I am fitting a green Oak post in the corner of a porch (9' longx6" square) which has a flat roof, it is not structural as the porch (approx 2mx2m) cantilevers and is part of a roof extension t6he post will be concreted in the ground but not very deeply (6"). However the post still needs to be fixed to the roof of the porch and my question is if I bolt it or screw it on will the post shrink enough as to cause damage to the roof by pulling on it?
Green oak will shrink for sure .. if the rest of the structure is not Green oak ... why use it ? ..... why not use a seasoned timber which has done most of it's shrinking.
Around 0.5% along the grain and 3-5% across it from green to dry.
So 9ft will drop around a half an inch or so.
My guess is it will pull out of the ground slightly, if your cantilevers are strong.
Then a fixing at the top that allows up to ½ inch of movement would make sense. For example, a metal bracket with slotted holes.
Dear NP I did some calcs using the BRE Tech Note 38 and came up with different figures to yours and wondered where your source was AFAIAC it seems to be reasoned as follows: from [RH 90] a mc of 20% down to a mc of 1`2 % (RH 60) there is a tangential change of 2.5% of the CS which is 150 mm ie some 3mm The actual change is from the FSP which is about 30% so from 30 down to say 14 % ( likely outside equilibration point under "shelter") is
16% ie double that cited above and thus nearer 6 mm No data on longitudial but will not argue with 0.5% seems to me that he is best to put it in with a thick felt or simiar dpc around it to give some flexibilitychris
You comonly see green oak post supports fitted with a separate horizontal plate of the same material at the top (usually another quite chunky piece of timber) - apart form the nice aesthetics, it may also act as an easily replacable shim.
Would you expect the beam to be dimensioally stable after that - or would it expand/shrink by any significant amount due to humidity, rain etc.?
In fact, if you socketed the post into this top plate, and fixed the top plate to the porch, shrinkage of the post wouldn't show.
Granted that longtitudinal shrinkage isn't much of a problem, what happens if the bugger twists as it dries out?
Downstairs all over the place! That's about what mine shrunk!
Across the grain Ive had about 3/4" in 8..about 12% but that was VERY green And indoors.
Outside in the porches its less. Warping has been a significant problem tho.
I wuuld make it oversize and be done with it..
You get less than 0.05% summer to winter along wood grain. Its worse across. Always get movement. Indoors and out. Just different directions..RH outside is worse in winter, RH inside is worst in summer.
Best idea yet if its not actually holding anything up.
Can't see it to tell, but expect zero shrinkage lengthwise, 5% radially and 10% circumferentially. That's the same for all species from "fully wet" to "fully dry", to a reasonable hand-waving approximation (a slighter tighter one is that total shrinkage varies, but the 0:1:2 ratio is pretty constant). Equilibrium moisture content (EMC) vs. relative humidity (RH) is likewise surprisingly consistent across species. Actual dimension vs. moisture content and initial moisture levels straight off the tree are the ones that do vary by species.
Watch out for crushing. If you crank the nut up every time it goes loose, eventually you'll have it tightened at the minimum dry summer dimension. Then when it expands in the autumn it will either crush the timber and take a permanent set, or even break the bolt.
A reasonable piece of oak (i.e. trunk, not branchwood) won't (significantly). Elm or larch would. If twist is that important, then look for quartersawn timber (flats) or boxed heart (big squares).
As always, if you care abou tthis stuff, either buy a copy of Bruce Hoadley or download the free PDF of the Forest Products Handbook. Both are excellent reading.
That 0 is an approximation. Its actually definitely >0, even for oak.
But it is millimeters over meters, so a guess at 0.1% is probably close.
Not break the bolt. Not with sane sizes. In practice the elasticity of the wood/bolt is usually enough. Best to use a wood peg though..same variance..
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