Why does wood keep drying out/shrinking?

[Doug71]

On my to do list is re staining a little garden shed I made maybe 10 years ago, a coat of stain is well overdue. The t+g boarding on it has really shrunk, some of the tongues don't even meet the grooves anymore. The boarding is unsorted redwood, think ex 19 x 125.

It just got me thinking, when you see anything old outside like garden gates etc they are always "gappy", joints have opened up and boarding shrunk, yet if I made a new braced and ledged gate there is a good chance it would bend as the boards expanded from taking on moisture if gaps were not left between the boards to allow for this.

It seems that wood will initially expand when it takes on moisture but gradually dries out/shrinks to smaller then it's original size overtime when exposed to the elements despite rain etc.

Is it because the wood is deteriorating? I guess it's something to do with the cell structure? I know it happens, just want someone to explain exactly why?

Thanks in advance, Doug

 

[Droogs]

before you get overloaded it is basically due to wood being made of straws that want to have the exact same amount of water in them as the air around them. as the air changes so do they. have a read of Sgian Dughs book "Cut and Dried" It explains it all in a good easy to read way as well as getting technical. You could also look at the wood database website that has good explanations of the process

hth