[Tim Watts]

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

--
Tim Watts

Hung parliament? Rather have a hanged parliament.

[Roger Mills[_2_]]

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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[Tim Watts]

On 22/05/10 15:31, Roger Mills wrote:

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.

Indeed. The joists are OK. Interestingly, the sagulator gives a much
larger deflection estimate for even a 4x2 ceiling rafter than I see in
reality. I can stand in the middle of one of these without any
discernable movement (and there's no floor screwed to the top yet) -
though there is are some "virtual noggins" by way of cross beams across
the top that are fixed through,

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.

I agree - but I have made it my primary design goal that there will be
no T&G as the boards I may potentially want to lift are all of them

I've built this place to support easy upgrading of comms circuits
(networking, TV etc) - every power socket is partnered by at least a
single backbox with dry conduit, so it's fully expected I'll want to
lift many of the boards perhaps ones every 2-3 years. I'm designing that
into the way the boards are set for lenght and the staggering of the
centre cuts so that I don't end up with trying to lift long boards with
both ends trapped under skirtings.

--
Tim Watts

Hung parliament? Rather have a hanged parliament.

[Tim Watts]

On 22/05/10 16:06, John Rumm wrote:


Both the calculation and the experiment seem to suffer the same weakness
- that you are treating the floor board as a free resting board
supported at either end.

The reality is different in a number of ways - firstly one board will
span a number of joists, and hence will not only be resisting your
weight in downward bending between the supporting joists, but also in
cantilever bending between the adjacent ones either side. Secondly the
board will be nailed or screwed down, which will enable it to resist
additional movement with tension as well.

That is true. I've gone for the worst case scenario. Useful in a way,
because I'll be loose laying the boards for a few months (with the odd
batten underneath to prevent slipping if necessary) to allow them to
season to the environment (ie shrink) before I set about putting in 2000
brass screws or however many it will be!. During this time, I'll have
DPM drawing pinned down on top so I can complete the rest of the work up
there - much messiness with plasterboard removal, reinsulation and
re-boarding. Can't do it at the moment 'cos the floor is too patchy,
unstable and would be dangerous trying to put steps on it.


I expect the reality will be better than you expect. 21mm floorboards
have always seemed pretty solid in my experience.

And it seems fairly decent wood by modern standards. I told the bloke
what I was after (floorboards) and asked him what he'd recommend for
cheapness, strength and an OK look if just oiled (with or without
colour). So far, I'm happy with the look. Nothing special, very
"floorboardy", will improve a little as it gets a patina. But that's OK,
we're not after "fancy", just pleasant and light and simple. I'll be
knocking the top edges to a micro bevel with 1-2 passes of a hand plane
which will reduce splintering and add perhaps a little interest to the look.

....

I would expect the 6" to be fine.

I suspect so. I can screw this length down to the worst case target area
and do some deflection tests just to satisfy myself. I'm not so bothered
by the bounce, but knowing I'm not overbending them means knowing I'm
several factors away from breaking them

--
Tim Watts

Hung parliament? Rather have a hanged parliament.

[The Natural Philosopher[_2_]]

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.



I was shocked to find how much bounce I had on a 100mm thick suspended
concrete and screed floor.

But its nothing to 8x4 period oak beams laid on their sides.

Like a couple of inches if you jump in the right place :-)

Its all pure taste. Its well within any building regs.



Cheers

Tim

[Tim Watts]

On 22/05/10 16:06, John Rumm wrote:


The reality is different in a number of ways - firstly one board will
span a number of joists, and hence will not only be resisting your
weight in downward bending between the supporting joists, but also in
cantilever bending between the adjacent ones either side. Secondly the
board will be nailed or screwed down, which will enable it to resist
additional movement with tension as well.

I cut my free sample of 2.7m into two yesterday, knocked of the edge
corners with a plane and screwed then down. They do seem to behave as
you say - quite firm.

I think we have a winner - looks perfectly OK bare. SWMBO agrees (good)
- so the only question now is which colour of Treatex to use (if any
colour) ;-

Anyway, I think I will lay each length as a pair or boards, in about a
1/3 room width to 2/3 width ratio, alternating. That should give a good
loading characteristic and still be easy to lift. Or maybe I'll
randomise it a bit for interest.

Cheers!

Tim

--
Tim Watts

Hung parliament? Rather have a hanged parliament.