If there was one type of flat roof material things would be so much simpler, but alas for those who are replacing an old flat roof, or constructing a new one from scratch, the number of different flat roof options can seem very confusing.
In this Renocompare Roof Guide we try to simplify your flat roof options. We’ll look take a look at the most popular flat roof materials, lay out the pros and cons of each, give you an idea of the average life expectancy of each and an approximate cost per square foot.
Built-up roofing is constructed with a first-layer cover board and alternating layers of liquid asphalt (tar) followed by fiberglass or organic fabric to reinforce the asphalt. Up to five layers or plies are used, and the final layer is topped with material that reflects the sun’s UV rays. Many BUR roofs start with a base layer of rigid foam insulation to improve the building’s energy efficiency. BUR construction is the most common type of flat and low-slope roofing.
BUR is often called tar and gravel roofing because the traditional topcoat is gravel or similar aggregate. Loose mineral granules or granules fixed to a cap sheet, aluminum coatings and elastomeric coatings are used in place of gravel on some roofs. When gravel, or stone is used to hold the roof in place, rather than being glued or fastened down, the material is called ballast.
Built-up roofing membrane has been used for more than a century, so its reliability is well known. This flat roof material durably withstands the elements including rain, wind, wind-blown debris, hail and intense sun/heat. As a dense roofing material, BUR can be walked on in moderation without damage. A built-up roof requires little cost or effort to maintain.
The cons of built-up roofing start with labor-intensive installation that increases costs. The vapors of some bituminous asphalt materials are hazardous, so care must be taken to protect installers. If the BUR isn’t properly installed, it will be susceptible to water penetration. If not properly coated with sun-reflecting material such as gravel or mineral granules, the BUR will dry out and crack in intense sunlight. Where granule loss is moderate to severe, usually in 5-7 years, the roof should be recoated.
Typically, 15-25 years. The general rule is that a properly installed BUR will last five years per ply, with 3-ply, 4-ply and 5-ply roofs most common.
$4.50-$7.00 per square foot depending on the number of layers and the topcoat material. Recoating with granules costs about $1.00 per square foot.
Modified bitumen flat roofing, or mod bit, is a membrane or mat coated with asphalt modified with polymers including elastomers for flexibility and strength. The membrane is produced in rolls and applied in two or three layers. The top layer, or cap sheet, is coated with mineral granules at the factory or on the jobsite.
Four methods are used to apply modified bitumen:
Starting with its advantages, modified bitumen roofing has been used for decades, so has a proven track record of weatherproof durability. The polymers in the material produce high tensile strength relative to other roofing membranes and helps prevent cracking. Mod bit roofs withstand wind very well. The variety of installation methods provide options based on the local climate, the facility being roofed and the type of roof deck.
As for disadvantages of modified bitumen roofing, when installers aren’t trained and experienced, installation errors including poor adhesion and seams that separate are common. Heavy traffic on a mod bit roof will loosen the granules, exposing the membrane to sunlight, causing it to dry out crack. Lost granules also make the roof less resistant to fire.
A three-ply roof will last 15-20 years when recoated with granules every five years.
$4.00-$5.75 per square foot plus about $1.00 per square foot to recoat with granules.
Further reading: Metal Roof Vs Asphalt Shingles Cost
Single ply roofing membranes are each one layer of waterproof material. The most common are PVC, EPDM and TPO:
Polyvinyl chloride single-layer roofing is the most popular flat roof material in use today. Most PVC roofing is a single ply; IB brand PVC roofing is two plies fused together with a reinforcing scrim between them. All PVC flat roofs install as a single membrane or layer.
The PVC is a thermoplastic; its overlapped seams are heat welded or fused together to create a water-impermeable barrier. Most PVC membrane is attached mechanically using plates and fasteners spaced at intervals that are hidden beneath the overlapping widths of material. A minority of PVC roofing jobs require the material to be fully glued to the roof deck.
The breaking point of PVC is more than 300ppi vs. the industry-standard requirement of 200ppi. Heat-welded seams are stronger than 300ppi. Most membranes are produced in light colors to provide better sunlight reflection that will increase material longevity and reduce cooling costs within the structure. This is a low-maintenance roof.
The main objection to PVC is its price (see below). Beyond cost, if the seams aren’t properly welded, they’ll come apart easily and are difficult to glue together. Repairs can be difficult and costly to make, though they’re not required often. PVC is not an ecofriendly roofing material because it releases toxins such as dioxin over its lifetime and in landfills.
This roofing can be expected to last 25-35 years. Some brands include a lifetime warranty against defects.
$6.00-$10.00 per square foot
Ethylene propylene diene monomer roofing, or EPDM, is a single-layer membrane that looks, feels and performs like rubber. It’s often called rubber or EPDM rubber for this reason. EPDM flat roofing is a fully-glued membrane with overlapped and glued seams.
The advantages begin with the lower cost of EPDM relative to other single-membrane materials, partly due to how fast it installs. When punctures or tears occur, they are easily repaired with patches or roofing cement. Sun-reflective mineral ballast can be added on top of rubber roofing to enhance its energy efficiency, and the material can be painted too. EPDM production has low environmental impact, and some brands use recycled materials, and all are recyclable.
The disadvantages of EPDM roofing are its poor longevity compared with PVC and the fact the seams come apart within a decade and are very hard to repair. Finally, since EPDM is relatively easy to install, roofers specializing in installing other types often “sideline” in EPDM, and the quality of their work can be poor. Make sure you choose an EPDM specialist for installing any flat roof material.
While the material can last 25-30 years, the adhesive at the seams usually fails in 10-15 years, forcing roof replacement. For this reason, most installers use wide rolls of EPDM rubber to reduce or eliminate seams.
$5.50-$8.00 per square foot.
Thermoplastic Olefin roofing installs as a single layer but is made with two bonded plies with a reinforcing scrim between them. The bottom ply is black and twice as thick as the white top ply. The thinner top ply is reflective and contains stabilizers to prevent deterioration from UV rays. Seams are overlapped and heat-welded.
This flat roof material is highly reflective, a “cool roof,” so it cuts cooling costs in sunny regions and qualifies for Energy Star certification. TPO costs less than EPDM and PVC roofing, it is lighter and easier to work with. This material can be installed with fasteners or adhesives and is recyclable.
The cons of TPO roofing start with the inconsistency of the quality from brand to brand. The two plies come apart easily in cheap TPO. With materials in 6’ to 10’ widths, your roof will likely have at least one seam, and seams are susceptible to failure. Finally, with TPO growing in popularity, roofing contractors are starting to install it with little experience. Be sure the company you hire has been installing TPO for many years.
TPO formulas have changed in recent years and still vary among manufacturers. Therefore, being exact about longevity is difficult. Most current materials are expected to last 10-15 years.
$4.50-$7.00 per square foot.
Silicone spray coating is manufactured in two grades. High-solids silicone spray is applied in one or two thick coats over existing BUR, modified bitumen, EPDM, PVC, TPO and metal roof and to wood, concrete, OSB and Hypalon underlay. Low-solids silicone spray coating is applied to polyurethane foam (SPF) to prevent sunlight from causing the SPF to deteriorate. A coating of 20-35 mils is sufficient for most applications. Check with your local building department about whether your roof can be coated. This will depend on the number of roof layers already present.
The fact it can be applied to so many surfaces is the main advantage of silicone spray coatings. When the silicone cures, it forms a single layer that covers existing cracks and prevents leaks. The coating is reflective, so this is an energy-efficient roofing solution. When the coating ages, a fresh coating can be applied.
The potential disadvantage of coating a roof is the cost vs. the life extension it gives the roof. Worst-case scenario, two coats will cost as much as a new roof and may last just 14 years. Coatings are a short-term fix; the money might be better spent on a new roof if you’re planning long-term.
Depending on the experience of the applicator and the condition of the underlying roof, silicone coatings last 7-12 years.
$2.50-$4.00 depending on how competitive the market is in your area.
This is glass-reinforced plastic roofing, or GRP, that is installed over OSB that must be in good condition. Pre-formed drip-edge pieces installed on all edges. Tape is applied to the decking seams. Fiberglass mat is installed on top of the OSB, and liquid resin is applied to saturate the mat. When the resin cures, a resin topcoat is applied that bonds to the first. The result is a tough, single-layer laminate with excellent weatherproof qualities.
The advantages of GRP roofing start with its one-piece cured finish that prevents leaks even under ponding. Fiberglass resists impact, wind and fire, and handles foot traffic if applied to a deck or balcony. Light-colored GRP reflects heat and improves energy efficiency.
The disadvantages of fiberglass GPR roofing are the cost and the potential for slips and falls when the material is wet or icy.
This material is expected to last at least 40 years and possibly much longer. Exact lifespan isn’t known due to GPR being used widely in the US only in the last 20-30 years.
$11-$14 per square foot, significantly higher than other flat roof materials but with much better durability too.
Your roofing contractor will supply all the materials needed for your roof installation and will likely get them at wholesale pricing.
Existing roofs require supplies for maintenance and repair such as ballast, mineral granules, roofing cement and EPDM patches. These supplies are widely available online and at Lowes, The Home Depot, Menards and regional or local building supply stores.