Modified Bitumen Roofing in Southern California

Modified bitumen (often just called "mod bit") starts as asphalt that's been modified with either APP (atactic polypropylene, a plastic-based modifier) or SBS (styrene-butadiene-styrene, a rubber-based modifier). That modification is what gives it flexibility across temperature swings instead of the brittleness you get from straight asphalt.

It comes in rolls, usually reinforced with fiberglass or polyester matting, and gets installed in one or more plies depending on the roof and the budget. Cap sheets often come with a mineral-granule surface for UV protection, or a smooth surface meant to be coated afterward.

APP modified bitumen is torch-applied - we heat the underside of the roll with a propane torch as it unrolls, melting the asphalt so it bonds directly to the substrate below. It takes a trained hand; too little heat and the seam doesn't bond, too much and you scorch the sheet or create a fire risk on an older, dry roof deck.

SBS modified bitumen is more often installed with cold adhesive or hot mopped asphalt, which is a better call on buildings where an open flame near the deck isn't acceptable - tenant-occupied buildings, roofs near vents or fresh air intakes, anything with combustible material nearby. We pick the install method based on the building, not just habit.

Modified bitumen is one of the easier low-slope systems to patch. A torch or cold-adhesive patch ties into the existing membrane cleanly when the surrounding material is still sound. That's part of why building owners like it - a small leak doesn't always mean a full re-roof.

On roofs with a smooth (non-granulated) cap sheet, a reflective elastomeric coating extends the roof's life and cuts down on heat load, especially valuable on inland buildings where rooftop temperatures climb fast in summer.

• Torch or cold-patch repairs at isolated leak points
• Reflective coating on smooth cap sheets to extend life
• Flashing resealing at parapets and curbs
• Granule loss inspection to catch UV wear early

Solar racking on a mod bit roof needs to be flashed into the membrane properly, not just sealed with roofing cement over a lag bolt. We've re-roofed buildings where a previous solar install did exactly that, and every attachment point had turned into its own leak.

Since we do both trades, we plan penetrations and racking layout as part of the roofing scope, so the membrane and the mounting hardware are integrated correctly from day one.