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 offer their pros and cons.
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So why can’t you put traditional roofing materials on a flat roof? Well, technically you can, but they are almost guaranteed to leak! 😉
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.
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.) 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 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 seams of EPDM are not as strong as the PVC, heat-welded joints either. EPDM 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 energy-efficiency, as they are more readily available in white and light gray (as well as black) 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 negative of TPO, EPDM, and PVC membranes is, although they 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.
Credits: Commercial Roof USA
- 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.
Credits: Blue Hills Builders
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 reminder of each material’s pros and cons:
- PVC Membrane – arguably the most durable of the single-ply membranes with a lifetime warranty, but also very expensive.
- EPDM – affordable, but not as durable as PVC.
- TPO – an energy efficient membrane, but a new technology requiring experienced contractors.
- 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.
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 then one that is steep and requires 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. Labor 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 help with energy costs. The membrane is installed in rows (6-18′), each of which must be hot-air welded at the seams. Many contractors choose to mechanically fasten the edges to help prevent uplift, all of which contribute to labor costs. All in total, installation of a PVC membrane costs roughly $7.00 to $10.00 per square foot.
- 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. The EPDM membranes install like a giant sticker but must be done slowly and with precision to avoid bubbles. Typical EPDM rubber roofing costs are about $4.00 to $7.00 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 $5.00 to $8.00 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 $3.00 to $6.00 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 $5.00 to $7.00 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 $3.00 per square foot and acrylic $6.00 per square foot. Silicon is the agreed upon premier spray application, but the material costs can drive installation prices up to $6.00 to $10.00 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.
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 replaced it will direct the decision as will if the roof is going on new construction. 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 material.
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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