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Background

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).

Question

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
@ChrisF Hehe, my DIY knowledge is very basic! If MDF is a poor choice, I'm keen to understand why - along with suggested alternatives if they make sense. I guess my question still stands regardless of material - or perhaps its material dependent? Thanks for taking the time to read and respond so far! Really appreciated. – InvertedAcceleration Aug 6 '11 at 12:51
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
@Mike Thanks again for more info. – InvertedAcceleration Aug 7 '11 at 7:31
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1 Answer

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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.

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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
@BMitch, it is, it's just that in the UK "Tongued and Grooved Chipboard Flooring" is the standard cheap flooring material. OSB tends to be used as a sheathing material in the UK eg Cladding the outside of a timber frame structure, then covering the OSB with a suitable exterior finishing material (pretty much like what is done here in the US from what I know of building practises here). – Mike Perry Aug 7 '11 at 2:37
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