How a built-up roof works
Formerly the membrane was the waterproofing agent and the bitumen was the adhesive. With the advent of synthetic fibers the roles reversed: today the bitumen is the waterproofing agent and the membrane holds it in place.
Built-up roofs can be installed with coal tar pitch, asphalt, cold-application adhesive or modified asphalt. Every system uses some combination of base sheet, ply sheets (felts) and cap sheet, then a surfacing: gravel, asphalt, coatings or mineral granules.
Reinforcement: what's between the bitumen
Glass-fiber mats
Glass-fiber redefined the built-up roof and has almost completely replaced organic felt across the United States. It reduces blistering and ridging, and unlike organic felt it does not absorb water — so moisture doesn't get trapped in a new roof system.
Organic felts
Cellulose fibers, shredded wood and felted papers. Susceptible to water absorption, which leads to increased blistering and ridging.
Saturated, coated and mineral-surfaced sheets
Saturated felts are bitumen-impregnated and generally serve as base sheets. Coated felts add finely ground minerals for durability and cold-weather resistance. Mineral-surfaced sheets take that a step further, embedding granules in the topside — the cap sheet.
The bitumens
- Asphalt — the dense residue left from petroleum distillation. Types run from I (dead level) to IV (special steep). Generally Type II in the field of the roof and Type III on penetrations and base flashings.
- Coal tar pitch — distilled from high-temperature coke oven tar. Naturally resistant to temperature change, water, moisture and vapor transmission. Its low melt point makes it self-healing. It must be surfaced with gravel to hold the bitumen in place, and is not recommended above ½" in 12" slope.
- Cold process — an inexpensive tar/asphalt semi-liquid, applied by brush or roller. Used mainly on smaller jobs where a more substantial roof isn't required.