Modified bitumen roofing for Phoenix low‑slope roofs

A typical residential mod bit roof starts with a base sheet nailed or adhered to the deck, then a cap sheet on top with mineral granules embedded in the surface. The sheets come in rolls, and they overlap at side and end laps by set distances. Those laps are fused with heat, set in cold adhesive, or self-sealed, depending on the product. The finished roof is two layers of tough, flexible membrane with factory-controlled thickness.

The modifiers are what make it work in Phoenix. SBS-modified sheets stay rubbery and flexible, which helps on roofs with movement. APP-modified sheets behave more like plastic and handle high heat well. Both types hold up far better than plain asphalt felt in Valley sun. Our crews choose the type and attachment method based on the deck, the building, nearby structures and how much traffic the roof gets.

Layout matters as much as material. We plan the sheets so laps run with the flow of water toward the scuppers, never against it, and so end laps are staggered rather than lined up in one weak row. Around parapets, the membrane turns up the wall and under a counterflashing or new cap, and AC curbs get a full wrap rather than a strip of sealant. These are the places where a quick install saves an hour and costs the owner a leak two summers later.

Torch-applied mod bit uses a propane torch to melt the back of the sheet onto the layer below, and it is still common on commercial jobs across the Valley. It is fast and creates strong bonds, but open flame on an older wood-framed roof carries real fire risk, especially near parapets, wood fascia and dry palm fronds. We use torch application only where it is safe and appropriate, with fire watch after the work.

Cold-applied systems use adhesives rolled or sprayed under each sheet, and self-adhered sheets have a sticky back protected by release film. These avoid open flame entirely, which makes them a popular choice on homes, near neighbors and on roofs with lots of wood detail. In Valley heat, self-adhered sheets bond aggressively, so crews work early in the day to control placement. We explain the trade-offs and pick the safest method that still gives a lasting bond.

A dark mod bit cap can get extremely hot in July. Light-colored granulated caps and white reflective coatings lower that temperature, which slows asphalt aging and helps the rooms below. Many owners coat a mod bit roof a few years after installation, or choose a factory-coated white cap from the start. If you are thinking about a coating later, our reflective roof coating page covers how that works over mod bit.

Most mod bit leaks we repair start at laps, flashing and penetrations rather than in the middle of a sheet. Heat cycling can open poorly sealed seams, and foot traffic around AC units wears granules away. Keeping scuppers clear, walking on designated paths and having the roof checked before monsoon season catch most problems early. A seam that opens in June is a simple fix, but after a season of storms it often means wet insulation. Granule loss shows up as shiny black patches and is worth recoating before the asphalt beneath starts cracking. For older tar and gravel roofs considering an upgrade, see our built-up roof repair and replacement page for how the two systems compare.