Plywood bookcase mistake

[Roger Marty]

I have little experience with plywood. My only use has been drawer bottoms.

I decided I needed a quick bookcase in the basement. Instead of joining/prepping hardwood, I decided to give plywood a try. My usual woodsupplier was closed yesterday, so I decided to buy Home Depot's finest-- a 4x8 sheet of "PureBond" 3/4" thick Birch for $54.

The whole thing was a mistake! I did a rabbit joint at the top and bottom and reinforced with screws. Shelves of dadoed-- also reinforced with screws. The result is about 68" high and 34" wide.

Even with all the screws (which I completely avoid with solid wood), the result is still... FLIMSY! Sigh. Bad design for plywood or simply bad plywood?





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[Cary Falk]

While HD plywood is not the best I think your design needs a little help. A piece of 1/4" plywood on the back would do a lot to stiffen it up. If you don't want to put a back on it then it needs a face frame to stiffen it up. You didn't say how deep it is. At 34" wide the shelves will probably sag without any support(books are heavy).

[Jerry Miner]

It's flimsy because it has no back. Lots of plywood cases like this exist--and are doing quite well. 1/4" ply to cover the whole back would be ideal, but even a "stiffener" cleat at top and/or bottom would help a lot. A nosing on those shelves would add stiffness to those.

[Jim Morgan]

Did you use glue? Rabbets and dados by themselves have little strength to resist movement, compared to dovetails, finger joints, or mortise & tenon. Rabbets seem like a particularly poor choice for the case joinery - you would be better off with simple butt joints reinforced with dowels, Dominos or biscuits. Some kind of diagonal bracing on the back (which a solid back would provide) will keep the corners square.

[Mike Cutler]

Roger

Please don't take this wrong, but if that is the final result, it is the design that is the central issue.
We all bash the quality of HD plywood, but 3/4" plywood in the vertical plane is very strong. Solid wood would have done the same thing. I promise.
You need to add shear strength to your design. In other words, you need to strengthen the shape to avoid it being able to "rack", or move side to side. You have a reflex parallelogram at this point,meaning that all of the joints can move in the same plane, at the same time, at the same rate.
The strengthening, or stabilizing, can be done in many ways. You could put a back on it made of 1/8" or 1/4" plywood, and screw through to all of the shelves, plus the top and bottom, which would definitely strengthen it. Corner braces at each joint along the vertical would also work.
Put a ply wood back on it and put a 4" wide length of material across the top connecting the top and sides. You may need this anyway, if it's going to stand against a wall. Trim molding is generally 3/4" thick, so a 3/4" stretcher across the back, at the top, would allow the shelf to fit flush to the wall. If you have kids you want to attach this to the wall anyway, and that stretcher at the top will become your screw plate.
34" wide shelves made of ply is going to stretch it a little bit from a sagging perspective. Your shelves may sag if they are heavily loaded with books over time. To counteract this a piece of 1x2 trim wood fastened to the underside of the front of the shelves would stop this.

You have the basics of a very sturdy shelf. Frame the face with nice 1x2 material in a complimentary wood and tie that back together and that shelf will last your lifetime.
Nice joints by the way. I know how hard it is to get dado joints in a tall shelf, that line up with each other correctly.

[Wayne Lomman]

Put a thin ply back on it and it will be solid as a brick s...house. Staple/flathead nail the back on at 150 centres everywhere. It will hold it square and support the shelves. You don't even need the dadoes then. Cheers

[David Eisenhauer]

They all said it. I have built tons of those without screws (all glued) and a 1/4" back rabbeted into the back of the sides/top/bottom. I "dot" a little glue in the narrow rabbet ledge to prevent squeeze out and then either pneumatic staple or hand nail with brads the back in place. The back will ensure the case remains square and prevent "rickety" from showing up. Yours is fixable as is. Some book cases I built had a FF, some just banding to hide the raw plywood edge. Your call. 34" wide would cause me to add in a FF with approx. 1"- 1-1/4" wide rails to help prevent sagging.

[Jim Becker]

You need something to structurally reinforce this to prevent racking...a back or braces...whatever. The same would be true if it were built of solid wood. A "box" will not keep square with lateral pressure unless there's something to keep it from moving in that direction.

This is all unrelated to the perceived quality of the actual plywood...there's a very long and recent thread about that kind of thing.

[Roger Marty]

Thanks. I already have a rabbit cut into the back to take a 1/4" plywood backer.

Can you say more about what a "face frame" looks like? As well as a 1x2 trim wood to reinforce the shelf?

My last bookcase was made of solid oak. Similar construction. Very sturdy.

[Mike Cutler]

Thanks. I already have a rabbit cut into the back to take a 1/4" plywood backer.

Can you say more about what a "face frame" looks like? As well as a 1x2 trim wood to reinforce the shelf?

My last bookcase was made of solid oak. Similar construction. Very sturdy.

Roger

A face frame is simply a frame that attaches to the front, to enhance the appearance, and hide the plywood edge. Many kitchen cabinets are an example of face frame construction. For the project you have it would look like a wide ladder. The outer upright edges of the "ladder" would align with the outside edge of your plywood sides. The "rungs" would align flush with the top of each shelf. This face can be glued, screwed, pin nailed, stapled, or any combination of these. to the plywood core edge.
"1x2" is a nominal dimension for wood. The actual piece of wood would be 3/4" thick, or 4/4, and probably be 1-1/2" to 1-3/4" wide..

[Roger Marty]

Roger

A face frame is simply a frame that attaches to the front, to enhance the appearance, and hide the plywood edge. Many kitchen cabinets are an example of face frame construction. For the project you have it would look like a wide ladder. The outer upright edges of the "ladder" would align with the outside edge of your plywood sides. The "rungs" would align flush with the top of each shelf. This face can be glued, screwed, pin nailed, stapled, or any combination of these. to the plywood core edge.
"1x2" is a nominal dimension for wood. The actual piece of wood would be 3/4" thick, or 4/4, and probably be 1-1/2" to 1-3/4" wide..

Ah, thank you. I've learned something new for my next plywood bookcase.

In this case, I had already ran the shelfs and sides through the router table to slightly round off the corners. So I won't be doing a face frame on this one. I did install a 1/4" plywood back. Hopefully that makes it sturdy enough. I remain slightly concerned about sag (even with lots of screws to the backing plywood) and am considering putting a 1" reinforcement underneath the front of each shelf.

[Roger Marty]

BTW-- I've done a few boxes with box joints. And they stay square with nothing else reinforcing it.

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[Jim Morgan]

BTW-- I've done a few boxes with box joints. And they stay square with nothing else reinforcing it.

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Correctly cut, finger (box) joints, like dovetail joints, are self-squaring. There is a world of difference between these and the rabbets used in the plywood bookcase.

[Osvaldo Cristo]

I have little experience with plywood. My only use has been drawer bottoms.

I decided I needed a quick bookcase in the basement. Instead of joining/prepping hardwood, I decided to give plywood a try. My usual woodsupplier was closed yesterday, so I decided to buy Home Depot's finest-- a 4x8 sheet of "PureBond" 3/4" thick Birch for $54.

The whole thing was a mistake! I did a rabbit joint at the top and bottom and reinforced with screws. Shelves of dadoed-- also reinforced with screws. The result is about 68" high and 34" wide.

Even with all the screws (which I completely avoid with solid wood), the result is still... FLIMSY! Sigh. Bad design for plywood or simply bad plywood?

I have been very successful with plywood shelves. My experience with simple bookcases intended originally to be an interim solution and turned "definitive" can be reached in that thread.

Basically I used a very basic project, no dados, no glue... only some wood screws... and my results are great.

BTW I agree with some previous posts about the need for some shears to reinforce the construction for an empty book shelve. My experience shows the load can also help a lot as it can minimize proportionally the spurious lateral forces with a significant higher downwards force. My bookcases are rock solid... but there are 1.5 ton of books driving the resultant force downwards...

All the best.

[Ted Phillips]

You might think about using solid pine for the back instead of plywood. Get some thin pine (think 1x4) and shiplap them vertically on the back of the case. Then screw the case sides and each shelf to the shiplapped back boards - don't glue them. This has the advantage of adding a lot more strength to the structure while allowing for seasonal wood movement. Plus, it looks a lot neater than plywood. No racking, no shelf sag.

TedP