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The conventional wisdom is that hardwood floors need a 1/4-3/8" gap to the wall (or bottom plate) around the perimeter, hidden under the baseboard. Ostensibly this is for thermal and/or humidity expansion.

For a floating floor, I get needing a gap, it may move a little even just by being walked on.

But for a floor glued or nailed down to a subfloor, what's the point? Your subfloor is subject to the same environmental effects, so it's going to expand too, isn't it? I mean, an OSB subfloor may not expand at exactly the same rate as hardwood, but it should be close—it's only 5% "not-wood."

Thoughts?

Rohit Gupta
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Huesmann
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3 Answers3

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When an irresistible force meets an immovable object, in this case you get floor buckling. It's been observed in practice countless times.

Feel free to repeat the mistakes of others, or learn from them, as you like.

If it's the baseboard you don't want, you can hide the gap under drywall or plaster, with a little more finishing effort, whether that be time or money.

What actually happens with a nailed-down floor is that everything shifts a bit (nails have limits) when it first swells, and then as it shrinks tiny gaps open up (one reason that narrow boards are more commonly used than wide boards - the gaps between are smaller and less noticeable for narrow boards) and after that the gaps between boards change size with the seasons.

Ecnerwal
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Humidity is a much bigger factor than temperature in wood expansion. And most of the humidity-related expansion is across the grain, not along the grain.

The floor and subfloor are subject to the same environment, but no, you can't assume that they'll react the same. If your subfloor is solid boards (common in old houses), then the flooring is likely perpendicular to those, so it'll expand in a different direction than the subfloor. If the subfloor is OSB or plywood, then it won't expand much at all. Also, different types of wood expand at different rates.

Mike Baranczak
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Good question.

Oak has a coefficient of linear expansion along the grain of 0.0000027, which is per deg F. for a 20 ft run of oak that equates to .000054 ft per deg F, or 0.0011 ft (0.013 in) for a 20 deg F change in temperature. So yeah, I agree, that the 1/4" to 3/8" gap recommendation around the floor seems like it's about 10X bigger than it needs to be.

I would like to see the rationale, with some analysis to back it up, for the 1/4" -3/8" gap recommendation.

SteveSh
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