The most common roofing material used on your average home today is certainly asphalt shingles and probably every shingle roof done in the US today starts with the installation of black paper called roofing felt underlayment.
What’s interesting – remarkable even, is that, in perhaps 90% or more of instances, the installation of this paper is a waste of time and resources.
The felt we’re referring to is your basic asphalt saturated, thick black paper which is most often used in two thickness, 15lb or 30lb which refers to its weight per 100 SF. The use of felt, dating back over 100 years, probably originated with wood shake, slate and tile – the three “original” roofs. With tile roofing, and particularly two-piece Spanish tile, the system was imperfect. Spanish tiles were handmade, not uniform, and often (intentionally) not laid down in a uniform manner. During any heavy rain, even on a newly laid roof, water would get past the tile so the felt helped to prevent actual leaks. In the case of wood shakes, it helped stop leaks when, after time, the shakes would split open. And so felt served an important role here as well.
Back in the day, homeowners often did their own roofs, or they hired very small and inexpensive roofing crews which could take weeks to months and in these cases felt was adopted for use as a way to “dry-in” a roof that might be exposed to the elements for some time, or for very low sloped roofs which need extra protection from wind-blown rain which may, from time to time, defeat an asphalt shingle system.
At some point the manufacturers of shingles (and felt) and subsequently roofers began pushing the use of felt on all roofing projects to increase sales, even when most roofers knew that it was mostly unnecessary.
But don’t take my word for it. The following is taken from the 2007 Master Shingle Applicator manual from CertainTeed (excerpted from pg 23): “CertainTeed does not require that shingle underlayment be used under their shingles for the standard shingle warranty coverage to be in effect, on slopes of 4/12 and more.”
There can be no more a definitive statement, than the manufacturer of the product itself, stating their warranty will be honored if you don’t use their own product!
Now it is considered that on low slopes of 2 to 3 in 12 pitch (9-14 degrees), that double roofing felts are necessary (by the manufacturer) and I agree since I have seen that this does seem to be the case. Shingle systems can be defeated at about a 10 degree roof pitch when the worst of circumstances are present, such as a heavy, wind-blown rain storm and the use of double felts can help prevent that.
It has been suggested that these roofing felts are a constituent part of a Class A fire resistant (level) roofing system. But I find that fairly dubious. As far as I know, there is nothing (chemically) in these felts which prevent fire and the only reason they are a constituent part of a Class A fire resistant roofing system, is because the Uniform Building Code states felts must be used in a shingle system to begin with so it became part of the test but it’s the shingles that are fire resistant. I am pretty sure if you soak paper in asphalt, you have kindling or fire starter, not something that is resistant to fire.
Matt Glass wrote this article on roofing felt and he is part of the Glass Family who owns and operates J and J Roofing in Los Angeles.