I'm replacing the rotten floorboards upstairs. The joists are approx 2 feet (60 cm) apart and the new floorboards are
2 cm thick. Are the joists too wide apart? Would the floorboards flex too much with that spacing?In principle I could fasten some extra joists between the existing ones to take the weight. Is this worth doing? Any other suggestions please?
The area will become a bedroom and bathroom (with shower only, no heavy bath).
Run Superbeam (my copy is on my other computer). It will tell you all /sorts/ of interesting numbers. (If I were you, I would assume that the bed legs will be a point load exactly in the middle of a single floorboard.)
Somebody should be along shortly with the Building Regs answer (even if they don't apply to you).
How thick were the original floorboards? Were they the tongue and groove type? Are the new ones? What section are the joists, and what is the span? Are you planning to support heavier loads than were supported by the old floor?
I think you need to answer these question before anyone can comment sensibly.
The original floorboards were tongue and groove as are the new ones. The joists that will support them are only 3 inches cross section but supported immediately below by strong joists going the opposite direction at 12 inch spacing. The span of the upper set of joists is quite small at only around 10 feet, but as I say they are very well supported. The old floorboards did not support anything as far as I'm aware, it was treated as an uninhabited space. The new floorboards will be directly inside a bathroom so will have to support a shower cubicle, toilet, wash basin and small cupboard only.
As it stands the new (2 cm thick) tongue and groove floorboards would be supported on the upper joists at intervals of approx 2 feet.
I think you will be fine mate.
I think in that case that I would generally go with the 24" spacing, but put intermediate supports in just the areas where the highest loads will be.
24" spacing is a *bit* wide for joists, but using 20mm T&G boards, any point loads will be spread across several boards - and 20mm is thicker that your average modern floorboard (in UK)anyway.
Dear David You are over the limit at 24" but with T and G well laid I agree with the others that you will be unlikely to have a problem. It all depends on how stiff you want the final surface. Personally, if it were mine and all I had to do was to put some 3" timbers in between over the main beams to get down to 12" centres then I would do that. As the area is to be a bathroom and shower I would not use any form of chip board T&G - "water proof" or not! I would use appropriate ply especially under the shower and bearing in mind the tendencies of showers to leak if the substrate can move, make the floor stiff and the wall surfaces finish (also ply) INSIDE the shower tray Chris Chris
Building regulations recommend 22mm with 18mm as a minimum IIRC.
M.
The message from David in Normandy contains these words:
FWIW my 1965 Builders and Architects diary (kept for all the useful information it contains) gives 3/4" t & g at 16" centres, 7/8" at 18" and 1" at 21" so your floor boards are more than a little light by those standards. But your floor is only going to be lightly loaded so I don't think you should have any problems. Just think what sort of heavy loads a domestic floor has to carry in some circumstances.
If your bathroom had been going to contain a bath I would have suggested an additional plank under each foot just to span from one joist to the next if any of the feet were going to inconsiderately plant themselves anywhere close to midway between joists.
Thanks. Your figures confirm my gut feeling that some of the spacing was too wide to be really solid.
I bit the bullet today and put some extra joists in between the widest spaced joists. The original joists must have been put in before tape measures or rulers were invented as none of them were consistently spaced, varying from around
19" to 28" apart.I also put some noggins between the joists where the toilet is intended to go - at least it should be possible to screw it down tightly through the floorboards and into the noggins below. I don't want a wobbly loo.
You've not given enough info here or later. One would need all 3 dimensions of each layer of the structure to know the result. However inadequate strength is most unlikely, bounce is the issue that can crop up. FWIW 2x4 joists at 16" spanning over 10ft with half inch floor boards are plenty strong enough for anything but baths, even though theyre far below what BR now insists on. BR sizes are primarily dictated by noise transmission, not strength, bounce or plaster cracking.
NT
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