Hello all,

I need to put a secondary floor in my shed, the existing one is OSB but is sagging between the supports (no cross bracing helpfully) as the previous owner didn’t keep it particularly water tight (New roof and felt has cured that).

I was planning on putting a new floor layer in over the existing one and wondered how thick I should go, common sizes are 9mm, 11mm and 18mm?

Yes, I should really rip it to bits and remove the existing base, but I really can’t be bothered.

18mm

If the old one is sagging due to being a bit shagged, it would be best to get it out, especially if it’s damp. Put some builders paper down and put a new 18mm (minimum) in.

18mm. Everything else just sags.

OSB will always have a tendency to sag imo, if you want to lay something on top I would go for 15 or 18mm moisture-resistant chipboard T&G flooring.

And in 2400mm X 600mm it’s probably easier to lay than sheets of 1220mm X 2440mm OSB too.

OSB is entirely the wrong thing to use for that. Get some 18mm shuttering ply or some chipboard. The strands in OSB are too big and don’t pack densely enough to provide support.
Take the existing out and put some cross members in if you can. If you absolutely have to leave the old in stick a damp proof membrane inbetween old and new to stop the damp from coming up.

Are you doing this under the shed?

http://www.unilad.co.uk/video/guy-spends-9-years-digging-out-basement-with-remote-controlled-diggers/

moisture-resistant chipboard T&G flooring

Interesting, I would actually be able to get that in the car aswell, not to mention it’s a cheaper than OSB and plywood in particular.

DPM underneath it is a good shout.

Would it be worth putting in a proper cross member’d set of “joists” too?

The problem with a DPM is that if you ever spill something it will take forever to dry out. And anything below the DPM, such as the timbers supporting the flooring, will rot if there is a damp problem. You would be better off ensuring that the roof doesn’t leak and that air can freely flow under the flooring/timbers.

you should really redo the base

get new cross members on it and fit your new floor

then place it on top of treated bearers with dpc between bearers and cross members – a good breeze, no way to draw water, and if water gets in it’ll drain away