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I'd like to understand why wood movement in a floor necessitates large expansion gaps around the outside of the room. I'm not asking why wood as a material undergoes changes due to moisture - I'm interested in how the floor system as a whole actually behaves once installed.

Take for example common wood-strip flooring such as 3/4 oak. Here's a diagram showing a typical recommended installation along a wall parallel to the floorboards:

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How is it possible that the entire floor could ever move by approx 3/4" at the walls without being totally destroyed? Nails would be torn out, boards split, etc. if the floor moved that much, no? So is the gap really useful?

Thanks!

(What I would have thought was: given the way boards like this are nailed, the tongues would remain in place relative to the floor, the grooves would slide around, and the floor as a whole would not move at all. Gaps would just open and close between individual strips.)

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  • Floating floors move as a whole system, if boards throughout are nailed down then the floor can't move as a single unit (which can lead to a host of problems if the ideal range of in-service conditions are exceeded).
    – Graphus
    Commented Nov 25, 2018 at 19:17

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You're correct that the 3/4" gap isn't going to be entirely closed and then open again seasonally.

Part of that big a gap is convenience... you can have the baseboard installed and still pull a floorboard out. It's easier to lay a floor with loose gaps. (And given the wows and flutters in long walls, the gap will be wide in places and narrower in others and you can still have a straight run.)

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