I am a complete beginner and need to lay a concrete slab for my new cabin (about 18 ft by 16 ft in size). I'm planning to lay a slab 6in thick.

I'm wondering if there's any particular type of wood I should use to make the form for a concrete slab?

I'm in the UK so constrained by whats commonly available here.

  • 2
    Almost anything that's stout enough. I've used ripped hardboard siding for curved sections, and doubled 1/4" underlayment for tightly radiused corners. It just needs to stand up to some moisture for a day.
    – isherwood
    Mar 17, 2021 at 16:49
  • Will it have a footing as in concrete going in the ground so when the ground freezes it will not lift the slab/cabin? Will this be a living space?
    – Jack
    Mar 18, 2021 at 4:06
  • @Jack It'll be a workshop and storage shed. I'm not sure about the freezing/lifting part - it hasn't come up in anything i've read up on. The plan is to put about 8" of masonry rubble below the concrete so there's a stable base and then I thought a moat of gravel to break up the concretey look all around the structure. Does that count as being in the ground?
    – immutabl
    Mar 23, 2021 at 15:13
  • I do not know how bad the winters are where you are, but typically the slab has enough of something going deeper in the ground, whether it be more concrete along with block work, and yes gravel, can work too. If the frost line is shallow enough let gravel do the job, for a shed..... If it were a living space it would definitely need to be more substantial. If it were my work shop, at that size I would do a little more than gravel unless you do not experience severe freezing temperatures. Do you get many tornadoes out your way? That would be a reason to go deeper.
    – Jack
    Mar 24, 2021 at 2:27

1 Answer 1


Normally, you'd want to use any type of straight 2"x 8" planks. Why 8" instead of 6"? Because the 6" is really about 5.5" and if plans call for 6" slab, the 6" planks would be too small. You'll have to trench a little to get the planks down to 6" above grade. You'll want to get some rebar and drive it in every 3 feet or so to strengthen the frame. You'll also need steel mesh or rebar under the concrete for strength and to prevent cracking. Depending on who's doing the work, you might want to break it down to smaller pours instead of one large one which would be about 5.3 cu yards.

Depending on what's available in your area, you might be able to rent the forms from a large home store.

  • 2
    It's common for forms to be slightly less than the height of the slab. 2x4 lumber (3-1/2" high) is used all the time for 4" slabs. You'd just need to backfill a bit of soil against them (which is a good idea anyway, for stability).
    – isherwood
    Mar 17, 2021 at 16:48
  • 2
    Use many support stakes behind the form as concrete is about 3 X more dense than water and will put a lot of pressure on the form while it is liquid. Mar 17, 2021 at 18:34

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