The absolute best materials for your flat roof - pros, cons, and costs

Dan Hahn

By Dan Hahn

October 26, 2020

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When you find it’s time to replace that big flat roof you have on your home or business, you’ll need to determine the best flat roof materials for your project. In this article, we let you in on the details of each flat roofing option, include a list of pros and cons for each, and how much you can expect to spend. Let’s take a look at each of the following options.

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It’s time to replace that old flat roof with much better roofing materials

If you’ve got lots of water pooling on your flat roof due to warps, rips, and tears in the roofing material or seams, it’s time for a roof replacement.

To get to the best results when replacing your roof, you’ll need to know what type of material to use and how it’ll fit into your budget. We’ll help you figure out what kind of roofing material would be best for your project and provide a cost guide for each. First, let's consider the unique features of the best flat roofing materials.

Why flat roofing materials are different than pitched roofing materials

Pitched roofs have a physical advantage over flat roofs in their ability to let rainfall cascade down into gutters for drainage to the ground. Therefore, pitched roofing materials like asphalt shingles are engineered to ensure water flows well over the surface. Flat roof shingles tend to do more harm than good when it comes to protection from the elements. Water tends to seep between each shingle and can penetrate your roof membrane.

While flat roof systems are more affordable from the outset, they can be more expensive over the life of your building. Leaks are harder to diagnose and repair in a flat roof, especially on pebble roofs where you can’t actually see the roof structure.

Until recently, flat roofs have had to be installed in multiple layers to protect against moisture damage and water leakage. Flat roofing materials have improved drastically over the past several decades to provide better waterproofing. Below, we review the best new roof material options for different types of flat roofs.

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The best flat roofing materials

PVC roofing

Installed cost per square foot: $6-12

PVC Roofing

PVC Roofing

Image source: Wedge Roofing

Polyvinyl chloride (PVC) roofing membranes have been heralded as one of the best flat roofing materials in the single-ply roofing industry. The material is made from processed petroleum or a natural gas and salt mixture. It comes in big rolls with a bottom ply, flexible fiberglass mat, a weathering film, and acrylic finish.

Pros of PVC roofing

Let’s explore why they’re one of the best options for most flat roof applications:

Longevity - PVC roofs are strong and very durable. They have a minimum breaking strength of 350 pounds per inch. That’s well above the 200 pound-per-inch standard set by the American Society of Testing and Materials (ASTM).

When installed properly, PVC lasts 20 to 30 years on a flat roof and doesn’t need much maintenance.

Resistance to the elements - PVC roofing has a strong fire-resistance rating, and their welded seams make for superior wind uplift resistance. If your region is prone to strong winds, PVC membranes are a smart choice. In tests, it can withstand 2-inch sized hail simulated with ice and metal.

Even if you’re replacing a roof on a restaurant, you won’t have to worry about grease vents damaging a PVC roof. The protective exterior weathering film ensures no damage can occur to the understructure.

Waterproofing - There’s a reason PVC pipes have been used for decades to carry water around. They don’t easily leak. When using the same material on your flat roof structure, you can count on watertight performance even if you experience temporary ponding, or even exposure to plants and bacteria.

When the sheets of PVC are rolled out, the seams are sealed with hot air. That makes PVC roofing membranes virtually impermeable to water.

Energy savings - Because PVC membranes come in white, they reflect most of the intense sunlight throughout the day. Since your building will not need to do as much work to cool itself, you’ll save a substantial amount of energy, and money. As an added benefit, the vinyl can be recycled after serving its life on your roof instead of winding up in a landfill.

Color options - PVC comes in several different colors. While white may make the most sense in warmer climates, you can go with a darker shade of rust, brown, or gray to match the natural surroundings. You can also select a darker hue to save more on heating in the winter.

Cons of PVC roofing

Toxicity - Toxic chemicals like dioxin are released when PVC is manufactured. That chemical has been related to several adverse health effects, including hormone changes, immune system damage, and even cancer.

Replacement complexity - When restoring an old roof with PVC, more labor and care are required to completely remove the old roof material. Any debris or sharp material can puncture through the underside of the PVC and cause future leaks.

Built-up roof (BUR)

Installed cost per square foot: $3-6

Built-up Roofing

Built-up Roofing

Image source: Jensen Exteriors

Built-up roofs (BUR) are made out of layers of materials bonded together by asphalt and hot tar, then topped off with rocks or gravel. It’s like putting multiple roofs on top of each other for more protection.

Ballasted roofs use bigger rocks that hold the waterproof membrane in place underneath.

Tar and gravel roofs use smaller rocks which are sealed down using a liquid that hardens as it cures, or with a blowtorch which heat hardens the material.

Pros of a built-up roof

Safety - If you have lots of foot traffic or mechanical systems on your roof, BURs make it safer for maintenance staff to walk around. Foot traction is superior to other roofing options.

If a neighboring structure is on fire, there’s a low chance a BUR will catch fire. The top layer of built-up roofs are made from several layers of rocks and are less likely to catch fire.

Longevity - Each layer of material you add to your built-up roof will extend its life by about 5 years. With 5 layers of fabric, you can expect to get about 25 years of longevity.

Strong seal - Since the roofing material is layered over itself, there are no seams to worry about being sealed with a torch or glued down. That means a strong seal you can trust.

Hail? Who cares? - Any kind of flying debris like hail or downed tree branches won’t be an issue. The thick layer of stone and gravel is a strong shield for your roof membrane.

Cons of a built-up roof

Weight - All those layers of roofing material plus the stones on top are heavy. Especially if you’re using a ballasted roof, you need to make sure your underlying roofing structure is strong enough to withstand this pressure.

Installation speed - Layering fabric, heating bitumen, waiting for it to cool, and layering an additional layer takes a long time. Now, multiply that process by 5 other layers, and you can see that a BUR will take a lot of labor hours to complete. That can be costly, relative to other roofing material options.

Leak repair - If you see the interior of your BUR roof leaking, finding the location of the leak will be a challenge. The process is more time-intensive than with other types of roofs. Your roof repair technician will need to dig out a large depth of rocks to find the source of the leak, making for a potentially frustrating, costly repair.

TPO roofing

Installed cost per square foot: $2-5

Image source: Firestone Building Products

Pros of TPO roofing

Cost - With many of the same benefits as PVC, like its heat-reflective white coating and heat-sealed seams, TPO roofs come in at a more budget-friendly installed cost of about $3.50 per square foot.

Energy Savings - TPO roof coatings are white, which allows them to reflect heat and keep the surface temperature lower than that of a dark roof. For commercial buildings, air conditioning can be a huge cost. That makes TPO a smart choice for building operators and owners who have an eye on their long term budget.

Maintenance - You don’t have to pressure wash a TPO roof, since the single-ply roof membrane’s surface is laminated. It is very challenging for any algae or fungus to grip to.

Weight - TPO is a single-ply, lightweight material that performs well over time on low-slope roofs. It weighs an average of just half a pound per square foot, making it an excellent choice for older structures that may not be able to withstand the pressure and load of a built-up roof.

Cons of TPO roofing

Many new brands - Since TPO is relatively new, vendors are continuing to tweak the polypropylene blends in the single-ply membrane. That means while two TPO offerings may have a similar price from different manufacturers, one of them may be a superior product. However, not enough time has passed to see how each material performs.

Maintenance - Repairing a TPO roof usually requires a heat welder and professional expertise. Moreover, TPO can become slippery in snow, rain, or frosty conditions. This isn’t a DIY friendly flat roofing material to work with. It is best to work with a roofing contractor to solve any maintenance issues.

Heat and fire resistance - Seams on TPO roofs can bust, crack, and leak when the temperatures get extreme up on your roof. The top laminated layer of TPO roofs has shown inconsistent performance in these conditions across vendors. Moreover, TPO is not as naturally fire-resistant as PVC in field tests.

Rubber roofing (EPDM)

Installed cost per square foot: $1-4

Image source: EPDM Roofing Association’s website

Ethylene propylene diene monomer (EPDM) really should be abbreviated EPDT, since the roofing material is no longer a monomer, it’s a terpolymer. Since this rubber roofing material has had a long history, the name stuck.

EPDM is a very durable synthetic rubber roofing membrane. It’s made out of an oil and natural gas mixture and installed either with liquid adhesive, screws, or ballasted with gravel and stones.

Pros of rubber roofing (EPDM)

Longevity - A properly installed EPDM roof will last much longer than other flat roofing options - up to 50 years!

Ease of installation - EPDM material comes in large single layer rolls which are easy for your roofing contractor to work with. The rubber membrane is elastic and flexible and you don’t need to use blowtorches to seal the seams together.

Resistance to the elements - Since rubber is elastic, it naturally expands in the summer and contracts in the winter. It can withstand temperature extremes easily without wear. Rubber is also a fire retardant and naturally impact resistant.

Affordability - With the average installed price per square foot around $2.50, this roofing option is much more affordable than other flat roofing systems.

Easy repairs - Unless using a ballasted system weighed down by lots of rocks, it’s straightforward to find roof leaks and repair them.

Cons of rubber roofing (EPDM)

Shrinkage - While EPDM is easy for roofers to install, the seams can break apart and the rolls are more prone to shrink away from each other, exposing the roof underlayer. If not installed properly, EPDM roofs can leak in a very short amount of time. That’s because the glue holding the material down is not welded like a TPO roof. This is more of a risk on larger, industrial-sized installations than smaller roof types.

Heat - Most EPDM rubber is black. In the summer, it will absorb heat from the sun and lead to higher building cooling costs.

Aesthetics - A rubber roof doesn’t look pretty. It’s made out of the same rubber weatherstripping material found on car doors. If you’re looking for a sleeker look, TPO or PVC would be a better option.

Modified bitumen roofs

Installed cost per square foot: $4-6

Image source: Armor Shield Construction

Modified bitumen is a single-ply rolled roofing system made from asphalt with added polymers. It has a base waterproofing layer and a surface layer that bonds to a material that resembles regular asphalt shingles. Europeans invented this roofing system in the late 1960s and it’s relatively easy for homeowners to install on smaller roofs.

Pros of modified bitumen roofs

Aesthetics - There are as many color variations of modified bitumen roofs as there are composite shingle types. That means you can easily select a variety of materials to match your building exterior, which may have parts that are covered by asphalt shingles.

Installation ease - Modified bitumen now comes in rolls that have a self-adhesive, sort of like a sticker sheet. That makes it easier, safer, and less smelly than having to use a torch down method to heat seal the material to your roof membrane.

Seal to the roof - There are no seams to worry about with modified bitumen roofs, so you have a superior waterproofing solution that is leak-proof.

Cons of modified bitumen roofs

Fragility - Other roof types are more resistant to hail and falling branches than modified bitumen roofs. The material is fragile enough to be broken when installers simply walk around on your roof trying to put it in.

Longevity - Modified bitumen roofs only last about 10 to 20 years before they need to be replaced. Because TPO or PVC hold up much better to standing water in puddles than modified bitumen, you may want to consider those options instead, especially if you live in a region with consistent seasonal rainfall.

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