I am starting my first serious DIY project - converting our garage into a playroom for the kids. The garage floor is concrete.

My plan is to run timber (Sawn Kiln Dried Timber (L)3000 x (W)75 x (T)47mm - B&Q) joists across the concrete floor. Then put down an MDF floor, put down carpet mat and carpet. I am going to fill the gaps between the joists with floor insulation (Jablite Flooring Polyboard).


I don't want the floor to sag between joists, but I don't want to go overkill with the number of joists.

What factors should I take into account when working out how many joists to put down? In other words how much of a gap should I leave between joists?

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    Not sure MDF is a suitable material for flooring - will have to do some research to back that up though. – ChrisF Aug 6 '11 at 11:39
  • Thos sticks are about 2" x 3", FYI. – Jay Bazuzi Aug 6 '11 at 16:54
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    @InvertedAcceleration, MDF isn't strong enough to be used a flooring material (it wasn't designed for that sort of use), also "standard" MDF swells terribly if it comes into contact with moisture... – Mike Perry Aug 7 '11 at 2:17
  • @InvertedAcceleration, no worries. Question for you, how are you going to deal with the increased floor height in the garage ie Door threshold areas? – Mike Perry Aug 7 '11 at 15:09

The standard flooring material is OSB, not MDF. MDF doesn't have enough strength to support a floor load. I can't remember if it's 3/4" or 5/8" OSB, so someone will hopefully chime in with that spec.

Typical spacing for joists is 19.2" (just under 3/8ths, there's usually a black diamond on tape measures for this point). It corresponds to 5 joists per 8' span.

Note that if the joists will lay directly on concrete, you need pressure treated to prevent moisture damage. And when nailing into pressure treated wood, you need galvanized nails to avoid a chemical reaction that would eat through the nail.

  • Sorry for all the US measurements, us Americans don't know how to play well with the rest of the world. – BMitch Aug 6 '11 at 15:36
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    A waterproofing layer on the concrete could allow untreated wood. tar paper, shingles, plastic. – Jay Bazuzi Aug 6 '11 at 16:53
  • In the UK & Ireland, Tongued and Grooved Chipboard Flooring is the "standard" cheap flooring material used in domestic situations. Also "standard" practise in the UK would be to put DPC between the concrete floor & joists (@ 400mm centres). – Mike Perry Aug 7 '11 at 2:13
  • @Mike, that looks pretty similar to OSB. The O stands for oriented, I believe because they alter the direction of the chips between layers. And the edges for flooring are TnG, as you have there. – BMitch Aug 7 '11 at 2:25
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    @Mike, thanks for the international lesson. Part of the fun of this site is learning something new every day. – BMitch Aug 7 '11 at 2:47

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