Wood movement guesstimates

[John Schreiber]

The wood movement amounts I get from the Shrinkulator don't sound right.

Here's the facts:

I'm working on a workbench made of southern yellow pine. The glued up top is 30 1/8" wide as measured today. The grain is primarily radial as it is glued up.

The wood was "kiln dried" construction lumber when I bought it and it has been in my garage for about four months. Before it was glued up, it was stickered - so it should be pretty close to equilibrium for the shop.

Outdoor relative humidity here in Central Illinois varies from mid 40% to 100%. The bench will be in my shop/garage, so it's basically outdoors. When I put those figures in the shrinkulator, it says I should expect over an inch of movement.

I'm getting ready to dimension the base and that seems excessive to me. How much should I really expect?

[Jamie Buxton]

Kiln-dried construction lumber is typically dried to 19% moisture content. It has probably dried somewhat in the four months it has spent in your garage. If it is just left in your garage for years, it will probably get to a moisture content around 8-9%. Try those numbers in your calculations and see if you like the result any better.

[Chris Friesen]

While it may hit 100% humidity, it usually doesn't stay there for long. Also, the finish on the bench will slow down moisture movement somewhat.

My guestimate would be to plan for 1/2" of movement, mostly on the shrinkage side.

[Neal Clayton]

that pine has an abnormally high sap content, which is, of course, partly water.

unless it's completely finished/sealed on all sides, the short answer of when it will stop shrinking is...maybe never.

the heartwood from those trees will stabilize pretty quickly, but the sapwood never really does until most of the sap is gone. that takes more than 100 years apparently, because i can still draw sap out of 100 year old heart pine doors outta my house while stripping paint from them with a heat gun.

it may take a good long while for them to shrink that much, but an inch or more isn't uncommon from that type of lumber.

[glenn bradley]

The construction grade KD fir that I used to wrap my bench top was like-wise prepared. It has shrunk to the point that I will eventiually replace it with more stable material. The top itself is 3" of laminated MDF and is remaining constant.