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Large wood desk top with miters.

1/21/17       
Mike Fuson

I'm building a large home office and the client is wanting the desk top made of wood, probably 8/4 wenge, sapele, or mahogany. It's a horse shoe shape, 12' in the middle, left leg is 6', and the right leg is 3'. I'm thinking that this has to be done with miters because of expansion and contraction? Also thinking that this would need to be done in one piece in the shop for the best looking miter? And then what would be the best method to join the miters? I've never done a top like this, only a few island tops. Any thoughts would be appreciated.

1/22/17       #2: Large wood desk top with miters. ...
Pat Gilbert

Mdf core with whatever veneer.

Connect with draw bolts or these (see pic)

1/22/17       #3: Large wood desk top with miters. ...
David R Sochar Member

A bit of a thought experiment:

The desk top is to be 30" wide, as a guess. So you will have 3 pcs of wood - two each with a single miter across that 30" and one with a miter at each edge.
You fabricate in your shop that has reasonable heat, and maybe little dry in humidity. The wood - Sapele - arrives at 6% MC, and you make excellent, accurate miters and glue and fasten the tops together. Or you leave them loose to assemble on the job - with or without the glue.

The client's house is kept at a fairly high Relative Humidity - say 40% - and within a year or two, the Sapele has grown to a 9% MC - 3% gain.
This means (via Shrinkulator) that the 30" wide tops are now 30-1/4" if flatsawn, or 30-1/8" if quarter sawn. They should all move the same in width. Not a whole lot, but movement all the same.

What happens to the miters? As the wood expands - both halves of the miter equally - that perfect 45 degrees you cut will become a bit less than 45 degrees, and the tops will try to open at the toe or long points. The "U" shape will no longer have parallel sides.

Okay - you anticipated this, so you splined, biscuited, and dowelled the miters, and added a 3/4" Birch ply reinforcement plate on the underside of each miter, with glue and screws. The finish is equal on top and bottom. So movement should be stopped, right?
Well, if the movement is stopped (not always easy to do) - the wood at the miters cannot swell, the miters hold, your concern pays off and the joints are tight.

But the homeowner travels to Europe for a year, and the housekeeper decides it is always too damp in the house and sets the humidity for 15% to better match her desert homeland. After several months, the desk tops will show cracks along the grain on either side of the miter - may be in the wood or on glue joints, but cracks the same. The miters will be relatively tight, but the crack will lead into the field of the tops a few inches or more.

Why? The wood did 'swell' when it absorbed moisture, but it could not go anywhere - could not swell at the miters, so all the fibers all crushed just a little bit all along the joints while they tried to swell.

Then, when it dried out, they retained that crushed fiber profile, and the cracks opened along the grain, radiating out from the miter until they get away from the fixed miter joint and can physically expand and contract.

Solution? I have my thoughts, and I know how I have dealt with this exact same problem, but I'm curious how many readers agree with me, or disagree or can add to my thoughts above.

1/22/17       #4: Large wood desk top with miters. ...
Warren E

I thought about it alot when i did the last reception desk with solid cherry top and two mitres on 30" solid. mitres were clamped and lamelloed from top and bottom and all glued. Used cherry solid and set it on ply bolted it in the middle to the ply and outsides but loose and waxed the ply. Top was about 1 1/16" thick. No issues at all about edging opening up after about six years. Carpenter here told me it wont work, old guy didnt really comment when asked or I cant remember his answer now years later. wood came from my shop storage which was not great to a office that was heated nicely, seemed if it moved if it moved together and had no issues.

1/22/17       #5: Large wood desk top with miters. ...
Pat Gilbert

Were the tops allowed to move with changes in the humidity?

1/22/17       #6: Large wood desk top with miters. ...
Warren E Member

of course, bolted tight in the middle, not tight on outside edges and surface between the solid and the ply totally waxed so the solid could move as it wished.

it was all one piece the whole solid top, the ply being under it helped with moving it as well figured it would put less stress on the joint.

1/22/17       #7: Large wood desk top with miters. ...
Pat Gilbert

Then that is a different situation than Mike's situation.

1/22/17       #8: Large wood desk top with miters. ...
Warren E Member

his are mitres on 90 degrees mine were mitres on 45 degrees. Thinking back I might and probably did my tight bolts on the inside of the mitre and let the middle and outsides float as at the time the logic seemed better.

Too long now I think I reasoned that inside mitres had more potential to open so bolted that tight. Then outside edges shrinking back would come back together and stay tight

1/22/17       #9: Large wood desk top with miters. ...
Pat Gilbert

You said the top could float. To me that means the top could expand unimpeded. Which would be different than Mike's situation because of the walls that would impede expansion of the top.

1/22/17       #10: Large wood desk top with miters. ...
Warren E Member

I read his post, what walls? I dont see anything there that says its fixed between walls so it cant move?

1/22/17       #11: Large wood desk top with miters. ...
David Sochar

Pat, Irregardless of the floating tops, it is the miters that actually change in angle as the tops swell or shrink in net width.

This is the point, the reason I have never done it. But am I over cautious?

1/22/17       #12: Large wood desk top with miters. ...
Larry

I'm with David on this one. If you are lucky the tops may not move enough to be an issue but the joint will ultimately show a crack line. How big? How much is acceptable? How lucky are you feeling?

1/22/17       #13: Large wood desk top with miters. ...
Pat Gilbert

You're right Warren.

Dave, yes that is one problem the containment would add to the problem as with wood flooring that pops off of the floor.

I have had this problem with pieces that were only 4" wide changing the angle by 10 deg, of the pieces before they were assembled.

1/22/17       #14: Large wood desk top with miters. ...
Adam

I did an island L top 10 years ago. qtr white oak. It was roughly 3' wide 6' legs. I glued up the whole thing out of 4/4 and cut out the inside corner. That way the grain is continuous thru the miter. Looks awesome.

It is fastened hard on the cabinet side. The side that floats is supported by a frame with turned legs. You can't tell how much its moving because the reveals on the leg side is generous. I fastened it as loose as possible with 1 1/4" fender washers.

Obviously I had a lot of latitude on design, but it is possible to achieve large miters. I can see from above that its not perfectly square, but no one else knows to look for it.

1/23/17       #15: Large wood desk top with miters. ...
Chris H

Website: http://www.csaw.com

You should probably take a look at the Lamello P-system connectors. They are great for clamping miters, and pull with up to 200 pounds of clamping force. I'm the Lamello product manager for the US, and I used to run a shop near DC for ten years, so I can answer anything you need. Very easy to use system, tons of benefits and applications - but I don't want to go too far into the weeds here!

Lamello Zeta P-2

1/23/17       #16: Large wood desk top with miters. ...
Mike Fuson

Thanks for all the responses. I'm thinking it would be much easier if I can talk the client into maybe a quartz countertop. I can see this possibly causing a call back... Thanks again.

5/13/22       #17: Large wood desk top with miters. ...
Skylaryer Member

Just how long until these price cuts make their way into the aisles of big boxes? Michael Goodman, director of specialty products at Sherwood Lumber, tells Fortune each dip will take "about 60 to 90 days to get into the market."


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