wobblydoggy":239z3nzn said:
It is going to be roofed over, but not covered on the sides.
I expect in the heat of the summer it will dry out, but yes in winter I suppose it will take in water
Typically you'll find wood that is outside but sheltered from direct sun, rain, etc will hover between about 25% or 27% MC in the winter to perhaps 17% in the height of summer here in the UK. We calculate wood shrinkage from fibre saturation point (FSP) to oven dry, i.e., 30% - 0% MC, with 30% used as an average for all woods bearing in mind that FSP varies from as low as about 24% in some species to up to about 34% in others all dependent upon the structure of the wood. In your case therefore you are dealing with potential movement over roughly 30% of all the shrinkage you might experience if the wood dried from FSP (30%) to oven dry (0%) because you're looking at the range between about 17% and 27%. European oak has a shrinkage factor of 8.9% tangentially from 30% MC to 0% MC, and 5.3% radially. Therefore 30% of 8.9% equals 2.67%, and 30% of 5.3% equals 1.59%. In other words, if your wood is 200 mm square to start with, and at that point it's at about 22% MC it might grow or shrink by about 1.34% (2.67%/2) tangentially and 0.8% (1.59%/2) radially because 22% MC is in about the middle of the expected MC range.
I guess that might all be too much information, but there are further complications. Thick oak posts are very unlikely to have a constant MC throughout their volume - all wood has a moisture gradient, even small and thin pieces. Thick oak beams that have been air drying for a few years are likely still to be pretty wet at the centre, possibly very close to FSP, whilst that stuff nearer the shell might be down as low as 18 or 20% MC.
In general then, I'd say get on and make your framing, pegged as you plan, but you will get shrinkage, and being oak you're likely to experience a fair amount of splitting, especially radially. It's the nature of oak, and if you look around you'll see lots of examples where oak framing has been used to make buildings, lean-tos, porches, barns, sheds, etc, where there is significant evidence of splitting. Slainte.
