Flat Roof Considerations

Worker welding pvc roof membrane by heater with welder machine

Have you ever stopped to wonder why almost every house has a sloped roof – gable, hip, etc – and almost every business has a flat roof – often hidden behind a parapet with a big sign on it? If you have, you would wonder why so many homes have problems with flat roofs. Let’s discuss that from a homeowner’s perspective.

Front view of 10 houses showing 9 with sloped roofs and 1 with flat roofs.

A flat roof can be easy to build if you just need to put a roof over a patio. Install 4 posts (columns) in footings, then form a roof-ceiling with 2x12s and lay a plywood underlayment over them. The most inexpensive roofing material for a flat roof is roll roofing (also called “modified bitumen”) – essentially, very large, long asphalt shingles that are nailed into the roof plywood. Any handyperson or homeowner with a ladder and hammer can assemble this. It’s fairly cheap to install and gets the job done. It’s a great choice for sheds, porches, carports, and barns.

Until you are the next homeowner, and it becomes a problem. The first place to leak is usually where the flat roof meets the sloped house roof. While you are up there applying goop over the seam, you may discover the flat roof surface has become discolored or even has water sitting up there. We call this ponding, and it is not what you want on your roof. The roof is so flat, that the water doesn’t run off it, so it stays, creating a small pond less than an inch deep, but the extra weight causes the underlayment to bow between the rafters. Which causes more water to stay, which is even heavier, and takes longer to evaporate.

Roll roofing is installed like large asphalt shingles on a flat roof surface. Edges are nailed or heat sealed together with a blow torch.

Not every flat roof with roll roofing will leak, but there are better options out there, which require a more skilled installer with closer attention to details.

Let’s step back for a moment and consider what “flat roof” really means. It could actually mean a roof with zero slope, as level as the floor. But in roofing terminology, it also means roofs with very slight slopes, like a sloped walkway, but not as steep as a driveway apron (where a driveway slopes down to the street across a sidewalk). Building a “flat roof” using wood framing (lumber or trusses) is actually easier if the rooftop has a zero slope because the ceiling will also have a zero slope. So how do you make sure rainwater runs off that?

We call it “tapered exterior insulation” and it’s just what it sounds like. Insulation made into boards that are slightly thicker at one end than the other are laid over the roof plywood. This is how many flat roof commercial buildings are insulated – with the insulation on the outside, instead of the fluffy insulation inside attics that most houses have. Then a roof membrane (a large, smooth sheet of roofing material, usually white, and made of rubber or plastic blend) is laid over the insulation. The sheets are heated or glued together, which makes the entire roof watertight. The edges are secured to the roof with a system of clamps and clips.

Roofing membrane – sheets are laid over the roof then the seams are heated together.

PVC and TPO are the most popular choices for roofing membrane over tapered exterior insulation. Skilled installers are familiar with the details required to make this type of roof watertight and long-lasting. There is a wide array of manufacturers of these roof membranes, with slight differences between them in installation, details, and the thickness and proprietary blend of materials. However, all of them should last 20-30 years, similar to the asphalt shingles on a sloped roof.

Look for a roofer who specializes in or is familiar with these types of roofs. You will want to pay a little extra for an experienced installer. And enjoy the sound of the rain falling on your roof, without the stress of wondering where it will leak next!

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