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?
Best Answer
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.