Flattening Subfloor


Old 10-24-08, 11:44 AM
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Join Date: Sep 2008
Posts: 37
Flattening Subfloor

Hello all, I have posted many a question related to my hardwood floor laying project but here is another. I have 3/4" plywood subfloor to which I will be laying another 3/4" layer plywood before installing the hardwood floor. As with flooring, the subfloor needs to be flat, not neccessarily level, to 1/8" over 6 ft. When I install the second layer of plywood, is this when I should flatten my floor? For instance, lay down the floor and check flatness, add shims, then attach to main subfloor with screws. Or should I wait until after I attach the second layer before flattening?

If I should flatten the subfloor as I am laying it, how should I add shims under a 4x8" sheet of plywood?

Also, how should I go about measuring flatness? I understand using a straight edge and making sure that the low spots are not more than 1/8". But then do I slide the straight edge half its length, and flatten then, and continue across the room. One thought I had was to create a 6ft x 6ft square and flatten, move the square over 3ft, reflatten, essentially flattening the two boxes to one another. Better yet, how do the pros measure the flatness across the room?

Hope I didn't ramble too much and provided enough information,

Last edited by tideman76; 10-24-08 at 01:30 PM. Reason: added information and another question
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Old 10-24-08, 09:19 PM
Join Date: Nov 2000
Location: Austin, TX
Posts: 5,074
Use at least an 8 foot long straight edge. Find the highs where the straight edge teeter-totters. Grind/sand/plane those down.

Now, find the lows. Using the straight edge, fill under the straight edge with layers of roofing felt or even shingles if it is that deep. Build pyrimids, so to speak.

Then lay your second layer of plywood.
Old 10-27-08, 07:46 AM
Thread Starter
Join Date: Sep 2008
Posts: 37
Thank you for you help answering several of my questions through different posts. WIll post back when I run into problems.


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