I have a lot of tongue and groove, reversible shiplap / v-groove boards from a big box store. I would like to use them for a plank ceiling that is 25x8ft. It is unclear to me how these boards allow for expansion since they sit flush together with seemingly no expansion gap on the V-groove side (see image). If I use the shiplap side down, do I need to rip the tongue to allow for expansion?

I plan to treat the boards before installation. I live in an area that stay low humidity year round.

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1 Answer 1


Barring very unusual circumstances, they are more likely to shrink from the "as installed" state than expand from it.

If they do expand, the small and sloped contact area works in your favor.

  • All true, they should set them tight. That is what the lap is all about.
    – Jack
    Apr 12, 2021 at 13:10
  • 2
    to clarify the point, unless your lumber is marked "KD" or "Kiln Dried" it's probably still quite wet inside (not that you can tell by feeling it - it will feel dry). As it sits, more and more moisture will escape (you'll never see it), and the wood will shrink not expand. Over a period of weeks to months, it will reach an equilibrium point where it's as dry as it can get. It will then expand (from it's shrunken state) in the humid summer and shrink (as much as it can) in the dry winter.
    – FreeMan
    Apr 12, 2021 at 14:49

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