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If every board is nailed down on one side of each board, then gaps would open between each board when they shrink. Dust and debris would fall into cracks and any liquid spills would get between the boards, dripping down into the "tongue and groove" seam, then down to the sub floor (or at least into the boards themselves). If every board is nailed on both sides, the boards would either crack or buckle when expanding or contracting, because they would not be allowed to move. The only way I can see to allow expansion and also have no gaps would be to glue every single board together into a massive room filling panel and then nail the panel only on one side of the room, allowing the entire panel to expand as a whole. But this is not how floors are done. I see many solid wood floors that are nailed everywhere, with no gaps, and seem to have no problems. How is this possible?

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    "I see many solid wood floors that are nailed everywhere, with no gaps" No apparent gaps doesn't mean no gaps at all. Even when the gapping appears to be consistently very tight (too tight to allow for any meaningful movement) the floor should not extend right up to solid, immovable surfaces at all edges. Instead it's typically shy of all walls and the skirting is installed on top which effectively hides the gap (which can be larger than you'd expect!) along with any unevenness, neatening up the appearance — which is exactly what skirting's role is. See excellent answer from @Caleb for more.
    – Graphus
    Jan 6 '21 at 18:07
  • "But this is not how floors are done." Well, actually, that is how many engineered snap-together flooring products work. The trick is to make sure that the flooring is installed with (I think) a 1/4 inch gap around the edges, and then install baseboards over the gap. Then the expansion and contraction of the floor is hidden by the thickness of the baseboards. Jan 17 '21 at 3:11
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I see many solid wood floors that are nailed everywhere, with no gaps, and seem to have no problems. How is this possible?

Solid wood floors do experience seasonal movement, but you may not notice it because many floors are made up of boards that are only 2 1/4" to 3" wide, so any shrinkage between summer and winter is distributed over many joints.

For example, a 2 1/2"-wide flat sawn oak board going from 12% to 7% moisture content shrinks by around 0.04", or just over 1/32". The difference is half that for quarter sawn boards, and since floors tend to have some flat sawn and some quarter sawn planks, the average value is probably around 0.03". Also, the one face of the board that's most exposed to the air is usually given a couple coats of polyurethane; that won't stop seasonal movement, but it may attenuate it somewhat.

The reason that wood floors fastened with nails, cleats, or staples is that those fasteners allow for seasonal movement. In contrast, engineered flooring is so stable that it's often glued in place.

If you look closely at a solid wood floor in the winter, you'll see that there definitely are gaps there. You may not find them at every joint, but you'll probably see a gap every few boards. Look at the same section again in the summer and those gaps will have tightened up.

Note: Please don't take the numbers in my example too literally. I chose the range 7-12% because that's about the range of seasonal moisture changes of the hardwood in my shop; the numbers may be different for you if you're in a different climate, and the particular facts of flooring (wood species, only one face exposed to the air, finish applied, etc.) would likely affect the numbers as well. The important point is that using narrow boards is one way to minimize the size of gaps by distributing the seasonal movement over many joints; many tiny gaps is often more desirable than a few larger ones.

So there is no way to avoid gaps in solid wood floor?

Seasonal movement is a fact of life. Relative humidity levels change with the seasons, solid wood expands and contracts with moisture content, and wood moisture content is affected by humidity. The main question is how to deal with that movement.

You could glue the floorboards together and float the entire floor, meaning that you don't attach it to the subfloor. That might work for a while, especially while the moisture content is increasing and the floor is expanding. But when the moisture content starts dropping, all those edge joints will be in tension as the floor is contracting, and chances are some of them will fail. At that point you'll get several large gaps instead of many tiny ones. Also, this method means that you have to allow for all the movement at the perimeter of the floor, so you need baseboard molding that provides sufficient space for the floor's movement.

You could put the floor together with tight joints and quickly seal the whole thing so thoroughly that no moisture gets in or out. But it's hard to prevent any change — even gymnasium floors, which typically have a lot more finish than residential floors, undergo seasonal movement. And most people who want a wood floor aren't thinking of something that looks like a basketball court.

You could carefully control the temperature and humidity in the room so that they stay constant and avoid the factors that cause seasonal movement in the first place. This really isn't a way to design the floor specifically, but rather a separate mitigation strategy.

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  • Thank you. 1/32 inch is a pretty large and visible gap. What prevents the gaps from filling with debris, and the boards from buckling up in the summer when they expand but are not able to because the cracks are full of debris? I don't see people vacuuming their wood floors to keep the cracks clean.
    – Xuan
    Jan 6 '21 at 22:59
  • @Xuan, 1/32" is not a large gap at ~5', which might be considered the typical minimum viewing distance for anyone who'd care about such things. Is your query exclusively about how modern solid-wood flooring installations work on a technical level, or broader, about how floors work more generally? Because gaps waaay wider than this (wider than a 1/16th) are commonly seen in old houses where the floorboards have permanently shrunk, and it was, still is, not uncommon to stuff those gaps with tow and sundry other fillings (principally to contain draughts BTW).
    – Graphus
    Jan 7 '21 at 10:08
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    @Xuan I would think that a regular schedule of dusting and vaccuming would take care of most of the minor dust and debris that gets into the floor gaps. Any which isn't vaccumed out is simply compressed when the floor expands. As far as drips, most people* wipe those up before they seep all the way past the tongue and become unreachable. (*Except the 3 year-old and under set, they rarely clean up after themselves.)
    – FreeMan
    Jan 7 '21 at 18:24
  • So, the answer is that my observations are incorrect and all solid wood floors have gaps? @Graphus He said over 1/32" or 0.04" between each board, not every 5'. That means every single board in the floor will have a gap between it large enough to fit 10 pieces of paper in.
    – Xuan
    Jan 7 '21 at 21:13
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    @Xuan, "@Graphus He said over 1/32" or 0.04" between each board, not every 5'." You misunderstood, I said a 32nd is not a large gap at a viewing distance of 5', not that such a gap would only occur every 5' or so. We're getting beyond the scope of what the Comments system here is designed for — if we go much further the system will automatically flag things and say "Comments are not for extended discussion" — so I'll only respond to one other thing, "So there is no way to avoid gaps in solid wood floor? " Think parquet flooring and you have one answer.
    – Graphus
    Jan 8 '21 at 10:56

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