How flat should floor be before hardwood installation?

Old 02-01-07, 11:41 AM
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How flat should floor be before hardwood installation?

I am installing 3/4 solid oak hardwoods in my living room/dining room area. This house is from 1930 and frankly, the 3/4 inch thick, 5 inch wide, tongue and groove subfloor has all sorts of valleys and peaks in it (the valleys are more typically from the sagging in the middle old floor joists). Nothing is over 1/4 inch but if you put a 8 foot straightedge anywhere on the floor you can see spots where the floor has sagged. I am putting on 1/2 inch of CDX plywood over the original subfloor, and will be using felt paper/ roof shingles to level the floor out, but the fact remains that I doubt I will get the floor perfectly flat. My question is- how much deviation from flat is acceptable? How much will the 3/4 hardwoods "conform" to an uneven surface?

I started laying out shingles and felt paper to try to attempt to start leveling out and it looks like 70% of the floor is going to be covered in them before I put the 1/2 CDX underlayment on.

Old 02-01-07, 11:44 AM
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Pull a tight string line, across the floor in every place imagineable.

This will help you with the shimming placement.
Old 02-01-07, 11:57 AM
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You want this to be as flat as possible - sweat the details and you'll be happy with the finished product.
Old 02-04-07, 12:55 AM
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I had a similar problem with my living room; though the cause was simply uneven joist installation, it was fairly clear that the builder had engineered the floor for carpet and linoleum.

In my case, I ripped up all of the existing subfloor to get at the joists directly. I attached strings to fasteners (nails, drawing pins) at each corner of the room (several corners due to being an irregular shape) and drew the string tight to form a grid of strings.

Provided the strings are tight, you will get a pattern that is within the thickness of the string in terms of flatness, across the entire floor.
I routed out channels through some joists where the string was sitting on top, such that the string could float and indicate the correct height for the joist. Then it was a matter of drawing a string along each joist, one by one, and using this as a guide for either planing (electric, don't even try with a hand planer, it's impossible) or shimming with strips of 2x4 ripped with a tablesaw. In general I shimmed any joist that needed to be raised by 1/8" or more with a ripped 2x4, and fine tuned with trimmed roofing shingles + felt.

Once this was done, I glued and screwed (every 6") replacement subfloor (5/8" CDX T&G in my case, for 16" O.C. joists) and I had a re-engineered subfloor that is as flat as it gets, with no detectable rises/dips over the length of my level (8ft). The subfloor even before covering with 3/4" hardwood was as solid as a rock to walk on, with the hardwood on top, the floor feels like one solid piece of wood. The subfloor prep work was *very* time consuming as I could only really work on it during weekends (I think it took me around 6 months of picking at it to do an 800 sqft area wall to wall, albeit with many rectangles), but worth it. You'll never have creaks/pops or a wobbly feeling underfoot (assuming you also acclimate properly and all that).

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