Flooring

Discussion in 'Remodel Forum & Blog' started by ingeborgdot, May 7, 2010.

  1. ingeborgdot

    ingeborgdot New Member

    Messages:
    119
    Location:
    Kansas
    Is it any better to put down two layers of 3/4" plywood or is it better to put down one sheet of 1 1/2". I have to have it that thick to match the floor height of the other room?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    You've found 1-1/2" ply - boy, I wouldn't want to try to hump that stuff around? I've seen some just over an inch thick and it is a bear to carry and place. Ply is fairly heavy, and 1-1/2" stuff would be REALLY heavy. It is true that thicker plywood is stronger than two sheets of thinner stuff that equals the single thicker piece, but you aren't really interested in max strength I don't think if all you are trying to do is get the floors even.

    So, I'd opt for the two thinner sheets. For maximum strength, install them both across the joists, but on the top layer, offset it by 1/4 the distance between joists (this means the top layer's end joint will NOT be on a joist - on 16" OC, the end would be 4" from the center of the joist). This provides the maximum bridging strength across the joist top and ensures you don't have two edges aligned that can squeak from deflection. You'll need lots of either ring-shanked nails or screws to mate the two layers together - don't skimp on those. Do use construction adhesive to bond the bottom sheet to the joists, but do NOT use construction adhesive between the two sheets (you want full contact, and it's almost impossible to get that with construction adhesive, and a full-coat of a wood glue just isn't normally worth the effort and the time to spread and place the sheet before it starts to skin over takes practice and speed). Leave the recommended end and edge gaps so there's room for expansion without cupping or tenting, and you should be fine. Assuming the joists are strong enough, this is the same prep as required for a stone tile installation for the subflooring. For maximum strength, use a T&G ply for the layer directly on the joists - regular, square-edged stuff is fine of the top layer.
  3. ingeborgdot

    ingeborgdot New Member

    Messages:
    119
    Location:
    Kansas
    Makes sense. Thanks
  4. ingeborgdot

    ingeborgdot New Member

    Messages:
    119
    Location:
    Kansas
    Is there an advantage to gluing and screwing instead of just screwing?

    Also, what is the proper distance between screws?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    A laminated floor (glued together) IS stronger, but it must be done right. Spreading a liquid glue like Titebond II can get messy, and you need to get things together fairly fast or it skins over and won't make a good bond. If you don't get it screwed or nailed fast enough, you'll have problems, too. Do it wrong and it's likely worse than not gluing it. When two layers are installed to meet the requirements for a stone tile, it is done to bridge the ends and edges. If you can picture those edges at a joist, when they deflect, they act like miniature levers, and that can telegraph and crack a stone tile (primarily because it has lots more internal imperfections than a manufactured, ceramic tile). Placing a second sheet over that first layer, offset properly, means that layer spreads the load caused by any jacking at the joints, and provides increased strength to defeat deflection between the joists. this additional strength is rarely needed to justify the extra work and money...the second sheet of structural material is plenty. If you attach them together properly with fasteners, you'll have a strong, squeak-free floor. There's an article on this in the 'Liberry' at www.johnbridge.com. I think the fastener schedule was 6" on edges, and 8" in the field, but don't quote me on that, find it yourself.
  6. ingeborgdot

    ingeborgdot New Member

    Messages:
    119
    Location:
    Kansas
    So, you are saying that if screwed properly no liquid nails is needed for the subfloor.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    NEVER use something like liquid nails in a floor except to anchor the bottom sheet to the joists. It is nearly impossible to get a full-contact spread of the stuff across a large panel. As a result, you'll end up with small gaps where the first sheet is not in contact with the second. So, do not use anything you squeeze from a tube there. If you really want to glue them together, and it isn't required, use a liquid wood glue, spread it out like thinset with a trowel to get 100% coverage, and be fast about it so you can get it glued together like a piece of furniture. Then, you'd have a stronger floor, suitable for any surface finish material (assuming your joists are stiff enough). Otherwise, just screw them together. Ring-shank nails can be used as well and are less likely to cause jacking (where the screw doesn't start into the second sheet immediately, and the threads hold it up from making contact). Screws are stronger, but a good ring-shank nail is nearly as good, and much faster to install. You don't want to use straight nails, though. Either work from one end to the other, or the center out.
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