I have 2 parallel brick walls, 2.5m apart. Between and at right-angles
to the walls are decking joists (themselves supported off the ground by
beams bolted along the walls, to which the joists are attached with
angle brackets).

I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?

Thanks for any advice!

Hi Richard

I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?

If timber expands or contracts it does so on the width rather than length -
any longitudinal expansion over a 2.5 metre span is going to be so small I
doubt you could even measure it.

Dave

In article , "David
Lang" says...
Hi Richard

I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?

If timber expands or contracts it does so on the width rather than length -
any longitudinal expansion over a 2.5 metre span is going to be so small I
doubt you could even measure it.

I thought you were supposed to allow roughly 10% on width and 1% on
length.

"Rob Morley" wrote in message
t...
In article , "David
Lang" says...
Hi Richard

I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?

If timber expands or contracts it does so on the width rather than
length -
any longitudinal expansion over a 2.5 metre span is going to be so small
I
doubt you could even measure it.

I thought you were supposed to allow roughly 10% on width and 1% on
length.

The thermal expansion coefficient of pine along the grain is aboy 5 x 10^-6
per degree C
(http://www.forestry.caf.wvu.edu/prog.../wdsc340_7.htm).

So for 2.5 m, which is 2500mm, and a change in T of 20C the expansion is:

2500 x 20 x 5 x10^-6 = 0.25 mm

Wouldn't worry about it

On the other hand, I don't know how big the elongation with seasonal
moisture cycling might be,
though much lower along the grain than across it

On Tue, 3 May 2005 19:01:48 +0100, "Norman Billingham"
wrote:

The thermal expansion coefficient of pine along the grain is aboy 5 x 10^-6
per degree C
(http://www.forestry.caf.wvu.edu/prog.../wdsc340_7.htm).

So for 2.5 m, which is 2500mm, and a change in T of 20C the expansion is:

2500 x 20 x 5 x10^-6 = 0.25 mm

Wouldn't worry about it

On the other hand, I don't know how big the elongation with seasonal
moisture cycling might be,
though much lower along the grain than across it

Generally up to 0.2%, see para 3:

http://www.fpl.fs.fed.us/research-areas/rwu/rwu4714/shrinkage_of_wood_faq.html

So ~5mm over 2.5m should do it.

cheers,
Pete.

David Lang wrote:

Hi Richard


I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?


If timber expands or contracts it does so on the width rather than length -
any longitudinal expansion over a 2.5 metre span is going to be so small I
doubt you could even measure it.


Uou can, but its slight. I would estimate 0.1-0.25% Thats with himifity.
Tempareture is almost irrelevant with timber - certainly the strains are
absorbable by the wood unlike e.g. a steel rail.

Dave

wrote:

I have 2 parallel brick walls, 2.5m apart. Between and at right-angles
to the walls are decking joists (themselves supported off the ground by
beams bolted along the walls, to which the joists are attached with
angle brackets).

I have cut the joists so that each of their ends is only about 5-8mm
away from the brick, and I am now a bit worried that I have not left
enough room for their possible expansion on hot days - might they start
pressuring the walls and even bowing them outwards if they expand by
more than the small gap I have left?


Expasnion in wood is far far gteater due to mositure uptake as humifioty
rises: Ourtdoors thats a winter issue as compared to inside, where its a
summer issue.

The expansion over a lenght of timber along the grain is at the absolute
maximum 1% or so. And that is shrinkage from green to approximately dry.

That would equate to at the very worst 25mm overall. But thats from
fully green to fully dry. Unlikely to see more than quarter of that due
to wet/dry cycling in kiln dried timber.

Additionally the only ill effects from expansion would be bowing in the
timber. If its decking supports, it can only presumably bow upwards, in
which case ignore it - it will so slight and in any case far less than
teh natural timber bowing due to being gut cross grain etc.

In short, you are worrying needlessly about the wrong things. Your
decking will move significantly due to many things, but expansion due to
heat in the long grain direction of the timber is not one of them


Thanks for any advice!