Hello:

I am finishing up the timbers for the shed described in Jack Sobon's book. I am down to cutting the two 16 foot 8 X 8 plates. The one I am working on is square with no wind. However, it does have a crown of 3/16" at the centre point. I have bored the mortises but have not cut the inset to 7" from the reference face. I have placed the crown up. The rafter seats are not cut yet.

My question is: Do I cut the insets (at the mortises) and the rafter seats according to the reference face (top of plate) then hope the weight of the centre post and the roof pushes the crown flat? Or, do I drop a chalk line from end to end then reference my insets and the rafter seats from that? I know crown is undesireable in a plate, but I am stuck with these last two timbers. Maybee 3/16" crown is nothing to worry about?

Thanks... Norm Hart, Carp, Ontario

Hi Norm.

Snap the chalkilnes.

The layout technique that Jack is describing is called square rule. What you are doing is laying out your joinery on an imaginary perfect timber inside the actual timber. Usually, you can treat one face of the actual timber as a face of the 'perfect timber' - the reference face. But the 'perfect timber', of course, has no crown. So you must create your own references using the chalkline. In my opinion, 3/16" on a 16' timber is enough to worry about. Good luck. CB.

Norm:
I guess you'll get different answers from different people. And I’m not saying that daiku’s answer is wrong.
But I would say let the roof weight and the draw bore of the peg holes pull the plate down and remove the crown. Cut your mortises with the correct offset location from your reference face and rafter seats on the surface of the plate and do not use a chalk line.
Then in the assembly process, pull your plate down using the draw bore method, thus removing the 3/16" crown. With the rafter seats cut on the surface, correctly attach the rafters and roofing. The weight of the roofing will help hold down the crown.
If you're not sure if the relish will hold on the end of the post tenon, then pull down the plate first, by putting a come-a-long on the plate connected to the sill over or near the middle post and pull it down until the draw bore holes nearly line up, be sure to pull down straight and then pound your peg in.
Then release the come-a-long and continue your raising or release it later if you wish. Again the weight of the rafters and roof deck and roofing will add to removing the crown.
Hopefully others will add their opinion to this topic.
Good luck with your project. Jim

I'd snap the line. Pulling it and drawboring it will work but why put added tension in the timber if you don't need to.

The only way to end up with a straight top plate is to lay out from the reference face and pull the crown out when you peg off the joints. If you cut the post-top gains to a snapped line, the crown will always be there in the finished frame.

You didn't mention what species you're working in, although I think you could pull a 3/16" crown out of about any species, without adding objectionable tension.

Folks...thanks for the replies. I was away to Upper Canada Village this weekend and am only back now. The species I am using is white pine. It was logged November 2002 and sawn in December. In this project the central post is pegged to the plate. However, the stub tennon on the bottom of the post is unpegged (as is the usual way of doing this).

Norm Hart

Is your sheathing vertical or horizontal? If it's vertical just put a board over the post and nail the plate, post and sill together, temporarily or permanently.
If your using horizontal sheathing then put one or two boards across the bottom to hold the post down to the sill until you've finished off the rafters, roof deck, and roofing. The added weight of all these materials will hold the plate, post and crown down.
In either case do this before you release your come-a-long, and you'll be alset.
It's only 3/16ths of an inch!
Jim

Ping the lines !

Crown, bow and wind, the organic indescrepencies responsible for 99% of all the joints that will not come togeather despite the seeming exactitude with which they were cut, are all easily overcome by describing those two perfect planes of reference through any timber, no matter how awful it is.

I grant you 3/16ths is not much, in a shed to be sheathed with boards, but throw that same crown into a plate the length NH is talking about to be put into a home to be enclosed with panels and you have a problem, you have telegraphed your crown to the surface of the structure. Harder to enclose with unfogiving materials and likely even percievable to the human eye.

Do a little expieriment for me, draw the section of your plate on a piece of paper full scale then draw two reference planes through that section at 1 1/2" from the reference faces, now draw the ascending and descending lines of your step lap rafter seats ( I don't recall but something tells me Jack's shed has these and angles are a perfect way to bring this home ) Finished ? Now draw a line 3/16ths above the theorectical surface of your stick and extend tha ascending/descending lines through it and you will see exactly how far your joinery is off.

That same drawing and those lines can be used to not just predict, but to know, ecactly where the entrance and exit wounds of your joinery should be, and the crown/bow/wind is canceled.

A perfect solution for our imperfect world.

Let the timber do what it want's and needs to do, put your joinery where it belongs !

Thanks Will for giving me even more insight to this situation. I may be looking at this the wrong way, however, snapping chalk lines, I think, can set up a problem of its own. It looks to me that the primary reference face of the plate is the outside vertical face and the top of the plate would be the secondary ref face (or vice versa). The two ref faces meet to form an arris. This arris lies in the plane created by the outer surface of the rafters. If a chalk line is used to "remove" the crown then the plane formed by the rafter tops will drop below the arris by 3/16" at the centre of the plate. I suppose I could simply plane off the protruding arris before sheating the roof. I realize that by not using the chalk lines I end up with a curved (by 3/16 in the middle of 16') roof surface if it does not flatten itself. Again, I may be looking at this the wrong way. In any case I have not used chalk lines. I think either way since it is 3/16" and a rough shed it would work out.

I certainly see clearly why it is said that crown is undesireable in plates!

Norm Hart

The arris does not have to coincide with two surfaces of the wood. If you measure down say two inches (could be anything reasonable) from the top on the vertical face at each end, and then snap a line between those two points, you can use that line, instead of the crowned edge of the timber, to measure from. You would then layout the rafter seats by measuring off that line 2". In the middle, where it is crowned, you would still be measuring 2" off a straight line, which means you remove more material for the rafter seats there.