Underlayment is a thing covering of material such as asphalt impregnated felt, with low vapor resistance, which is installed on top of roof sheathing and underneath the roof finish material. The purpose of the underlayment is threefold. First, it protects the sheathing from moisture and humidity during construction.
Secondly, it gives additional protection from weather through prevention of the intrusion of wind-driven rain, snow and sleet.
Lastly, the underlayment prevents the shingles from directly contacting resinous portions of the sheathing surface.
Heavy felts or coated plastic sheeting should be avoided, as these tend to act as a vapour barrier, which would allow mpositure, frost and humidity to be trapped between the covering and sheathing.
Depending on the type of shingles used and the slope of the roof, 15 pound roofers felt may be used. This is also known as Tar paper, roofing paper and felt paper, but it is all the same stuff.
Underlayment shoould never be installed on a damp rood, as this may trap moisture, thus damaging the roof.
Recent advances in roofing materials have brought the addition of synthetic underlayments to the construction industry, although they are not yet in wide use. These synthetics boast a greater resistance to moisture, rips, and ultraviolet light than roofing felt. They are made from materials such as polypropylene, polyester, or fiberglass fabric.
Certain manufacturers also offer fire-rated synthetic roof underlay when fire-resistance is required.
One of the benefits of synthetic underlayment over traditional tar paper is the weight savings. It is around six times lighter, which makes it much easier to install. Also, because it is fully waterproof, which felt underlayment is not, builders can begin interior work while waiting for the roof shingles to be installed.