Floorboard thickness for big joist spacings

Does anyone know how to calculate suitable board thickness for joists at 1000mm centres? I have some stones with a roof in France I want to board out, and the joists are approx 220mm x 150mm at 1000mm centres. Currently the rotten boards look like waney edge oak boards, around

30mm thick, but too soft now to know if that was ever a good idea.

I'm assuming ply would be the best material, assuming eventually I'll want to lay some smarter looking regular floor boards on these.

Thanks.

Reply to
sheepish
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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.

Reply to
The Natural Philosopher

You could try

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which might help calculate any deflection.

Reply to
The Medway Handyman

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

600mm centres). It needs proper calculation to be certain - not my estimate.

But if your span is

Reply to
dom

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.

Reply to
dom

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.

9x6 is very large joists, so unless its a huge building those will be more than strong enough.

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

Reply to
meow2222

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.

Reply to
David H. Neumann

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

NT

Reply to
meow2222

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

Reply to
mjh-nospam

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.

Reply to
The Natural Philosopher

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

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Reply to
meow2222

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

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Reply to
meow2222

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

Reply to
The Natural Philosopher

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