ponding Water: If the roof occasionally experiences ponding water, a silicone coating should be used. No coating will hold up to ponding water on a regular basis, but silicone can withstand it periodically.
- Climate: Silicone coatings cure by drawing moisture from the atmosphere, while acrylic coatings cure as the moisture in the coating evaporates. Silicone coatings, therefore, cure far more quickly in humid conditions and acrylic coatings cure much faster in arid climates.
- Existing Coating Type: Acrylic coatings will not adhere to silicone coatings. A roof that was previously coated with acrylic can be recoated with either acrylic or silicone, but a roof that was previously coated with silicone can only be recoated with silicone.
- Cost: Acrylic coatings typically cost less than silicone coatings and may be the right choice when the budget is particularly tight.
- Perception: Silicone coatings are generally perceived to be more premium products. If the building owner is planning to sell the building soon, a silicone-coated roof can be an attractive feature.
Not all roofs are candidates for coating, though. Fluid-applied roofing systems are not “miracles in a bucket.” They will not make an unsound roof sound; unsound roofs must be replaced. They will not magically fix pervasive leaks, ponding water, saturated insulation and other performance issues; these problems must be addressed before the roof is coated, or they will only get worse. Step one on any prospective coating job is, therefore, evaluating the roof to determine whether it should be coated and identify necessary repairs. More on that in our next post.
We now have all three layers of our low-slope roof sandwich. With the options available, there is something on the menu to suit every taste. So, which one to choose for your roof, and how should it be assembled? We’ll answer those questions in the final two installments of this series.
By: Rob Keating, Regional Director – Northeast