Bituminous waterproofing systems are designed to protect residential and commercial buildings. Bitumen (asphalt or coal-tar pitch) is a material made up of organic liquids that are highly sticky, viscous, and waterproof. Systems incorporating bituminous-based substrates are sometimes used to construct roofs, in the form of "roofing felt" or "roll roofing" products.
Roofing felt
Roofing felt (similar to and often confused with tar paper, but historically made from recycled rags rather than heavy kraft paper) has been used for decades as waterproof coverings in residential and commercial roofs as an underlay(ment) (sarking) beneath other building materials, particularly roofing and siding materials, and is one type of membrane used in asphalt built up roofing (BUR) systems. 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.
Function
The rapid application of waterproof or water-resistant roofing underlay protects the roof deck during construction until the roofing material is applied and is required for roofs required to meet Underwriters Laboratory (UL) fire ratings. The separation of the roof covering from the roof deck protects the roof covering from resins in some sheathing materials and cushions unevenness and old nails and splinters in re-roofing applications. The underlayment also sheds any water which penetrates the roof covering from an ordinary leak, a leak from wind-driven rain or snow, wind damage to the roof covering, or ice dams. However, the application of underlays may increase the roof temperature, which is the leading cause of ageing of asphalt shingles. Not installing an underlay may void the roof covering warranty.
Weights and grades
Felt paper is available in several grades, the most common being Type 1—commonly called 15-pound (15#) or No. 15 (#15)—and Type 2—commonly called 30-pound (30#) or No. 30 (#30). The weight designations originated with organic base felt weighing 15 or 30 pounds per 100 sq. ft. ( or per ). However, modern base felts are made of lighter-weight fibre, so the weight designations, though common colloquially, are no longer literally accurate.
Another basic designation is organic or inorganic. Organic felt paper has a base material made with formerly living materials such as rag fibre, hessian (burlap), or cellulose fibres (wood, or jute). Organic felt papers are now considered obsolete, having dwindled to just five percent of the market by 1987. Inorganic base products are polyester, glass fibre developed in the 1950s, and historically, asbestos mat. Polyester mat is weaker and less chemically stable than fibreglass but because it is cheap is gaining market share. Polyester mat is primarily used with the more flexible modified-bitumen felt products. Asbestos mat was the first inorganic base material but was outlawed in the 1980s for health reasons but is still in use on some buildings. Inorganic felts are lighter, more tear-resistant, more fire-resistant, and do not absorb water. Another type of felt paper is perforated for use in built-up roofing and is not for use as a water-resistant underlay.
Heavier material is typically used for underlayment of longer-lived roof materials in order to match their longer life span, and on less sloped roofs, which are more susceptible to leaking. For example, two layers of No. 30 felt might be used under a slate or tile roof, whereas a single layer of No. 15 might be adequate for a steeply raked roof of 24-year asphalt shingles. s0
Manufacturing process
Roofing felt is manufactured in roll format. Rolls of base felt are pulled on rollers through large tanks of bitumen mixes until they are saturated with the tar-like bitumen mixture, producing rolls of water-resistant but breathable material.
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.