floor joist spacing

I wish to extend a floor 1.8m to an existing wall. I'm confined to using 4 x 2 joists which I'm assuming would be a bit light if used at standard spacing so what spacing would I need to use, or what are the alternatives ? The critical dimension is the 4 inches Standard grade flooring chipboard will be placed on it.
Paul Mc Cann
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See building regulations Approved Document A for tables of joist spans, sizes and spacings. It is available by searching on http://www.odpm.gov.uk.
http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/documents/page/odpm_breg_600457.pdf
As a typical example, a 97mm x 47mm joist with 400mm spacing will span 1.92m, securely supported at both ends with 0.25 - 0.5 kN/m2 dead load.
Christian.
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Paul Mc Cann wrote:

Use your 4x2 but incorporate a sleeper wall and wall plate along the span for extra support.
J.B.
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Author-Address-Header@[127.1] says...

Forgive my ignorance but what do you mean by a sleeper wall and wall plate ?
--
Paul Mc Cann

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He is assuming that it is a ground floor, so you can just lie the thin joists on sleeper walls, which are masonry walls with lots of holes in for ventilation/damp resistance built for the express purpose of supporting joists that would be too shallow. If it isn't a ground floor, then it won't work.
Christian.
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@nospam.yahooxxxx.co.uk says...

Thats what I thought he menat but my mind was running along the lines of the actual problem which is not at ground floor level. I'm actually trying to extend a steel framed mezzanine floor which already uses 4x2s across the steel horizontal beams.
A prevous poster kindly pointed me towards load calculation tables and a quick google (which I suppose I hould have done first) threw up a few more such tables. They all suggest that 4x2 will span 1.8m @ 400mm centres but it looks awfully light to me when viewed in situ.
I think I'll just space the 4x2s a little closer, I have plenty off them anyway.
--
Paul Mc Cann

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...

If the floor ends up too bouncy, screw a length of steel strip to the side of some of the joists.
Colin Bignell
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says...

for
won't
=================I've just replaced a downstairs suspended floor (house is about 60 years old). The joists are 4" x 2" at 16" centres (virtually the same as your proposed metric figures). The sleeper wall is 6 feet from the outer wall so the joist span is also 6' . Since this was apparently the standard 60 years ago and there has been no failure in my joists I would think that your dimensions / lengths are perfectly acceptable. I would suggest that you use galvanised 'joist hangers' on the outer wall and overlap the inner ends of your new joists with the old. Use either coach bolts or coach screws to secure the overlaps.
If your new extended floor is intended to take unusually heavy weights then it may be necessary to use heavier joists but for domestic use your proposed sizes should be perfectly safe.
Cic.
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wrote:

A sleeper wall is built under the floor, usually with some spaces in the bond to allow air through. You then put a DPC on the top and a wall plate (i.e. piece of timber) on top of that. Finally, the floor joists run perpendicular and on top of the plate.
However, when you say that you are confined to 100x50mm joists does it mean that you are short on height?
.andy
To email, substitute .nospam with .gl
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floor 1.8m to an existing wall. I'm confined

Have you considered using some steel joists ? Not all would be needed to be so. But you'll need an expert to calculate how many.
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Paul Mc Cann wrote:

Mailly ventilation, to keep a cross-flow going via the air-bricks through the underfloor void. The wall plate is a piece of 4x3 or even 4x2 laid on top of the sleeper wall on a DPC of slate, felt or whatever, that the floor joists bear on.

Ooops! The highest dwarf wall in existance, perhaps?!

Can't you put in a new piece of steel to match existing supporting the floor?

If you've 4x2 over an RSJ (or whatever) could you use (say) 8x2 notched at one end to fix over the steelwork and give a surface under to finish?
A photo is worth many words...
J.B.
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