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Butt-jointing plywood with glue and screws

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Forum topic by Brett posted 02-16-2012 03:14 PM 12719 views 0 times favorited 15 replies Add to Favorites Watch
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Brett

621 posts in 1336 days


02-16-2012 03:14 PM

I need to butt-joint two pieces of plywood with glue and screws. Will plywood tolerate having a screw driven into its edge (even with a pilot hole), or will the plywood split? What about butt-jointing normal wood, with the screw driven into end grain? How well will that hold up?

-- More tools, fewer machines.


15 replies so far

View SnowyRiver's profile

SnowyRiver

51451 posts in 2133 days


#1 posted 02-16-2012 03:17 PM

I guess I am not sure if you are screwing it on the face, or the edges. I use a pocket hole jig for 3/4 ply. It seems to work fine. I havent tried to glue ply thinner than that though.

-- Wayne - Plymouth MN

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crank49

3421 posts in 1624 days


#2 posted 02-16-2012 03:24 PM

Butt joints have almost no strength, even if the plywood does not split.
Could you possibly rabbit the edges so you would have a lap joint?

Or, as an alternate, this is a good application for biscuit jointery.

I think they make special screws for pocket hole joints in plywood that have flat heads to reduce splitting.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View CharlieM1958's profile

CharlieM1958

15696 posts in 2871 days


#3 posted 02-16-2012 03:34 PM

It really depends on the overall situation.

I have used butt joints, with screws going into the edge of plywood many times on shop furniture, jigs, etc., and they work perfectly fine. However, I would not use that type of joint to support a great deal of weight, such as a shelf for a planer, unless the overall structure included some other type of reinforcement. I’d give pretty much the same advice for solid wood as well.

-- Charlie M. "Woodworking - patience = firewood"

View BobM001's profile

BobM001

388 posts in 983 days


#4 posted 02-16-2012 03:38 PM

Multiple offset biscuit joints in a “saw tooth” pattern. Or if you have the room and your biscuit jointer blade is SUPER sharp stack two at each position with the width of a biscuit between each one. The “side grain” of the plys will glue OK, it’s the end grain where the issue lies.

-- OK, who's the wise guy that shrunk the plywood?

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Mosquito

4683 posts in 945 days


#5 posted 02-16-2012 03:52 PM

I’m not too knowledgeable when it comes to plywood, so I have to ask…
Is the “side grain” of the plywood the edge of the plywood that is running parallel to the grain (typically the 8’ sides on 4×8?) That’s my guess, but was just making sure…

-- Mos - Twin Cities, MN -- Stanley #45 Evangelist - www.youtube.com/MosquitoMods

View richgreer's profile

richgreer

4524 posts in 1727 days


#6 posted 02-16-2012 03:57 PM

I don’t think you are ever going to get a real strong joint when you put 2 pieces of plywood together. However, in some applications a real strong joint is not needed.

If I wanted a strong joint with plywood, I would run both pieces into a groove in a corner piece of solid lumber.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View CharlieM1958's profile

CharlieM1958

15696 posts in 2871 days


#7 posted 02-16-2012 04:12 PM

You can also, if you have room, use a glue block to support the joint. (For example, a piece of 3/4×3/4 stock running the full length of the joint.)

-- Charlie M. "Woodworking - patience = firewood"

View Brett's profile

Brett

621 posts in 1336 days


#8 posted 02-16-2012 06:53 PM

The diagram below shows a rough picture (it’s a jig for sharpening chisels on a bench grinder). The bottom and side pieces are either plywood or real wood. The gray thing across the top is threaded rod. There are other parts, but I’ve left them out.

If I use plywood for the two uprights, I’ll need to drive screws up through the base into the edges of the plywood. I can use real wood if plywood won’t work. In either case, I can probably cut some dados for the bases of the uprights to give them more stability.

Or, maybe I should mount the uprights on the sides of the base and drive the screws horizontally through uprights into the base:

The base is a big slab of 2”x12” pine, so this design would require driving screws into the end grain of the base.

-- More tools, fewer machines.

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patron

13034 posts in 1994 days


#9 posted 02-16-2012 07:06 PM

if you have the room for it
a third piece as a brace
will keep the sides from ‘wobbling’
loosening the joint
as in box construction
or cabinets

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View NiteWalker's profile

NiteWalker

2710 posts in 1230 days


#10 posted 02-16-2012 07:08 PM

Brett, I would go with option #2 above, but take it a step further and rout a very shallow 1/16” rabbet on the end pieces. Not for additional strength, but to make assembly easier. With plain butt joints, things like to slip around when glue and clamps are applied.

Predrill for the screws, glue it up and everything will work great. Just use long thick screws.

If you go with option 1, I’d definitely bring the ends in about 1/2” each and cut 3/8” or even 3/4” dadoes since you’re using 2×12” pine.

-- He who dies with the most tools... dies with the emptiest wallet.

View Brett's profile

Brett

621 posts in 1336 days


#11 posted 02-16-2012 07:21 PM

NiteWalker, if I go with option 2, should the rabbets in the end pieces be made where the base touches the end pieces? (Sorry for the beginner-level question.)

-- More tools, fewer machines.

View Jimmy0x52's profile

Jimmy0x52

31 posts in 953 days


#12 posted 02-16-2012 09:50 PM

Kreg Jig.

It’s made for this specifically.

-- Jimmy, Austin, TX - Software developer by day, woodworker by night

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NiteWalker

2710 posts in 1230 days


#13 posted 02-16-2012 10:51 PM

Brett, yep, rabbet the end pieces where they attach to the base.

And the kreg jig would work better for option 2 as well. Definitely go that route if you have one.

-- He who dies with the most tools... dies with the emptiest wallet.

View crank49's profile

crank49

3421 posts in 1624 days


#14 posted 02-16-2012 11:14 PM

I thought you were talking about a true butt joint, two pieces of plywood butted end to end.
The right angle connection you are planning should be fine.

If you need extra holding power for that joint you can put a large (3/4” to 1”) dowel through the “end-grain” piece in the path of the screw and about an inch away from the end. That makes a very strong joint.

I believe I’d go with your first example and make the base piece about an extra 1” long so the screws would not be so close to the edge.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Loren's profile (online now)

Loren

7550 posts in 2301 days


#15 posted 02-16-2012 11:51 PM

Plywood screwed through the edge doesn’t perform as well
as most solid wood in screwholding, especially near the edges
where the plywood can split if the joint is stressed.

Confirmat screws offer the best performance in this sort
of application. The holes are best drilled with a special
bit.

-- http://lawoodworking.com

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