rate of expansion/contraction for oak

[Mister S]

Hi All

I am in the middle of using some well seasoned (reclaimed) oak to make the top and bottom for a box, about 20cm in width, sitting in grooves in the sides.

Any idea how much allowance I should leave for seasonal expansion/contraction? A figure of 1% (which would be 2mm) jumped into my mind, but I can't find a reference to rates of movement anywhere. I don't want to make the grooves unnecessarily deep as it would weaken the sides.

Any ideas?

Thanks
Steve

 

[Mister S]

Just found some web sites that calculate wood movement (American of course - the Shrinkulator :shock: ) , which need variables to be input to give an approximate value.

What would be a reasonable moisture content range for indoors in the uk? I was thinking 10% to 15% but I'm willing to be corrected.

Steve

 

[custard]

Steve, average relative humidity levels vary around the UK, in London the external range is from 86% in January to 70% in June, but in coastal areas it tends to be a bit higher but more stable. In Plymouth for example it's highest in January at 89% but lowest in May at 77%. To achieve equilibrium at 80% humidity timber needs to be dried to about 16%, at 70% humidity timber needs to be at about 13%.

However, I find that indoors humidity can be quite a bit lower, in my workshop (London, heated) it tends to be in the range of about 50-60% year round, with only occasional and short moves to 65% or 70% during summer thunderstorms. To achieve equilibrium at 50% humidity timber needs to be dried to about 9%, and to achieve equilibrium at 60% humidity it needs to be dried to about 11%. Consequently I aim for about 10% moisture content in the timber I use for indoors furniture, and 15% for outdoors furniture.

 

[Mister S]

Custard

Thanks for the comprehensive reply. As I suspected, "it all depends".

The calculators I have found suggest movement at the levels you have described of roughly 1% to 1.5%. I think I'll aim for that and make the grooves oversize accordingly.

Cheers
Steve