In our office, we just finished a project with steel studs and gypsum sheathing. We normally like concrete masonry unit wall back-up with hydrocide coating behind brick veneer, but economics has caused this old construction method to only be used on higher end jobs. On my drawings, I put 15# (fifteen pound) felt over the gypsum sheathing. When the contractor started placing the felt on the building, we noticed it was No. 15 felt. not 15 pound felt. After some research, it seems the roofing industry is trying to pull a fast one here. No. 15 felt is about 8 pounds a square yard, where 15 pound felt is about 12-14 pounds a square yard. I find it amazing that 15 pound felt is almost impossible to find in building supply houses, and is actually is a special order item. So we told the contractor that he had to use No. 30 felt which is about 11 pounds a square yard. After much arguement, he did. I find it funny that the roofing industry has done some pretty tricky marketing when it comes to felt. So from here on out, I note felt as #30 on my drawings.
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the rolls of 15lb. felt are right next to the 2x4s and the 1/2 ply in the lumberyard
15# per square YARD would be pretty thick felt.
It's 15# per square which is + 100 square FEET.
Fifteen pound felt used to contain 15 lbs. of tar per 100 sq. ft. Back in the 70s, it became #15 felt, and it no longer is as saturated as it once was. If you want the eqivelent of what once passed for 15 lb. felt, you now pretty much have to buy #30 felt, or an enhanced paper such as Roofer's Select.
Andy Engel, Forum moderator
I learned something today. Glad I was here to do it.
The problem I have found with using 30# or No. 30 felt on more sensitive applications ( I laid it under a new hardwood floor), is that by lapping it, creates a noticable (sp?) hump. Even after the floor had been sanded, the lump was still in the floor. Maybe I should have used a larger overlap, I dunno. The project was above a moist basement and I decided the 30# might be a better product.
Why not just use 2 layers of #15 felt with a full lap? You get no bumps, and just as much material in place. Any reason why this wouldn't work?
Tar paper under hardwood?
Tarpaper under hardwood is done frequently here in the northeast. Imagine a wet basement, no tarpaper under the flooring and a varnish finish. That floor would be very likely to cup as the unfinished bottom of the boards absorbed moisture from the basement and expanded, while the finished top absorbed little or no water from the house. It's common to see rosin paper or a felt that's about half the weight of #15 used for this purpose. I've never seen anyone use #30 under a floor though.Andy Engel, Forum moderator
I'm building over a crawlspace, so a vapor barrier under the hardwood is required. I wasn't necessarily recommending 30# felt. But I have the same concerns about the lapped joints -- in a big room, looking say from the front door, the "high spots" are going to show in the floor where the laps are . I'm using pre-finished flooring -- probably wouldn't be a problem if your floors are going to be sanded and finished.
Andy, what kind of felt is half the weight of 15# ? What do I ask for?
I don't know the answer to that one. I know that I've seen flooring installers use the stuff, so you might ask at your hardwood supplier. Andy Engel, Forum moderator
Roofers Select is pretty light weight stuff .
Thanks Andy , around here most use the rosin paper, I thought it was a manufacture spec.
Here's an experiment that I've never tried, but that I bet would be enlightening. Wad up some rosin paper, wad up some felt, and toss both into a bucket of water. Come back in a week.Andy Engel, Forum moderator
#15 felt under hardwood is spec'd out by pretty much all of the hardwood flooring installation/manufacturing associations.
The only time I'd advise using red rosin paper instead of #15 tar paper is if the flooring is going over radiant floor heat.
Hey Andy - I am ordering up supplies to install cedar claps on my house and I have gathered from some of the posts that you have made (thank you by the way) that the #15 is really about 7lbs of tar per square. The #30 paper is really what 15lb felt used to be. If I have that right I would rather see if I can use #30 felt under my claps. Where the heck can I find this stuff? My local yard looked at me weird when I asked them.
Also, do you have any feedback or experience with GP Primetrim?
Thanks
SJ
Know a little about alot and alot about little.
On the subject of felt under clapboards, I see a lot of recommendations of using house wrap instead of felt so that the wall can release the moisture from inside.
Steve
Steve -
I am not sure how the search feature works these days, but your questions RE: felt under claps and GP PrimeTrim have a long history in the archive. I happen to think highly of PrimeTrim (as do some others here) and I use it on almost all of my projects. Primetrim is not very strong, so it must be supported on 12"-16" centers if used in a horizontal position.
With regards to felt, there are many different opinions. I happen to strongly believe in 30# felt over either 15# or any type of housewrap. The best practice is to leave an airspace between siding and whatever you use, and definitely backprime or seal the back and ends of your siding.
I live in the northeast and I have almost always used red rosin paper under hardwood floors as have most of my installers. Under most circumstances this seems to work fine. One advantage rosin paper has over felt is that the hardwood slides into place much easier when your trying to make time. If you have a situation where you think that there is the potential for moisture reaching the hardwood from below then felt is a better choice. I had one old time installer insist that I get him slater's felt and i think that is what Andy is talking about. It is a lighter felt but also did not have the tackiness of regular felt which let the boards slide much easier. You should be able to locate it through a roofing or slate supplier. I can't tell you why I have never used it since: can't teach an old dog I suppose.
Below is the link for the Hardwood Flooring Mgs. Association -- pretty good info.
http://www.nofma.org/installation.htm