The oak may shrink 10% in the direction tangential to the growth rings, a little less in the radial direction but much less along the grain. So your rafter may get 10% thinner but it won't get 10% shorter.

You could think about slotting the fixing holes to allow for movement like they did with old furniture.....

Biff get serious steel does not expand and contract that much!

Posted By: biffvernonIf it shrinks by 1% you lose 35mm in your 3.5m rafter

ReallY? That certainly requires slotted fixings or something. But as well as shrinkage, what about warpage and bending on rafters conventionally installed on non-shrinking supports, i.e. not as part of an all-green structure that will shrink and move as a whole but sort of stay together till it settles down?

The coefficient of Thermal Expansion of Steel is about 11E-6/C. Concrete is similar which is one of the reasons that Reinforced concrete works as a compatible structural material. At 3.5 metres, the thermal expansion of most materials is negligable though large external surfaces subject to sun heating usually need to be controlled with joints. The comment about movement during day/night temperatures above is wrong.

If you fix your rafters to the structure, the forces generated by shrinkage contraction will be sufficient to either pull most frames fixed to it or to destroy the fixing: These shrinkage forces generally exceed the forces due to loading on a structure by several magnitudes. If you have made your frame sufficiently flexible to allow this movement, and accounted for rotational (flexural) forces that may be generated at the joint, then it may be possible. However, generally, green timber should always be left to dry in a similar humidity environment to that of its final intended use before being used in construction.

Jon, could you check the arithmetic for me as I've probably got it wrong - 3.5 metre steel with 40 degree C change.

3500 x 1.14 x 10^-5 x 40 = 1.6 Hence "A 3.5m steel in the roof of a garden room might move one or two millimetres. OK, not enough to worry about."

I've never built with steel. Have you ever built with green oak?

Thanks for checking the sum, Jon. I plucked the figure of 40 degrees C out of the air as I thought this might be the maximum conceivable diurnal variation in the temperature just below the glazed roof of a garden room. Engineers would, of course, want to include a margin of error and might like to consider that if the steel were to be painted a dark colour and in direct sunshine, the beam itself might actually experience an even greater temperature range. You are right to point out that "...average variances would usually be less than 40C", but I'm not sure how much one is comforted by the knowledge that the structure will not usually fail. It is the exceptional that hurts one's head.

I wouldn't use 'most types of finishes and joints' but would, as I said, "...use traditional framing joints with pegs." Traditionally, oak beams are used green. It takes several years to dry out an oak beam, the rough rule of thumb being a year for every inch of thickness, so using it green is the only practical option. Kiln drying oak is a silly idea.

The shortage of craftsman builders with specific experience could, in part, be addressed by encouraging the view that oak framing is not that difficult. It's carpentry, not rocket science, the tools are simple and it's good for one's biceps.

Posted By: jonit limits the range of insulation available (other than traditional)

That rules out multifoil then.

Hi tom

Unfortunately the drying shrinkage process is reverse exponential relative both to time and the size of the material used. If you used timber from different sources, or with different drying start dates, or with different sizes, the rates, and eventual shrinkage differences, would not be the same leading to warping of the form. Even if you managed to obtain pieces that all did shrink at the same rate to the same eventual ralative shortenings, the large joints required in the insulation would form huge cold bridges unless you managed to install a very flexible insulation and exterior finish.

Unfortunately, if you did that, the durability would go down leading to an increase in annualised embodied costs (and maintenance).

More or less Tom

Nothing to stop it being done (other than economics and finding a friendly engineer willing to put his PI on the line)

:-)

I expect oiseaux has built his garden room with the green oak beams by now. I'm sure it will be just fine :)

Thankyou all for your extremely detailed replies, my conclusion is to provide slotted joints to allow for movement. I only wish I had finished my garden room but alas with visitors, rain etc this has not happened. On a somewhat less controversial subject, I have noticed that the outer rafters which are exposed to the elements are showing a "tide mark" after the rain. Will this disappear and should I treat the timbers or leave to weather.

Thanks Biff - I was wondering if, from your working familiarity with oak, you also have any observations on my question

Posted By: fostertomjon, what would be the effect of using green oak, expecting it to shrink etc, in non-traditional frames? e.g shallow-pitch trusses in which each member is double, sandwiching steel flitch connectors for the joints? You'd have to allow for the ensemble to shorten, but could you rely on each piece shrinking more or less the same, so the whole would keep its geometry? especially as the doubling would help to even out exceptional movements of individual pieces?

- which is about structural shape-retention -
and jon's answer

Posted By: jonIf you used timber from different sources, or with different drying start dates, or with different sizes, the rates, and eventual shrinkage differences, would not be the same leading to warping of the form

of which only the bit about differing cross sections seemed to be insurmountable. It would be fantastic to feel free to use green oak, chestnut etc for such purposes.