Floorboard thickness for big joist spacings

Calculate? that's a bit 'advanced' innit?

Trouble is, what are your criteria? how much give do you think is acceptable.

In all likelihood the joist themselves are out of spec anyway. Consider just using normal flooring but adding more joists and herringbone bracing.

You could try

You don't say what the span is on the joists.

If we assume joists of 220x150mm at 1m centres are equivalent to joists of 220x50mm at 330mm centres in loadbearing ability - that would be a typical "design from tables" figures for a 4m span (at

But if your span is

That may be the simplest solution - add 50x220mm joists in between every pair of original joists, taking the centres to 500mm, then board out in 19mm t&g.

Once youve calculated you can get 1 piece and check it does as expected. Ply is indeed the toughest option for 2 reasons:

- not only is it the toughest of the wood products

- but also its sheet format means all loads are spread over a wide area of wood, unlike traditional boards.

I wouldnt want to use chip. Nor 1" boards.

Ply is not cheap, there are 2 optoins to reduce costs if needed.

1. Use OSB, whish is somewhat like ply but cheaper
2. Run some extra smaller joists at 90 deg to the existing ones, positioning them so each ply sheet is supported on all 4 sides. Then you could use half inch ply. I'd only do that if present joists arent worth looking at and will be covered.

NT

sheepish: The design requirement is stiffness driven. In your specific scenario, we can multiply the required design concentrated live load, P, by 1.206 to account for the required design dead load. The typical deflection requirement for floors is delta = L/360. Typical plywood modulus of elasticity is E = 8000 MPa. Assuming no piece of your plywood has a width less than b = 900 mm, then the required plywood thickness would be t = [108.5*P*(L^2)/(E*b)]^0.333 = {108.5(1335 N)[(1000 mm)^2]/[(8000 MPa)(900 mm)]}^0.333 = 27.1 mm.

That would be right for a conversion in UK. L/360 is an excessively tight requirement, and French regs may be very different.

NT

Can you run through the notation in this calc please. Esp what I do with L in the calc (or L/360) and what "^" signifies. My maths is v rusty!!

Thanks

Mike

L=length..in this case the unsupported span between joists.

the ^ is the exponent..so l^2 is understood to be length, squared.

I think this calc is a wee bit conservative, as it does not take into account the fact that the ply is not freely supported over the joists..it is in fact nailed across several.

Also, I am not sure that ply is stiffer than chipboard. Its stronger, but is it stiffer?

with the requirements to add boards over, I might be tempted to compare cots of the recommended 1" ply with say two layers of flooring grade chip laid over one another and glued..or even - if the boards are of any real thickness (not laminate) just one layer of 19mm chip.

very much. And OSB is a fairly similar product at lower cost.

very much. And OSB is a fairly similar product at lower cost.

Well I didn't see any stiffness figures quite there at all.