Bituminous waterproofing

Bitumen (asphalt or coal-tar pitch) is a material made up of organic liquids that are highly sticky, viscous, and waterproof.

Over time the felt's natural mesh used as a substrate for asphalt impregnation (derived from fabrics like cotton or burlap) has evolved into synthetic products performing the same function with improved durability.

Other changes with time have enhanced performance, with roofing felt remaining a heavier and more durable product than tar paper.

However, modern base felts are made of lighter-weight fibre, so the weight designations, though common colloquially, are no longer literally accurate.

[2] A heavier class of materials with a similar construction but designed for civil engineering, environmental protection, and mining applications are known as Bituminous Geomembranes (BGMs).

Modified bitumen is mixed with filler components such as limestone, sand, or polymers such as atactic polypropylene (APP) that gives rigidity and tear resistance or styrene-butadiene styrene (SBS), a rubber additive that gives more elastic benefits.

To protect its asphaltic base from ultraviolet degradation mineral granules are added on top of the felt, also decreasing the product's fire vulnerability.

Thin, removable transparent film is added to the base of rolled roofing during manufacturing on all torch-on products.

The complex chemical composition of bitumen makes it difficult to identify the specific component(s) responsible for adverse health effects observed in exposed workers.

From 1905 to 1988, The Paraffine Paint Co. of San Francisco had Malthoid as a trademark for waterproof and weatherproof building and roofing materials made of paper and felt in whole or in part.

[16] Malthoid was once common enough to be used as a generic description of flat roofing material in New Zealand and South Africa (item 26).