Sawn green oak tolerances

[JBaz]

I am starting a project to build a pergola (from scratch - not a kit) using green oak, which I've not used before.

The timber has arrived and going through it I have determined that the dimensions of the 4.8m lengths I ordered at 150 x 75 actually vary between 69mm and 81mm on the width and some have a bow of up to 30mm (measured with a line strung from each end. The 150mm dimension is OK.

I also have one piece (4th from right in picture) that varies between 66mm and 85 mm on the thickness with a bow of 55mm which I definitely want replacing, but I was wondering whether I should ask for some of the other lengths to be replaced.

Anybody got any experience with green oak and what tolerances you would expect?

 

[Adam W.]

I have a bit.

Firstly, if you ordered straight from the mill, then expect to get what you're given as it's sawn to order and comes straight off the stem and not from stock.

Expect bow, twist and splits, but if it's not sawn accurately then you may have grounds for complaint. Expect it to shrink as it dries. It'll shrink a lot on the tangental plane and not so much on the radial. 2:1 is about the right ratio for shrinkage.

It won't shrink in its length, however.

 

[Adam W.]

I'll add. It's obvious that they have slabbed at 150mm and then ripped it at 75mm. Because trees have a lot of stresses in their wood, the pieces which are ripped off the slab tend to bow as the tension is released. As the slab gets smaller the more they rip off it, it will end up bowing as well and is why you get a discrepancy in the 75mm dimension.

It's really upto your skill as a carpenter to get around that.

 

[Jameshow]

Nothing to add but keep us updated.

Cheers James

 

[Darrenp]

can you not use the bowed lengths for the braces ?

 

[julianf]

Green oak, as I'm sure you know, will move about so much anyway that your current inaccuracy may be well within the scope of inaccuracies by the time you are finished with it!

 

[Keith 66]

Its green, it will move all over the place, dont worry about it.

 

[owen]

They look perfect compared to some of the timber I saw at Homebase on the weekend, one piece was so bent I'm sure if you threw it away it would have come back.

 

[Adam W.]

I think it looks OK too.

 

[Phil Russell]

I built a pergola three years ago using 100mm x 100mm uprights ... treated timber. Good solid stuff but after a year they started to twist a bit, maybe by 15 degrees over the (I think) 7ft length. I found this strange as I had not seen fence posts twist before. Never mind. It has not affected the rigidity of the structure.
Cheers, Phil

 

[JBaz]

Darren

The braces are coming out of 200 x 75 sections, but I'm planning to build a set of steps in the garden as well, so I can probably use it for them.

My query to the forum is really what my expectations should be when I buy green oak cut to order. I've bought plenty of kiln dried hardwood in the past, but never green timber cut to order.

My gut feel is that a variation of +10mm and -9mm along a single piece ordered at a nominal 75mm is not acceptable, but I'm wondering what a reasonable variation is.

I shoukd add that I expected to plane the stock to a regular size in order to cut the joints, but taking all the 75mm stock down to 65mm is a lot of shavings and having now felt the weight of the green oak I don't think we will be able to put it through my workshop machines. Consequently I have ordered a Triton 180mm planer along with a Makita HS0600 270mm circular saw so we can take the tools to the timber. (I'll sell them on when I've finished with them and hopefully won't lose too much on them - cheaper than hiring and the kit is new.) I just hope we can use the workshop chisel morticer.....

 

[Adam W.]

I wouldn't bother with planed square edge, because after a month it won't be square any more.

Normally the joints in green oak are scribed, draw bored and pegged.

If you're not happy, why don't you just speak to the mill and see what they say.

 

[Jacob]

Set aside the bendy bits for shorter lengths where it will be less noticeable and just use the heap as it is.
If you are really going to plane it all then cut it all to lengths first, according to your design drawing. Common beginner mistake is to attempt to plane up stock as per P.A.R. but this is a very bad idea.

 

[LJM]

With out of square green oak, I have simply nominated the faces that I wanted to be coplanar and referenced them for cutting joints.

 

[Austin Branson]

Hello. Don’t worry. You’re not building an extra large piece of fine furniture. As Adam said, construction using green oak (and ‘steady’ oak) is scribing. Mortice and tenons with pegs. I was very uncertain when I built my verandah and bought several books. You will need steel draw pins and a long 3/4 inch drill bit. Timbers twisting and bending is part of it, and largely restricted by jointing other timbers onto it. The resulting frames are incredibly strong.

 

[Adam W.]

That's nice Austin....

 

[pe2dave]

My query to the forum is really what my expectations should be when I buy green oak cut to order. I've bought plenty of kiln dried hardwood in the past, but never green timber cut to order.

My gut feel is that a variation of +10mm and -9mm along a single piece ordered at a nominal 75mm is not acceptable, but I'm wondering what a reasonable variation is.

An obvious one - have you spoken to the supplier with those figures? Is he/she willing to do something about it (for reputations sake)?