Did you know the right roof covering starts with the roof’s pitch, or angle? Other factors matter too, such as whether it’s a commercial building and whether you do a lot of cooking in it. The chart below and the step-by-step infographic walk you through the basic process of determining the right roof for you.
What’s your roof’s pitch/angle? Most modern homes have a roof that’s considered steep, such as between 3/12 and 12/12. Most modern homes also have a section of roof that’s low-sloped, or less than 3/12. Commercial buildings, on the other hand, often have low-sloped/flat roofs with a pitch less than 2/12.
So, is your roof’s pitch less than or more than 2/12 (9.5º)? Is it basically flat (commercial) or steep (residential)?
Commercial
If your roof’s pitch is less than 2/12, it’s considered flat, and your options are PVC, TPO, EPDM, and Modified Bitumen.
Is cooking involved?
Do you have a flat roof and do a lot of cooking, such as at a hospital, restaurant, etc.? If so, choose PVC because it resists animal fat and grease and is fire-resistant.
Money a concern?
If there isn’t much cooking involved, is money a concern? If money isn’t a concern, choose TPO because it’s less expensive than PVC and still a great product. If money is a concern, choose EPDM or modified bitumen since these are the least expensive options for a flat roof. Notably, EPDM, also known as a ‘rubber roof’, is the longest-lasting.
Residential
If your roof’s pitch is greater than 2/12, your options are asphalt shingles, stone-coated metal shingles and tiles, and metal, such as standing seam and corrugated/PBC. (If your roof’s pitch is greater than 3/12, clay and concrete tiles are also an option.)
Snow?
Do you get a lot of snow, for example, more than a foot in one snowstorm? If you do, you should consider a standing seam or corrugated/PBC metal roof, as it sheds heavy snowfall quickly. Even if you don’t get a lot of snow, you should still seriously consider a metal roof, such as standing seam. There are more options today than there used to be. What’s more, metal roofing is the ‘coolest’ roof as it helps to lower your Summer cooling bill. It’s also recyclable, unlike asphalt shingles. Lastly, a metal roof often costs less in the long run than traditional asphalt shingles.
Looks, Longevity, or Cost?
If you don’t get much snow at once, do you want your roof to look good, last a long time, or cost the least? If you want your roof to stand out, consider a tile roof for its unique appearance. If you want your roof to last a long time, consider metal, such as stone-coated metal shingles or tiles, or standing seam, since these are among the longest-lasting roofs (psst – a metal roof often costs less in the long run because they last so long). If you want the least expensive roof initially, go with asphalt shingles.
Inevitable problems if you use a roofing material on the wrong roof.
Every roofing material is manufactured to perform certain tasks and behave a certain way. For example, steep-sloped roofing materials are meant to allow water to move quickly downward only. Low-sloped roofing materials, on the other hand, are manufactured to allow water to move in all directions, and at a much slower pace. When you use something in a way it wasn’t meant to be used, something is bound to go wrong and cost a lot of money to fix.
Low-slope single-ply membranes, such as EPDM, are mechanically attached (nailed) or fully adhered (glued) to the roof’s insulation or deck. Installers install single-ply membrane roofs by unrolling the roofing material (PVC, TPO, EPDM), securing each layer to the next, and securing it to the roof. When fully adhered, the seams become stronger than the membrane itself. When done, you end up having one large membrane that covers the entire roof. This is required for low-sloped roofs because rainwater moves much more slowly on low-sloped roofs than on steep-sloped roofs. Low-sloped roofs require impenetrable seams and membranes to prevent water from seeping in.
Leaks
Steep slope roofing materials, such as asphalt shingles, are mechanically attached at the top of each shingle. The sides of shingles are completely exposed to standing or slow-moving water. Although the bottom of one row of asphalt shingles is somewhat adhered to the row below, it’s not strong enough to keep standing water from moving underneath it. Roofing tiles are completely exposed to standing water on the sides and the bottom of each tile. This is one major reason you do not install steep-sloped roofing material onto a low-sloped roof.
Strength
What’s more, steep-slope roofing materials use fewer fasteners per square, if any. On a steep slope, they may not be enough to hold the membrane up and secure it to the roof. Single-ply membranes are designed to almost lay flat and use fewer fasteners. They were not designed to withstand their own weight on a steep incline. Steep-sloped roofing materials are designed to be installed on steep slopes, with more fasteners. As Kevin of Huuso points out, “Potential issues with installing EDPM/TPO membrane roofs on steep slopes are the lack of reinforcements and shrinkage. The weight would pull on the fasteners, causing potential wrinkles, tears, and damage to the membrane.”
Warranty Void
Also, no roofing material manufacturer, such as Mule Hide or Owens Corning, will warranty an incorrectly installed roofing product or a roof covering installed onto an inappropriate roof.
Appearance
Although some single-ply membranes come in different colors, they aren’t nearly as appealing as shingles or tile. Most low-slope roofing materials are white, grey, or black. As a result, these have very low curb appeal. Ami of Feller Roofing says, “Putting low-slope products on a steep roof looks strange.”
No professional roofer is going to install a steep-slope roofing material, such as asphalt shingles, onto a low-sloped roof, or a low-slope roofing material, such as EPDM, onto a steep-sloped roof. If you find a so-called ‘professional’ roofer who will, you should find a better roofer 🙂
Learn more about residential roofing material longevity, cost, etc., in our Guide to Residential Roofing. Learn more about commercial roofing material longevity, cost, etc., in our Guide to Commercial Roofing.
Summary of roof pitch and the right roofing material for it:
Roof pitch:
material option(s)
.25/12 to 2/12: EPDM, TPO, PVC, Mod-Bit
3/12 to 20/12: metal
2.5/12 to 19/12: tile (clay or cement)
2.5/12 to 20/12: asphalt shingles