Only two main types of felt paper are available for roofs: asphalt-based paper and tar-based paper, also called bitumen-based paper. Technological advances in roofing and water damage prevention mean that the basic tar-based felt papers can now be bought with upgraded features, such as impregnation with glass fibres or polyester. These modern enhancements improve moisture resistance and durability.
Asphalt is used often in roofing construction as an underlay to prevent water infiltration and as a temporary water protection agent during the early stages of the building process. According to Home Addition Plus, there are two weights available: 13.6 and 6.8kg. The 15-weight version is not as durable but is less expensive and allows the roof to "breathe," so is favoured among roofers.
A heavy-duty paper roofing material impregnated with tar, or bitumen, tar-based roofing material is highly resistant to water and often contains glass fibres or polyester fleece. Sand is applied to one side, so it can be rolled up without sticking to itself, and serves as a protection against the elements. Thirty- and 6.8kg. weights are common in the tar-based material also. Tar-based roofing felt paper is commonly transported in 30cm diameter rolls, according to the Transport Information Service
New designs of roofing paper use TPO (thermoplastic polyolefin), which is added as a waterproof membrane to prevent leaks. Compared with traditional roofing felt, TPO materials are advanced technologically and can reduce condensation inside the loft space.
Unrolling sheets of granular tar-based felt paper and using a torch to stick the sheets to the roof is a new invention. The paper used in the process is specialised tar-based paper that has been modified with granules made from rubber. The granules protect the roofing material from heat damage and ultraviolet light from the sun's rays. The granulated coating is applied using a hand tool with a cylindrical shape, as reported by Patent Storm.
Hessian & Asbestos
The "carrier" or base layer was always paper, but paper is not resistant to water, nor is it durable or resistant to tearing, so other carriers were tried and tested. Asbestos felt roofing was withdrawn on health and safety grounds but did provide effective coverage at one time. Another previous design involved the use of hessian, a heavy fabric material that prevented water penetration. Glass fibre was an improvement over the original designs but can still be brittle and tears easily.
Other Waterproofing Agents
Paint and tar were first to be used, then oxidised (ordinary) bitumen became the "gold standard" of waterproofing agents although it remained quite a brittle substance to work with. Since then, developments including chemically-enhanced bitumen, have become more common. Actactic polypropylene (APP) or styrene butadiene styradene (SBS) are additives used to improve the performance of standard bitumen. Roofing felt containing modified bitumen is referred to as high tensile or "HT" felt.
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