Unflat Plywood Subfloor

Old 11-25-03, 09:17 PM
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Question Unflat Plywood Subfloor

Hi there
I'm a novice home renovator learning as I go. I've completely gutted an 80 year old house and now am at the flooring stage. There is new 1/2" plywood down on top of pine planks. I have found it inconsistent in terms of flatness. I tested laying a few of my 3/4" hardwood planks down and they bowed down slightly in places and sat on hills in others, so that they wouldn't sit flat.
I don't want to have to rip up my plywood or get involved in any more major repair work if I can avoid it.

1) I was told that I could just shim dry sheath/sheaf (whichever it's called - construction paper, more or less) strips underneath the planks where there are spaces in order to level it out. Do you advise against this? Will this create problems for the nails/cleats in any way? My house is under 12 feet wide, in the kitchen more like 9 1/2 feet wide by the way. I know you would probably never recommend this solution but can I do a decent job this way or am I just asking for trouble?

2) The 4 x 8 plywood sheets are butted together with no gaps in most areas. Is that a cause for concern? (There is a decent gap from the baseboards all around the perimeter of the room.)

3) If you really feel I must do it another way what's the quickest practical solution.

Hope you can help
Old 11-26-03, 10:27 AM
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The plywood should have a 1/8 inch gap around each sheet. It is very stable, but still changes dimension.
How big are the spaces? One method for older houses is to increase the nailing schedule and follow the larger curves and dips. Old houses are seldom flat and square.
The major fix is to strip the entire floor to the joists, sister each joist with a 2x4 to establish a new level surface, and then install a new sub-floor. A huge undertaking, and one that may give an odd look to an older house.

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