On 22/05/2010 14:24, Tim Watts wrote:
Hi,

Bit of thinking in advance...

Question - what sort of design parameters should I be looking for in
terms of max point loading and acceptable deflections for square edged
floorboards upstairs?


Background:

I have with me a sample of some redwood floorboard (planed square edge
by design for easy lifting, no T+G thankyou!)

It is 144mm x 21mm finished dimensions (nominal 6x1"). I have a joist
spacing (open span lengths, not centre to centre) of 400mm, 450mm and
one of 500mm.

The Sagulator http://www.woodbin.com/calcs/sagulator.htm

suggests young redwood with a mid span point load of 130kg (yes I know,
cut down on the pork pies) gives a deflection of:

Span Deflection

500mm = 3.3mm
450mm = 2.4mm
400mm = 1.7mm

Which seems to concur roughly with me standing on the sample plank.

A Sofa or bed with 3 fat people will load 4 points or more so that's no
worse than me on one foot. No-one is putting a piano up there (unless
they are a masochist - wouldn't go round the stairs anyway).

And the 500mm span is a single exception, I could close that down to 450
by screwing a 50x50 batten on the side of one of the joists.

Couple of mm bounce seems OK to me - do folk agree?

I could go to the next thickness up whatever that is, or I could go to
8" width, but Alsfords told me the price starts going a bit exponential
much after 6" wide. At the mo, I'm looking at 500 quid inc to have
around 25m2 of new floorboards which seems OK.


Cheers

Tim


Can't give any absolute answers, but the main problem with excessive
deflection is that it cracks the ceilings below. In your situation, most
of the deflection will come from the boards themselves without bending
the joists very much - so may not matter, apart from feeling a bit springy.

I would seriously suggest using T&G though - which will spread the load
between boards and considerably reduce the deflection due to point loads
- and will probably reduce the joist deflections as well. OK, they're
then harder to lift - but you should be able to anticipate which ones
you're most likely to need to lift (areas with plumbing and electrical
connections underneath) and make little "trap doors" (with the bit under
the groove removed, and screwed down rather than nailed) at the time of
laying.
--
Cheers,
Roger
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