Decking joist expansion?

[richardclay09]

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!

[David Lang]

Hi Richard

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

[Rob Morley]

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

[Norman Billingham]

The thermal expansion coefficient of pine along the grain is aboy 5 x 10^-6 per degree C

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

[Pete C]

Generally up to 0.2%, see para 3:

So ~5mm over 2.5m should do it.

cheers, Pete.

[The Natural Philosopher]

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

[The Natural Philosopher]

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.