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Synthetic roof felt under hardwoods

StevenNC8b | Posted in General Questions on

Is there any reason not to use synthetic underlay instead of asphalt roof felt? It is 3/4 site finished oak


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  1. Expert Member


    Better than both are purpose-made flooring underlays, which you can find at any big box store.

  2. Expert Member
    Michael Maines | | #2

    My somewhat unpopular opinion is that flooring underlayments provide no benefit. I definitely wouldn't use tar paper or felt paper due to the VOCs involved.

    People claim they help prevent squeaks. Flooring squeaks when it rubs vertically against a nail, or possibly against an adjacent board; it doesn't slide along the subfloor which is the only way having a slip sheet would help.

    People also claim that it blocks vapor movement. The traditional material is rosin paper which does nothing to slow vapor. Tar paper/felt paper slows it down very slightly. If you have so much moisture under your flooring that you are concerned about warping the flooring, you should fix the cause, not rely on a membrane.

    I believe that underlayment began as a way to stop dirt and air from filtering through the flooring and board sheathing and became habit, later justified by guesses as to what it might be doing.

    1. paulmagnuscalabro | | #3

      You're probably not too far off on your thinking that underlayment began to stop dirt & air movement, Michael. I used to always wonder why old homes had rosin paper under the siding (or the case of my 1880 house, sheets of old newspapers), and it blew my mind a bit when I learned that it was just a way to slow air movement / stop the wind from blowing right through the house.

      1. Expert Member
        Michael Maines | | #4

        The house I grew up in was built in Maine in 1817 and had birch bark under the siding!

    2. Expert Member
      MALCOLM TAYLOR | | #5


      The justifications offered by the manufacturers certainly don't seem very convincing.

    3. Expert Member
      DCcontrarian | | #6

      People in the flooring industry don't even know why they do it.

      I have one room where there is Tyvek under the flooring because that was handy when the floor went down. It was done 17 years ago and has been fine.

      1. Expert Member
        Michael Maines | | #7

        It doesn't seem to hurt anything, so while I used to argue about it with contractors, I don't bother anymore. The sole advantage I have found when laying my own floors is that a membrane makes it easier to see if the floor is fully free from debris; it's no fun to get a board nailed off before realizing there was a piece of grit under it. With a narrow board you would notice but when laying wide planks over floor sheathing it can be hard to tell until it's too late.

    4. StevenNC8b | | #8

      It is 4" oak and I have been told that the wider the boards the higher the likelihood of cupping. I would like to do everything I can to prevent that. And synthetic felt is cheap. I just thought that it was better to go ahead and do it. You never know when a pipe will burst or some other unforseen accident. In that case you have to dry the crawl but in the process you would have high humidity at least for a while in the crawl.

      1. Expert Member
        DCcontrarian | | #11

        So I'm a little torn with the idea of putting a vapor barrier between the finish floor and the subfloor. And roofing underlayment is a vapor barrier.

        If the space below is conditioned, it's not normal to put either insulation or vapor barriers between finished spaces, and with a crawl space in particular the goal with conditioned space is to get it as much like the rest of the house as possible.

        If the space is not conditioned, the vapor barrier should go on the bottom of the joists, and the assembly should be able to dry up. You don't want your framing trapped between two vapor barriers.

  3. BrunoF | | #9

    If you don’t want any squeaks the flooring installers around here will recommend glue and nails / staples. Some even trowel on a continuous, thick adhesive layer that also works as a vapor barrier.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      If you do use adhesive, make sure it is elastomeric so the floor can move.

      1. BrunoF | | #12

        Agreed. The concept kinda makes me nervous especially if there’s ever any water damage or something that requires removal of the floor but from what I understand it results in a super solid end result.

        1. Expert Member
          MALCOLM TAYLOR | | #13


          Having spend decades installing floors with just staples, using slip sheets, and carefully avoiding solid attachment to the subfloor, like you I still have some residual nervousness about adhesive. But the glued down floors I've put in over the last eight years or so have performed really well. Most other floor types share the problem of needing their adhesive separating during replacement. I guess we just have to get used to wood being the same.

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