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I am planning to install prefinished solid 3 1/4" wide by 3/4" thick ash hardwood in my home using 2" cleats. I live in Canada, so the average indoor humidity difference between winter and summer is quite high, maybe 50%. I don't use air-con in the summer. It is early December now, so I expect each board to expand in width by approximately .01" between now and summer.

Is there any reason why I should not put small gaps of .01" between each board (using a removable shim) as I nail them down? I still plan to leave a 1/2" gap around the entire room, just in case.

I ask this because I don't understand how a tightly-fitted floor is supposed to expand at all. What happens to the nails around the periphery if the whole floor moves by a good fraction of an inch? Are they simply ripped or bent out of place? By 1/2 an inch?? Or does the 45 degree angle of the nails force each board (and progressively the whole floor) to rise up by the same amount as the expansion? And when they shrink back down what pulls them back into place? People walking on the floor?

In removing my old maple floors I saw no such evidence of major movement. So I ask, quite literally, what gives?

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    If you have that much humidity difference, you really should be using AC/dehumidifier. The cost of operating it more than pays for itself in preventing loss of value of your home due to moisture. – R.. GitHub STOP HELPING ICE Dec 8 '19 at 2:52
  • Call the manufacturer of the flooring, and ask them what they recommend. It's possible they might want a larger expansion gap around the perimeter, or recommend a product line that's more stable. I suspect that if you install it as you're asking about, it would void any warranty they might offer on the product. You could also go to a flooring store in your area, and ask for advice there, as others in your area would have the same issues – Joe Dec 8 '19 at 19:20
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Do NOT install expansion spaces between each board. You want a very tight fit between each board and then leave an expansion space around the perimeter of each room.

First you’ll need to acclimate the wood to your house. This may vary by location and season, but generally the manufacturers recommend about 10 days.

Second, seal the floor (again, according to the manufacturer’s recommendation).

Third, lay the “sleepers” out according to the size of your flooring and the allowable span. Sleepers are NOT fastened to the floor. They are designed to “float” or slide depending on expansion of the flooring.

Next lay out the flooring (loosely). This will give you an idea of color, etc., but will also help acclimate the finish flooring. Gaps at the perimeter is based on the size of the room. When we do gymnasiums, we leave a 2” gap and cover with an appropriate base. In homes, we generally leave a 1” gap.

See MFMA recommendations for acclimation and installation: https://www.maplefloor.org/ForArchitects/Installation.aspx

Btw, to test your finish flooring, hold a piece vertically and insert another piece into the T&G edge. If it fits tight, it’s okay. If not, you’ll have problems with the T&G edges when you walk on them. They’ll snap and pop and drive you nuts. If it’s not tight (and supports the other piece of wood when held vertically,) then I’d send it back. (See MFMA guidelines.) https://www.maplefloor.org/Home.aspx

Btw, yes, I know it’s the Maple FlooringManufacturing Association, but is consistent with ash, etc. too.

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    1" gap from what? If it's from the framing that should be okay, but I'd think you'd want to aim for less. – R.. GitHub STOP HELPING ICE Dec 8 '19 at 2:49
  • There are no sleepers required here - it is the main floor of a bungalo with a full basement underneath. Built in 1956, the subfloor is 1 x 4 pine T&G laid at 45 degrees over the joists spaced at 16". I have added an extra layer of 1/2 " plywood to stabilize the subfloor and to isolate it from the new hardwood. The old 3/4" by 2 1/4" maple hardwood (63 years old) was very squeaky and had shrunk, leaving gaps, some as large as 3/16". In gymnasiums they leave 1/8" expansion gaps every 8' or so. So, why not leave a tiny gap at every board? It's only .01" and is effectively invisible. – jLegris Dec 8 '19 at 3:54
  • @jLegris : because sand, dirt, water, and other things get down into those gaps. A larger expansion gap you can pull out the filler material and clean it if needed. (although you'd probably want to replace the filler if you did this, as it's likely be damaged as it was removed) – Joe Dec 8 '19 at 19:15
  • @Joe: But don' t small gaps on the order of .01" open up in any case? If you install during the humid season they'll open during the dry season. If you install during the dry season, the wood will expand during the humid season, shifting the whole floor a little and then it will shrink down during the dry season, leaving small gaps, again roughly .01" wide. Same difference. As I see it, by anticipating the gaps that are sure to appear sooner or later, each board has room to expand and contract independently, which is better than yanking their nails out during the first expansion. – jLegris Dec 11 '19 at 20:39
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Wood gives, and takes....

In my opinion, wood will compress when needed. Picture a hammer striking a board and leaving a mark, it compresses. This is quite the exaggeration, but for a point. The wood in the center of your floor, held in place by all the other nails surrounding it will compress. Although I do not know how much area is "held tight" by this theory, but it will negate the need to consider how much of the floor will actually move.

If you leave the standard 1/2" from the drywall so a 3/4" base will cover it, that will suffice very nicely. If you have a gap at the drywall/plaster to allow the floor to have more room to move, you can use the exposed plate under the edge of the drywall to start your gap, therefore you can use a thinner base without resorting to shoe mold.

Another point to consider is the "cut"or "saw" of the flooring. Quarter sawn, rift cut and plain sliced, factors in on the movement of the floor. That topic alone can be looked up as to how it affects wood movement. The terms I used may be different in different regions, but the nature of the wood when cut is the same.

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    Regarding compression... yes and no. Years ago I put a 3/4" solid hickory ceiling up in a large commercial office--maybe 30x40 feet. A few months later it was buckling badly due to introduced moisture, almost to the point of falling down. We pulled it all down, reinstalled it, and ripped nearly an inch off the last board so it would fit. Floors will do the same thing in many cases before they'll actually compress. – isherwood Dec 7 '19 at 19:31
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    Also, only the largest luxury hardwood base trim is 3/4" around here. Most is between 7/16" and 5/8", so base shoe may be necessary. – isherwood Dec 7 '19 at 19:33
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    @isherwood I take it you checked the moisture content, and the surrounding areas to be sure it was acclimated before install? Sounds like the wood was too dry for the area it was installed in? When the difference is too great between the substrate or location, compared to the flooring or wood used, this will happen. (Not meant to lecture, but for the OP's information)... On the 3/4" base versus what may be used, that is why I mentioned the gap at the drywall. I should have suggested to create a gap if there was none there. IF shoe mold was not desired. – Jack Dec 7 '19 at 19:48
  • "Introduced moisture..." Sometimes testing doesn't reflect outcome. – isherwood Dec 9 '19 at 13:49

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