Air Dried Or Green Oak For External Use?

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Plecc
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Air Dried Or Green Oak For External Use?

Postby Plecc » Fri Oct 13, 2017 6:32 pm

Hi all,

As the title says, should i use air dried or green oak for external use?

I have been reconditioning old victorian cast iron benches to try and bring in a little extra cash, all the previous benches have been fitted with slats cut from green oak sleepers and the bench ends have been heavy industrial style ends that only require 4-6 large chunky slats, finished with three coats of teak oil.
This method has worked well for me for the large almost board like slats, but i have a bench comming up that requires much finer slats that are about 1" squared, i think about 24 slats total.
Should I continue with this method for this bench? I am concerned that any warping or twisting as the wood dries will be accentuated by the smaller slats.

Up untill now i have only sold to friends and family, I would like to start selling to the public but my knowledge of the properties of timber in an outdoor enviroment is poor at best, my background is in engineering so im a little outside my comfort zone.
Please shout if you feel using green oak like this is a total sin, before i can sell to the public i have to convince myself that i will be supplying a high quality product that will look good for years to come. (with a good yearly coating of teak oil!)

How would a professional do this?

Thanks, Plecc

rhrwilliams
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Re: Air Dried Or Green Oak For External Use?

Postby rhrwilliams » Mon Oct 16, 2017 4:37 pm

I am not a professional. However green oak can shrink by as much as 20% so if you are using thin slats you might want something dryish to star with to avoid splits or shrinkage.

sgiandubh
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Re: Air Dried Or Green Oak For External Use?

Postby sgiandubh » Mon Oct 16, 2017 8:35 pm

rhrwilliams wrote:I am not a professional. However green oak can shrink by as much as 20% so if you are using thin slats you might want something dryish to star with to avoid splits or shrinkage.

I'm not sure where you might have got the 20% shrinkage factor from, but it is a bit generoust. European oak has a shrinkage factor of 8.9% tangentially and 5.3% radially in the range of moisture content (MC) that wood moves, i.e., from fibre saturation point (FSP) which is ~30% MC down to oven dry which is 0% MC. Above FSP there is no expansion and contraction in wood. In service wood generally only experiences a proportion of the expansion and contraction that can occur over the full range because, for example, in a residential building typical wood moisture content varies between about 7% MC and about 13% MC, which is only 6/30 or 1/5 or 20% of the maximum possible range of movement.

But you're right to suggest air dried wood would be a better choice for Plecc's project because it's likely to be somewhere close to 18- 20% MC when he buys and uses it, and this sort of MC will probably be at about the centre of the range of MC the wood will experience in service externally. Slainte.

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Re: Air Dried Or Green Oak For External Use?

Postby Meccarroll » Mon Oct 16, 2017 9:25 pm

sgiandubh wrote:
rhrwilliams wrote:I am not a professional. However green oak can shrink by as much as 20% so if you are using thin slats you might want something dryish to star with to avoid splits or shrinkage.

I'm not sure where you might have got the 20% shrinkage factor from, but it is a bit generoust. European oak has a shrinkage factor of 8.9% tangentially and 5.3% radially in the range of moisture content (MC) that wood moves, i.e., from fibre saturation point (FSP) which is ~30% MC down to oven dry which is 0% MC. Above FSP there is no expansion and contraction in wood. In service wood generally only experiences a proportion of the expansion and contraction that can occur over the full range because, for example, in a residential building typical wood moisture content varies between about 7% MC and about 13% MC, which is only 6/30 or 1/5 or 20% of the maximum possible range of movement.

But you're right to suggest air dried wood would be a better choice for Plecc's project because it's likely to be somewhere close to 18- 20% MC when he buys and uses it, and this sort of MC will probably be at about the centre of the range of MC the wood will experience in service externally. Slainte.



May I say, what I nice piece of information. I sometimes forget all this stuff and then you come along and re-kindle my interest, cheers from me ;)

Mark

sgiandubh
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Re: Air Dried Or Green Oak For External Use?

Postby sgiandubh » Mon Oct 16, 2017 11:18 pm

Meccarroll wrote: May I say, what I nice piece of information. I sometimes forget all this stuff and then you come along and re-kindle my interest, cheers from me ;) Mark

Many thanks for that Mark. I'm not the most prolific of posters around these parts as I'm sure you've noticed, nor anywhere else I suppose, but if I've managed to re-kindle your interest in all this timber tech stuff, and you feel the urge to absorb a bit more, then perhaps my hopefully soon to be released book on the subject might suit your purposes. The publisher is putting it all together as I type, and with luck nothing will go wrong with a release provisionally scheduled for early in 2018, ha, ha. Slainte.

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Re: Air Dried Or Green Oak For External Use?

Postby woodsmith » Tue Oct 17, 2017 8:54 am

I am a bit concerned about using 1" square timber for a bench. Unless you have some way of linking or reinforcing the timber I think you are going to have major problems with it. Plus teak oil would not be a finish of first choice for me. Oak is a difficult timber to finish for exterior use. Tung oil is often recommended as is linseed oil but I find whatever finish you use it will only look good for a short period before the wood starts to turn black then grey. Sikkens (Cetol filter 7 & HLS) is the best finish I've found but that looks less natural and even that is not completely resistant to blackening.
Keith

rhrwilliams
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Re: Air Dried Or Green Oak For External Use?

Postby rhrwilliams » Tue Oct 17, 2017 8:07 pm

The 20% shrinkage factor (I thought) was from a Trada document on Green Oak... however I have re-read it and I was mistaken. It referred to being oven dry at 15% shrinkage, but gives a more general shrinkage factor of 4.5%. My mistake.


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