No roof should be completely flat, or it won’t shed water. But “flat roof” is a convenient term for a class of multimembrane systems with very shallow pitches. At one time, built-up roofs (BURs) once represented half of all flat roof coverings. BURs consisted of alternative layers of heavy building paper and hot tar. Today, modified bitumen (MB) is king, with cap membranes torched on to fuse them to fiberglass-reinforced interplies or base coats. For that reason, future roofs are likely to employ hot-air welding, cold-press adhesives, and roll membranes with self-sticking edges. Increasingly, professionally installed sheet-material systems such as EPDM (synthetic rubber), TPO (thermoplastic olefin), and PVC systems are gaining market share.
In the final phase of a modified bitumen roof, an installer torch-welds a granular surface membrane to an interply sheet or directly to a base sheet. The granular surface is somewhat more expensive at installation, but it is cost-effective in the long run because it doesn’t need periodic recoating.
Once the granular membrane is down, its overlapping edges are often lifted and torched again to ensure sound adhesion and a waterproof seam.
Roofers refer to the molten material being forced out by the pressure of the trowel as wet seams—the mark of a successful installation.
The intersection of flat and sloping roof sections is worth extra attention. Run MB membranes at least 10 in. (vertical height) up the sloping section. Then overlap those membranes with the underlayment materials and the composition roofing material so water can run down unimpeded.
Fine Homebuilding Recommended Products
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Affordable IR Camera
This camera is super useful for tracking down air leaks in buildings. The one-hand pistol grip arrangement frees your other hand for steadying yourself while maneuvering tricky job sites.
Available in 2- to 12-in. widths, this is a good general-purpose flashing tape that sticks well to most things. It features a two-piece release paper, water-shedding layers, and good UV resistance.
This heat gun is great for drying joint compound, primer and paint when patching drywall and plaster walls. Plus it can soften adhesive, get a very cold small engine to start, and shrink heat-shrink tubing.
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