Which way would wood move?

[Sir Percy]

Hi,
does anyone know if wood expands more with grain, across grain, or evenly with temperature?
Thanks

 

[MattRoberts]

Wood will expand the most across the grain and the least with the grain

 

[custard]

Hi,
does anyone know if wood expands more with grain, across grain, or evenly with temperature?
Thanks

Wood hardly moves at all because of changes in temperature alone, it only moves because of changes in moisture content. When the moisture content of a board changes there is a almost zero change along the grain, there's quite a bit of change across the grain in a tangential direction, and even more change across the grain in a radial direction.

To give a specific example, take a piece of European Oak. At a relative humidity of 90% it will have a moisture content of 20%, if the relative humidity then drops to 60% the Oak will eventually stabilise at a new moisture content of 12%. Along the way it will hardly change at all in length, but it will shrink across the grain by 2.5% in a tangential direction and by 1.5% in a radial direction. That's one of the reasons a quarter sawn board is the most stable cut, the biggest dimensional change occurs on the thickness which is generally the smallest dimension of the board.

One final twist to the tale, dimensional changes in timber only occur once the wood has dried out to below about 20%. A freshly felled and planked tree could take a year or two to lose the free moisture contained in its cells, during this time it's pretty stable even though it's losing a lot of moisture, the dimensional changes only really begin once it crosses that 20% barrier. Furthermore, if you kiln the timber (rather than air dry it) down to exceptionally low moisture content levels then it will subsequently respond a bit less to any changes in relative humidity than a similar board that's only been air dried. It seems that the cells in kilned timber lose some of their ability to shrink and expand in response to changes in humidity.

 

[Sir Percy]

Thanks custard. That was quite educational!
The wood will be outdoors, and painted. So hopefully changes in humidity won't be a factor.

 

[Sgian Dubh]

One final twist to the tale, dimensional changes in timber only occur once the wood has dried out to below about 20%.

I'll add a slight correction. Dimensional changes in timber only occur once the wood's moisture content falls below Fibre Saturation Point (FSP), which varies from as low as about 24% Moisture Content (MC) up to about 34% MC, with this disparity in FSP accounted for by variations in the make up of different wood species.. Taking the average of all wood species gives the median figure of ~30% MC as FSP, which is the standard number used for calculating (estimating) such characteristics as expansion/contraction in response to moisture gain/loss. Slainte.

 

[MattRoberts]

Thanks custard. That was quite educational!
The wood will be outdoors, and painted. So hopefully changes in humidity won't be a factor.

On the contrary, being outdoors will expose it to constantly changing moisture and humidity. Any protective finish won't make it immune to movement. That's why most garden furniture has gaps between boards

 

[Phil Pascoe]

As above, and it tends to cup away from the heart when it moves, which can influence how you fix something - a skirting e.g. with the heart outwards will cup keeping the top and bottom edge tight to the wall. With outdoor stuff it can matter if you're trying to keep boards tight to a frame. You'll always have a bit of a problem outside as invariably you'll have sunshine on one side and not on the other, for one thing.

 

[Sir Percy]

Thanks custard. That was quite educational!
The wood will be outdoors, and painted. So hopefully changes in humidity won't be a factor.

On the contrary, being outdoors will expose it to constantly changing moisture and humidity. Any protective finish won't make it immune to movement. That's why most garden furniture has gaps between boards

Right, thanks.
I've basically got a set of boards set laterally in a frame, and have left gaps between the boards. Lengthwise, they are quite snug, so I think I'll have to ease them in a bit, then.

Thanks all!