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 like asphalt shingles and concrete or clay tiles are installed by overlapping one row on top of another. They 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 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 the 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 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 material 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 braces 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 adhesives 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 a 10/12 pitch.
Labor prices will vary depending on your area. You can get a good idea of the estimated cost to cover your flat roof based on the average material prices.
- PVC Membrane – aside from the membrane itself, an insulation board must first be installed to provide a suitable substrate and to help with the energy costs. The membrane is typically installed in rows comprised of six foot wide PVC membrane, plus an overlap of 6 to 8 inches. 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 prevent the wind uplift.
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.
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 mechanically attached one.
All in total, the installation of a PVC roofing membrane such as that from IB Roof (residential and commercial vendor) or Sika Sarnafil (mostly commercial vendor) will cost between $7.50 and $12.50 per square foot, depending on the membrane type, scope of the project, roof accessibility, and project location.
- EPDM Rubber – manufacturers try to avoid seams with EPDM membrane sizes that 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.
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 $10.50 per square foot installed.
- TPO – insulation boards are first fastened to the roof substrate. TPO also comes in rolls and can be mechanically fastened to the insulation boards or installed with the self-adhesive. Costs are approximately $6.50 to $10.50 per square foot installed.
- Modified Bitumen – modified bitumen is installed in multiple layers, 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 $4.50 to $8.50 per square foot installed.
- Built Up Roof – 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 $9.50 per square foot installed.
- Spray-On Roof – applying a spray to a roof sounds easy. The material can be applied right over an existing roof so little to no prep work is needed besides 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 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: