Merriam Webster describes felt as such:
Jon Alan Wright
Definition of FELT
a : a cloth made of wool and fur often mixed with natural or synthetic fibers through the action of heat, moisture, chemicals, and pressure
b : a firm woven cloth of wool or cotton heavily napped and shrunk
: an article made of felt
: a material resembling felt: as
a : a heavy paper of organic or asbestos fibers impregnated with asphalt and used in building construction
b : semirigid pressed fiber insulation used in building
We will discuss 3a, of course.
Originally there was just 15 lb., 30 lb., and copper felt. They were made from pulp made from rags, newspaper, and even old dollar bills. At the GAF plant on Singleton Blvd. in Dallas, TX, there was once a network of government agents guarding a convoy of armored vehicles coming from Treasury to guard the old dollar bills about to be mushed into roofing felt. 15 lb. weighed about 15 lbs. per square and 30 lb. did as its moniker stated also. Copper felt was a flashing, that I've actually seen, used for flashing. Copper was laminated to the asphalt impregnated felt. Watching the felt and asphalt get it on has scarred me for life.
There are other old felts, like asbestos, that we won't talk about because the censors at the EPA are in the next room. By the way, it was replaced by fiberglass felt.
Some felts were used for built up roofs, known as BUR, because they are used in layers with an interply of asphalt, to build up layers. Later they were punctured full of holes so the fumes and vapor could escape. This felt has been long gone from my life but could still be in use in colder climes. Non-fiberglass felts work well where it is so cold the fiberglass felts shatter.
The problem with 30 lb. as an underlayment is that it can wrinkle, even after it has been installed, and it is also a vapor barrier. This will accelerate the demise of a less than perfectly ventilated attic as it is moisture, and not heat, that is the Great Satan of Roofing.
On low slope roofs two ply of 15 lb. is mandated because it eliminates the weakness of a two inch lap that one ply of 30 lb. has plus the wrinkle factor. Any wrinkles on a roof between a 2/12 and 4/12 is a cardinal sin. 30 lb. under composition can be an original sin if it is not ventilated buy the rules governing fluid dynamics and moisture migration. These be Bernoulli, not to be confused with the great Texan Burnelli, and Venturi. If you are lost here you're going to flunk the test at the end of the blog. So you better go back to the beginning and start taking notes.
Sometime back in the late 1980's the 15lb. became #15 and 30 lb. became #30. This was because they weren't what they used to be. They lost their tonnage and the lb. suffix became a "#" prefix. Sorry Capitalist Pigs, of which I am a card carrying member.
So what's an ethical roofer to do? Fret? Cry? Stomp his feet until something better comes along? Assuredly we did and the roof god Techus provided us with an alternative to sin. Synthetics, something short of sin but in the same category. This is because there are some really bad ones and some okay ones and some really fantastic ones. You've got a complete cornucopia of synthetics and hybrids out there that could confuse even the most learned of roofing Philosophers and Sophists.
Some synthetics breath and others do not. Permeability, my friend, is the mother of roof penicillin. Whether it be in the movement of attic air or the containment of humidity in the undergarments of roofing, the issue is the same: moisture ruins organic and metallic materials.
The hybrids are asphaltic felts that lay flat like #15, ugh, but have the strength of #30.
Now, let's talk new construction versus re-roofing steep roofs. When roofers roof new homes they can use #15. They roll felt and lay roofing. They do not need to walk on the #15, and if they did they'd be mangled in some way or another. #15 will not support a person on a steep roof. When re-roofing a steep roof, the roofers must dry in the home rapidly to protect the contents and the finished product: sheetrock, paint, insulation, carpet, etc.
In the recent past all the way back to my ancient beginnings as a roofer, insurance companies paid for #30 on insurance claims pertaining to steep roofs. This protected the roofer from certain death. Now they cheap out and only pay for #30 if it is on the roof prior to replacement. I've told them that one day a roofer will break his neck and they will be included in the lawsuit. I hope some attorney comes across this and uses it in his filings.
There is so much more more to tell about felt but my felt pen is running dry.
Jon Wright Roofing, Siding, and Windows
1915 Peters Rd., Suite 310
Irving, TX 75061