Base-to-top joinery versus timber expansion

**DomAU** · 26th April 2022, 08:37 PM

Originally Posted by pippin88

Tangential expansion is greatest. In a bench this will often mean the top gets thicker, as posted by others above.

But radial expansion is around half tangential expansion (does vary substantially between species)

Longitudinal movement is essentially zero.

Even if radial expansion is only 4%, that is 36mm on a 900mm wide bench.

But 4% shrinkage would be highly unlikely. For example, radial shrinkage for say, Rock Maple, is 4.8%. If we assume that the moisture content changes +-4% between the seasons, then a 700mm top would expand/contract - 700 x 0.048 x 0.04/0.3= 4.48mm. And +-4% moisture change is highly unlikely for a thick top like that on a Roubo (the centre of the slab is unlikely to have time to change at all if the timber was seasoned to equilibrium m/c to begin with)

Cheers, Dom

**pippin88** · 26th April 2022, 09:04 PM

Ah, I did not understand the average 4/8% figures are for green to oven dry - not seasonal variation for already 'dry' timber.

**Tonyz** · 27th April 2022, 09:52 AM

IF !!! and I repeat if I was to attempt building a bench similar, how would I go with Jarrah & Oregon posts that were in a shearing shed so well air dried and been stored for around 10 years.

would I be looking at much shrinkage?

**Mountain Ash** · 27th April 2022, 10:02 AM

Hi T. All timber is affected by the moisture content of the environment that surrounds it. The timbers you have will be very stable and as posts, ie. thick, should not change much at all. If you rip them to smaller dimensions the timber will need time to readjust

**Wardrop** · 12th September 2022, 11:20 PM

Originally Posted by Fuzzie

I suspect any movement in the benchtop would be accommodated by the legs flexing along their length down to any bottom stretcher where it probably wouldn't resolve to a significant movement.

I came here to ask the same question as the original poster, and this is exactly the answer I was looking for. Obvious once it's pointed out. As long as there's nothing (other than the benchtop itself) rigidly holding opposing legs together, then the legs themselves accommodate the movement. The caveat is the joinery connecting the top to the legs needs to use proper strong joinery as that connection is what will resist the racking.

If you have a stretcher at the top of the legs, then you'd need to use a mechanical connection that accomodates seasonal movement.

Hmm, now to decide which route I want to go...

Thread: Base-to-top joinery versus timber expansion

Thread Tools

Search Thread

Similar Threads

Expansion gap in timber floor

Finishes on open grained timber versus closed grain timber

Timber expansion per meter - across the grain?

Timber expansion in Frame and Panel construction

Posting Permissions