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Old 12-05-2010, 05:38 AM   #1
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how should i flatten subfloor for hardwood flooring?

first, hi there and thanks for all the info everyone on this forum has provided. this is my first post, but certainly not my first time to the site.

i will be installing 3/4" maple hardwood flooring in a bedroom addition and the hallway approaching the room. the house is a 1940s war box that has settled a bit and the addition room is a bread box shaped addition over a CMU foundation that is almost as old as the house. there were some major roof water drainage issues before we bought the house that we have addressed to hopefully minimize any additional settling.

anyway the floor is out of plane up to 3/4" in it's 12' x 18' area and there is up to a 3/16" variance between one floor joist (16" o.c. 2x10's) to the next in the most extreme case. there's no consistent trend except for where it slopes up about 1/4" to 3/8" in a couple feet by the doorway to meet floor elevation of the entry hallway and the rest of the house. the subfloor is 1x8 fir shiplap laid diagonal to the floor joists.

i am resolved to not replace the subflooring and mess with the joists as i think this should be workable as is, and our money and time budget just wont allow us to go that deep. i've already added one screw to each board at each floor joist to try and tighten things up and discourage squeaks. also i would like the new flooring to meet the same plane/elevation as the existing oak flooring of the same dimension in the rest of the house so i don't want to throw down another layer of ply subfloor. although, with the slight difference in the addition floor level i could see using 1/4" or 3/8" ply as potentially the best way to make the room plane with the hallway floor.

so my questions are what is the best way to plane things out? a neighbor contractor friend suggested simply cardboard shim strips (drywall butt strips) for the low spots and sanding the high spots. does this sound good to you experienced with this sort of thing? or what about some sort of leveling compound? if so what sort of product? and if i do 1/4" to 3/8" ply to bring up the bedroom, are there certain types of ply to avoid or seek out? and then again,what about the transition because there is a gradual slope not an immediate 1/4" drop.

thanks for any suggestions and for making it through my wordy post.


Last edited by cbendock; 12-05-2010 at 02:21 PM. Reason: clarify post as question rather than how to
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