In preparation for installing my own hardwood, I've been reading these forums and have learned a great deal from them. I have great respect for the knowledge base found here and that's why I'm asking for a little advice.

I'm installing 3/4" solid pre-finished rustic maple over a 5/8" plywood subfloor (not T&G) which rests on joists spaced 16" oc over a 3' crawlspace which has plastic liner for moisture/vapor protection. I have screwed down the subfloor to eliminate squeeks, which worked very well, however my problem now is the subfloor is VERY uneven. I plan to sand the high spots and joints as outlined in one of the articles on this site, but that only takes care of some of the imperfections with the subfloor. I also intend to apply an underlayment and that is where my questions arise.

1. It is clear to me through these forums that a 1/2" plywood underlayment is preferred, is 7/16" OSB acceptable? ($5 less/sheet)

2. Should I flatten the subfloor before putting down the underlayment?

3. If I flatten the subfloor BEFORE installing the underlayment, can I use something like Ardex, or should I still use felt?

I would greatly appreciate any advice or direction any of you could provide me. Thanks

Bryan

1) OSB is acceptable but the plywood is better and prefered.
2) Yes
3) I use roofing shingles to fill in the low spots (the inexpensive 3-tab ones)
Also use thin plywood and 30 lb asphalt roofing felt
Tips: Use a hammer tacker to hold down shingles and felt in low areas. After filling in valleys (low spots) install your plywood with 1&1/2" staples. Staple 3" around the perimeter and 4" to 6" in a grid in the field.

Bryan,

If, I understand you correctly you are adding an additional 7/16 sheet of plywood over the existing 5/8 plywood. If I read this correctly then you are building up a sub-floor that is a total of 17/16 or 1.06 inches. That should be plenty as you really only need to clear ¾ for a strong sub floor. If the existing floor is 5/8 then you could possibly get away with ¼ as that would translate to a 7/8 floor or .87 which is over .75 or 3/4



Is the under-layment the additional plywood that will be laid? If so you have two options flatten the existing plywood and lay the second layer over it or you can try laying the next layer with either a joint offset or in a 90 degree rotation from the existing floor. This will sometimes eliminate seam imperfections but if you have imperfections that are not related to the seams then you will have to address them before you lay the second layer.



You could use a leveling compound like Ardex but if you hit it with a screw it is going to crack. I would use felt or shingles to shim the area.

I read the 7/16 but skipped the OSB sorry about that I would spend the money and get plywood.

Answered all my questions. Thanks guys, you are awesome. I will use plywood for underlayment (but i'll still use OSB under the carpet areas) and I'll flatten the subfloor with shingles/felt combo prior to laying the plywood.

Only remaining questions are do I need to use a full 1/2" or could I use 3/8" or 1/4" as suggested? 1/2" would show less imperfections, but if I do a good job flattening the subfloor, the thinner options would work well. Second, what grade of plywood? CDX, sanded, etc?

Thanks again!

CDX