When it comes to covering up a flat roof, your options are both limited and expansive. What that means in a nutshell is that your traditional roofing materials such as asphalt shingles, concrete tiles, and corrugated metal are out of the window. That being said, flat roof systems such as PVC, TPO, EPDM rubber, and others, each have their distinct pros and cons.
So why can’t you put traditional roofing materials on a flat roof? Well, technically you can, but they are almost guaranteed to leak!
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Systems designed for sloped roofs, such as asphalt shingles and concrete or clay tiles, are installed by overlapping one row on top of another. These systems work cohesively with the pitch of the roof to shed rainwater and snow as it falls from the sky.
Since flat roofs have little to no pitch, the water would work itself underneath the shingles or tiles, eventually rotting the substrate and causing leaks on your interior.
Covering a flat roof is a whole different animal (specialized skills and expertise are required) than shingling a pitched one. On a flat roof, generally speaking, you want to avoid any types of seams, if at all possible.
The biggest threat is of course going to be water, which WILL find any access though any hole or inadequately-sealed seams in a roofing membrane.
Your main goal when covering a flat roof is to create a barrier that will be impenetrable to water.
How do you make an item (besides a roof) impenetrable to water? You can either apply something physical such as a tarp, or coat it with a material to create a barrier like you would via deck stain or lacquer. Roofs follow this same premise –- either physically cover it with something like a PVC or an EPDM rubber membrane, or apply a coating such as tar or spray on silicon.
Flat Roof Costs:
It’s easy to assume that a flat roof would be far less expensive to apply materials to than a pitched one. For access reasons alone it would seem it’s a lot easier to roof a flat surface than one that is steep and requires a harnesses and a rope line to move around.
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Many roofing contractors will tell you, though, that working on flat surfaces is actually harder on the back.
Take into account that flat roofs often require the application of a layer of insulation, adhesives or hot air welding, and some types such as modified bitumen are installed with a torch that heats up the seal.
Installation costs can be just as high for a flat roof as they would be for a steeper roof with say an 18/12 pitch.
Professional warrantied labor prices will vary across different markets in the US. You can get a fairly good idea of the estimated cost to cover a flat roof based on the national average:
- PVC, short for Polyvinyl Chloride and Vinyl, is a single-ply thermoplastic roofing membrane known for its superior durability, longevity, and energy efficiency.
- The seams of PVC membrane are hot air welded, creating a permanent bond. The seams are stronger than the membrane itself.
- PVC roofs have been successfully installed on low-slope roofs (both residential and commercial) in the US since 1960s, with many installations known to last over 30 years.
- Residential grade PVC membranes typically come in 50 mil thickness, while commercial grade membranes are also available in 80 mil and 60 mil thicknesses.
- Aside from the membrane itself, an insulation layer/board must first be installed to provide a suitable substrate and to help with the energy efficiency of roof.
- Some installers will employ a fully-adhered or glued to the roof deck rather than mechanically-attached installation method to help prevent wind uplift. All else being equal, the fully-adhered membrane will cost more than a mechanically attached one.
- The membrane is typically installed in overlapping rows comprised of six foot wide PVC membrane, plus an overlap of 6 to 8 inches at each row. Each row of rolled membrane must be mechanically attached and hot-air welded at the seams, with overlapping rows of PVC membrane.
- Immediately next to the roof edge and throughout the outer perimeter of the roof, a three foot wide PVC membrane is used to help mitigate the wind uplift risk.
- The edges of each row are mechanically fastened (using plates and screws) to the roof deck before the seams are hot air welded to help prevent uplift.
- All in total, the installation of a PVC roofing membrane such as the one from IB Roof (residential and commercial vendor) or Sika Sarnafil (mostly commercial and industrial systems vendor) will cost between $7.50 and $15.00 per square foot installed, depending on the installer, membrane type, scope of the project, roof accessibility, and project location.
- EPDM, short for ethylene propylene diene terpolymer, is a rubber roofing membrane that is commonly installed on residential and commercial roofs. It’s considered a low-cost alternative to PVC and TPO roofs.
- EPDM rubber manufacturers try to minimize/avoid seams with their EPDM rubber membranes, as seams are oftem common points of failure with EPDM roofs. To help mitigate roof failures at the seams, wide span EPDM membranes are commonly used.
- EPDM rubber membrane sizes can reach 50′ wide by 200′ long. These huge pieces are great for avoiding possible leaks at the seams, but these wide rolls are also very difficult to handle during the installation.
- Similar to PVC, EPDM rubber membrane requires a layer of insulation installed in between the roof deck and the membrane for improved energy performance. This is especially important, as EPDM membranes are commonly dark in color and will attract a significant amount of solar radiant heat.
- EPDM membranes install like a giant sticker, but this must be done slowly and with precision to avoid air bubbles.
- Typically, EPDM rubber will cost between $6.50 and $12.50 per square foot installed, depending on the installer, project scope and complexity, and property’s location.
- TPO, short for thermoplastic polyolefin, is a single-ply thermoplastic roofing membrane that offers good overall performance and great energy efficiency for the money.
- TPO membranes are a newer product that made their initial appearance in 1990s, but were plagued by performance issues. Since then, TPO roofs have undergone reformulation and seem to be performing quite well over the last decade or so.
- Similar to PVC, TPO membranes are hot air welded, but the two systems have different chemical composition and hence cannot be combined.
- TPO membrane chemically bonds rubber, ethylene, and propylene, as well as numerous filler materials. In that sense, it’s somewhat closer to EPDM rubber than PVC, although, unlike EPDM, TPO membranes seams are hot-air welded (not glued at the seams).
- Similar to PVC, insulation boards are first fastened to the roof substrate. TPO also comes in rolls like PVC and can be mechanically fastened to the insulation boards or installed with the self-adhesive (fully-adhered).
- Costs are approximately $6.50 to $14.00 per square foot installed, which slightly lower than PVC. Overall, project costs can range widely, depending on the contractor giving you a quote, project’s complexity, and your property’s location.
- Modified Bitumen, an asphalt-based multi-ply flat roofing system, is considered a modern take on its close cousin, BUR or built-up roof.
- modified bitumen is installed in multiple layers (multi-ply roof), each of which is torched to the surface below at every ¼ turn of the roll. This is a very labor intensive process that absolutely must be performed by a professional.
- This type of roof does have cold-rolled technologies available now as well, but it involves a lot of application of roofing tar.
- Estimated costs are $5.50 to $11.50 per square foot installed, depending on the project and location.
- Built Up Roofing system is somewhat similar to the modified bitumen, with often an added layer of gravel on top.
- BUR installation includes applying multiple layers of ply sheets that are bonded together using hot asphalt. The top layer can be a reflective coating for energy efficiency or gravel for added durability.
- Costs range from $6.50 to $12.50 per square foot installed, depending on the project size, scope, and location.
- Spray-On Roof coating is a common way to extend the lifespan and improve energy-efficiency of an existing roof.
- applying a spray coat to a roof sounds easy. The material can be applied right over the existing roof, so little to no prep work is needed, besides a very thorough cleaning. It’s still important to apply the spray evenly and delicately though.
- The material itself will also determine the cost, as polyurethane foam can be applied for as little as $3.50 per square foot and acrylic $6.50 per square foot.
- Silicon is the agreed upon premier spray coat application, but the material costs can drive installation prices up to $7.50 to $12.50 per square foot or more.
Choosing an appropriate flat roofing material is a two-part process. On one hand you want a substance that has nice aesthetics while fitting within your budget. You also want to know the product will last:
The life span of your flat roof depends on a lot of different factors starting with proper installation. If your climate has rapid changes that see Spring, Summer, and Winter in the same week, the material will be put under more stress which will reduce its life span.
A roof that is accessed a lot will also wear down faster. Here is what you can expect from the popular materials in terms of longevity:
- PVC – 15 to 30+ years.
- EPDM – 10 to 15 years.
- TPO – 7 to 20 years.
- Bitumen – 10 to 20 years.
- Built Up Roof – 15 to 20 years.
- Spray-On – up to 20 years.
Why Do Flat Roofs Exist?
Before we get into comparisons of different flat roofing materials, it’s important to know why they exist in the first place since they’re seemingly such a hassle and an almost-immanent leak threat.
There are two main reasons behind why a contractor would call for a flat roof: 1) aesthetics and 2) convenience.
For example, when you’re adding on to a home (building an addition) with something like a three-seasons room, a flat roof simply looks nicer. Homes with unblended roof pitches can be an awkward eyesore.
In commercial buildings, flat roofs, outright, offer a more convenient place to install outdoor HVAC units rather than putting them in high-traffic ground areas.
Of course, any roof’s main job is to create a barrier of protection between the building below and the atmosphere above. Therein lies the conundrum associated with flat roofing. For all the aesthetics and convenience, the design doesn’t do a lot to avoid snow and water buildup.
To be fair, flat roofs aren’t completely ‘bubble-level’ flat. They work in much the same way as a gutter system, angled slightly or pitched a couple of degrees, so that water can flow into a downspout. Even so, flat roofing materials need to be able to absorb the brunt of the weather and to withstand ponding water or snow and ice until it melts.
Pros and Cons of Common Flat Roof Membranes
Whether you have a commercial or residential building, some type of roof covering is 100% mandatory. There are generally 5 to 6 different routes to take regarding the materials needed for roofing a flat surface:
- PVC Single-Ply Membrane – arguably the most popular of all flat roofing materials (some contractors even install ONLY PVC membranes.) for higher-end residential installations PVC membranes are a single layer of thermoplastic material. PVC roofs are especially strong and durable, featuring a minimum breaking point of 300 pounds per inch compared to the industry-recommended 200 PPI.
PVC seams are heat-welded (hot air welded) to form a permanent, watertight bond. The welded seams are actually stronger than the material itself! Most PVC membranes, especially white-colored ones are very energy efficient, because they reflect the sun’s energy instead of absorbing it.
PVC membrane can be installed either as a mechanically attached system — using hidden metal plates to secure the hidden or concealed/overlapped parts of the membrane to the roof deck, or as a fully-adhered (more expensive) aka glued onto the roof deck system.
Most residential and some smaller commercial installations are performed by mechanically attaching the PVC membrane to the roof deck. Both residential and commercial PVC installations often come with lifetime warranties.
- EPDM Rubber – commonly known in the industry as a rubber roof. One of the biggest advantages of EPDM over a PVC roof membrane is a much lower price.
PVC and EPDM typically go 1-2 in terms of popularity for flat roofing depending on what area you live in. The glued-together seams of EPDM are not anywhere near as strong as the PVC, heat-welded seams forming a permanent bond.
Darker EPDM membrane also has a tendency to absorb heat, which can drive up utility bills for the property owner.
- TPO – another single-ply roofing membrane that chemically bonds rubber, ethylene, and propylene, as well as numerous filler materials.
TPO roofs have been vastly growing in popularity ever since their introduction in the early 1990s. Part of what is fueling their recent rise is their energy-efficiency, as TPO membranes are more readily available in white and light gray (as well as black) colors compared to their closest competitors PVC and EPDM.
TPO is said to offer the best of both EPDM and PVC in one package. The material is closer to a rubber roof in cost, but also features welded seems for durability like PVC membranes.
- Modified Bitumen – one potential area of concern with single-ply roofing membranes including TPO, EPDM, and PVC, is that although these roof membranes are durable, sharp objects and collisions can still rip through the single ply of the membrane.
Modified bitumen features an advantage in this aspect since it is a multi-ply roofing material. A base layer is mechanically attached to the roof deck with plates or bars.
A ply overlap is then sealed to the base layer with a permanent adhesive. Finally a granule top surface is applied to provide aesthetic and energy efficient properties.
- Built Up Roofing (BUR) – better known by their ‘street name’ of tar & gravel roofs, BUR is just that.
Built up roofing can include up to four plies or more consisting of alternating layers of bitumen (asphalt, coal tar, or other adhesive) with a topping aggregate layer of gravel or other materials.
Built up roofs have been in use for over 100 years with their biggest advantage being the ability to withstand heavy foot traffic. Membrane roofs are durable… in a sense, but a gravel coating can withstand heavy collisions.
The downfall is of course the added weight requiring reinforcement to the supporting structure.
Credits: Wayne’s Roofing Inc.
- Silicon Spray – an example of the protective coating installed on a roof is a silicon spray. The biggest advantage of a spray on roof is foregoing any seams like you have with membranes. One would assume that spraying on a roof material would be much less expensive than applying a physical material, but in reality the silicon covering is one of the most expensive roofing procedures.
So, it’s evident that you have options when it comes to applying a material to your flat roof. How do you make the decision on which type to go with though?
There are some important factors to consider such as ease of installation and how environmentally friendly the manufacturing process is, but for most consumers the two most important decision makers are cost and performance.
Here’s a quick recap of each material’s pros and cons:
- PVC Membrane – arguably the most durable and long-lasting of the single-ply membranes, PVC is hot air welded (not glued or torched down), with a lifetime warranty available for residential installations, but PVC is also expensive. Durability and longevity can be further extended with an 80 mil membrane over 50 mil one.
- EPDM – affordable membrane, but not as durable as PVC or TPO. EPDM rubber has a tendency to develop leaks at the seams over time due to glue drying out under freeze-thaw cycles.
- TPO – an energy efficient membrane that is hot air welded (not glued or torched down) similar to PVC. TPO membranes have had problems in the past, but with a fairly recent reformulation of the membrane, manufacturers claim those problems are now in the past.
- Modified Bitumen – multi-layers, but also heat absorbing and difficult to install.
- BUR – compliant with heavy foot traffic, but very heavy.
- Silicon spray – seamless installation, but costly.
To find the perfect flat roofing material you really need to consider the most common issues your roof will face (lots of traffic, volatile climate). You’ll, of course, also have to consider your budget.
So, how will you decide on which roofing material is right for you? It’s recommended to meet with multiple roofing contractors so that they can examine your exact situation and the specific surface needing a coating.
If an old roof just needs to be replaced, it will direct the decision, as will the roofing system chosen for a new construction project. The contractor will lay out your options, then you’ll find the best choice based on your budget, location, and the ability of companies in your area to adequately install each system.
The end goal is to prevent leaks on a flat roof, luckily you’ve got a lot of different routes that can get you there.
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