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post #1 of 14 Old 02-20-2012, 03:51 PM Thread Starter
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12 1/2 foot desktop questions

Helping a friend build a built-in for his home office, I've got the filing cabinets and lateral file covered, questions are coming up around the desk surface that will sit over the top of the filing cabinets and the lateral file.

The plan is to rip a 48"x96" sheet of veneered plywood down the middle, edge the front face with 1.5" x 1.5" hardwood and edge end joint between the two sheets with the same hardwood.

Some of the questions coming up are:
How thick should the plywood be? (3/4" is what I'm thinking - veneered on both sides to relieve stress)
Should I back the plywood with MDF to build up the thickness?
How do I finish the desktop off-site and assemble on-site such that the seam in the middle isn't noticable?
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post #2 of 14 Old 02-20-2012, 04:21 PM
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You can start with 3/4" hardwood plywood and for the edge use 1.5" x 3/4" hardwood, with the 1.5" edge facing out. The center seam can be 3/4" x 3/4" and both pieces share that edge (attached to just one piece). Offsite, prefit both pieces with a block sander for the mating edges.


You coud use "tite joint fasteners" to bring the two sections together once on the site. A rear cleat on the wall, "L" shape (to screw down the tops), will hold the back. The tops should be supported along their length.







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post #3 of 14 Old 02-20-2012, 04:30 PM
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I would do this

You want to end up with a plywood surface that's 24", (actually 23 7/8" after a saw kerf) x 150" right? Does the seam fall on top of a support like a file cabinet?
If not, I would support it all the way undeneath with an additional 3/4" ply or 3/4" MDF. You can edge the front like you said, but I'd leave the ends off unless they are visible.

I would make a half lap seam so that each section laps over the other by 1". I would glue and assemble the top off site, finish it off site and make it structurally strong enough to carry it to the install without fracturing the seam by backing it with a section of 3/4" which can be permanent or unscrewed at the location. You could just back the entire length which will make it quite heavy, but that's the way I prefer.

You would like the joint to be invisible but because of the grain that won't happen, but match the grain as close as possible. Don't use a wild grain but try to find as straight a grain as possible when you pick out the ply.

If the ply were sitting in a rabbett for the front and rear edge trim, a backer may not be needed. You'll need a helper with a knowledge of lifting heavy panels, so carry the thing on edge or vertically until it's going on the final surface. bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 02-20-2012 at 09:14 PM.
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post #4 of 14 Old 02-20-2012, 04:44 PM
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Originally Posted by woodnthings View Post

I would make a half lap seam so that each section laps over the other by 1". I would glue and assemble the top off site, finish it off site and make it structurally strong enough to carry it to the install without fracturing the seam by backing it with a section of 3/4" which can be permanent or unscrewed at the location. You could just back the entire length which will make it quite heavy, but that's the way I prefer.
Carrrying a top that long with a glue joint like that will break. I know, I've done enough of them. I would assemble on the site, with the tite joint fasteners, and along the underside seam add a 3/4" plywood cleat, 12" (or more) wide by the depth of the top, covering the seam, screwed to both sides (approx 6" per side).






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post #5 of 14 Old 02-20-2012, 06:21 PM Thread Starter
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Quote:
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You can start with 3/4" hardwood plywood and for the edge use 1.5" x 3/4" hardwood, with the 1.5" edge facing out. The center seam can be 3/4" x 3/4" and both pieces share that edge (attached to just one piece). Offsite, prefit both pieces with a block sander for the mating edges.


You coud use "tite joint fasteners" to bring the two sections together once on the site. A rear cleat on the wall, "L" shape (to screw down the tops), will hold the back. The tops should be supported along their length.
.

Thanks for the welcome!

Tite-joint fasteners are exactly what I was trying to come up with - used them when installing some pre-fab counter tops a few years ago.

The wood used for the front edge face and center seam will be in contrast to the center panels (think wenge and oak or something similar)- rather than trying to hide the joint it will be emphasized intentionally. I should have been a little more clear in my initial description.

Fastening the two panels together with the tite-joint fasteners takes care of that problem. The next is how to literally finish the two separate surfaces as one yet still be able to separate them for transport/assembly. If I join them and lacquer/poly/.../etc, how do I separate without ruining that coating? I can't finish them individually and hope for the same build height at that union..


Structurally, the edge banding should help span the unsupported front - the longest unsupported run will be 33". In the rear, an L shaped cleat to attach the top - from the underside..then because walls are never perfectly flat, a small strip of trim will sit around the edges (not 100% clear on this piece, I just know every time I've put a long run of anything perfectly straight up against a wall there's always a gap or a bump or three). My inclination is to rabbet the back side leave a 1/4" lip up top and scribe the piece to the wall...depending on how far out of whack things are - this is a deal with it when I get to it item.
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post #6 of 14 Old 02-20-2012, 06:32 PM Thread Starter
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Forgot my manners.

Thanks both of y'all for the responses, they were really appreciated.
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post #7 of 14 Old 02-20-2012, 06:47 PM
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If you plate cleat the backside with 3/4" plywood about 12"-18" on both sides of the seam, you may be able to finish and install it. I'm not suggesting it, but I've had good luck and bad luck doing that.

Leaving a 1/4" section from a rear rabbet could be used for scribing to the wall.






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post #8 of 14 Old 02-20-2012, 08:34 PM
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Quote:
Originally Posted by woodnthings View Post
You want to end up with a plywood surface that's 24", (actually 23 7/8" after a saw kerf) x 150" right? Does the seam fall on top of a support like a file cabinet?
If not, I would support it all the way undeneath with an additional 3/4" ply or 3/4" MDF. You can edge the front like you said, but I'd leave the ends off unless they are visible.

I would make a half lap seam so that each section laps over the other by 1". I would glue and assemble the top off site, finish it off site and make it structurally strong enough to carry it to the install without fracturing the seam by backing it with a section of 3/4" which can be permanent or unscrewed at the location. You could just back the entire length which will make it quite heavy, but that's the way I prefer.

You would like the joint to be invisible but because of the grain that won't happen, but match the grain as close as possible. Don't use a wild grain but try to find as straight a grain as possible when you pick out the ply.

If the ply were sitting in a rabbett for the front and rear edge trim, a backer may not be needed. You'll need a helper with a knowledge of lifting heavy panels, so carry the thing on edge or vertically until it's going on the final surface. bill
If you do finish the whole thing off site then be VERY careful of how you carry it. DO NOT carry it flat. Carry it upright like a sheet of granite. And, if at all possible, have a third person help in the middle.

George
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post #9 of 14 Old 02-25-2012, 07:31 AM
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If you don't have a problem getting a 12' long top into the room, I don't see a problem with making the top and pre-finishing it. If the top is to go between two walls check the walls with a framing square to see if they are correct. You may have to make the top a little out of square to accommodate the walls. Anyway it's better find out this problem when you are building the top instead of when your there with a finished top. Assembly the seam with biscuits and glue and clamp it. You can put two or three pipe clamps together to clamp it. If you can, glue and screw some permanent cleats to the bottom over the seam. If not, screw some on until you get the top where it goes. Making a end to end seam will always show. The best you can do is make it a good one. 3/4" plywood is finish sanded after it is sized and rounds the end of the sheet a little. When you put two factory ends together it looks like the two pieces of wood are chamfered a little making the seam show up more. I would cut the factory ends off. When you put the 1 1/2" edge on use 12' long wood if possible. If not put the seam at the opposite end from the plywood seam. What info I'm not seeing is how much support the top will receive. If you have any spans more than 3' the 1 1/2" edge won't be enough to prevent it from sagging. You may have to make the edge a little wider and put a piece of steel angle behind it.
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post #10 of 14 Old 02-25-2012, 10:45 AM
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If you wish to use two layers of ply just use two 4' pieces & two 8' pieces. Stagger the joints opposite of each other. Each layer will have a 4' & 8' piece. Then apply your edging.

James
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Just because you can, doesn't always mean you should!
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post #11 of 14 Old 03-03-2012, 01:07 PM Thread Starter
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Steve - on something like this, I won't try to hide the joint, I'll accent it with some solid wood in between the sheets of plywood. Trying to hide it is an exercise in futility - been there done that.

We've bantered back and forth on the possibility of a torsion box for the desk instead of slabs of plywood - that's still up in the air and it would be the single longest torsion box I've ever built. Who has a 13 foot long flat surface I can borrow for the glue-up???

Other questions have arisen - posting about that shortly.
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post #12 of 14 Old 03-03-2012, 03:22 PM
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will 11 ft work?

Maybe this is crazy but why not use solid core doors?
They are 6' 8", that's 12' 16" and you need 12' 6", so 5" off each...... Then skin the doors with 1/4" plywood or greater if you want maybe 1/2" then edge the outsides. They come in 30" and 28" wide also.
Most of my benches are SOLID core doors. in 30" wide. These things are HEAVY in 1 3/4" thick, but you can get them in 1 3/8", a bit lighter. You then seam the plywood other than where the doors join. You can rout a slot for a spline to join the doors flush with one another. Then skin them. I like it. but then I thought of it. Sometimes I agree with myself even more after reading my own posts..... bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #13 of 14 Old 03-03-2012, 08:02 PM Thread Starter
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+5 points for thinking outside the box.

Solid core doors - flat, stable and HEAVY! I'll add this to the suggestions for the desktop - though this one by far would be easiest - thin solid core doors skinned in hickory.

I'm working on sorting out the rail/stile dimensions since the base cabinets will be built w/o toe kicks all the way to the slab and then laminate flooring and quarter round will be installed...if anyone gets a change, comments on my other thread re: check my math, give me some opinions - would definitely be appreciated.
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post #14 of 14 Old 03-03-2012, 11:51 PM
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Making both pieces of the top out of the same sheet of plywood there is a good chance the grain will match up pretty good going end to end. If you make a clean tight joint it should look pretty good. At one time I worked for a shop that cared more about speed than quality. We had to glue the factory ends together and use a gun that shot corrugated fasteners on the under side to make a long top like that. Had to use a lot of putty and it did not look good at all. I never got the opportunity to see them with a finish on them. I bet the painter was cussing us.
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