Wall unit bookshelf Shelving question - Warping walnut ???

First, you can use dowels or biskets. There will be less warping / sagging with glued up shelving if you take care to use opposing grain directions, however, the shelves over 36” will be more prone to sagging under the weight. IMHO, either rethink the widths or increase the shelving to 5/4 glue up that should be sufficient for the 4’ shelves . I have also seen what I call strong backed shelves where a strip is glued and screwed to the bottom centers of the shelves to make them more rigid taking care to cut those strips so grain is perpendiclar to the downward force. IMHO plywood would be worse on the 4’ expanse…

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Walnut is one of the best cabinet woods. It is pretty stable.

There’s a lot of walnut in that job. Dowels are not needed
in edge joints for strength, though they can be used for
alignment. Biscuits are a little easier to work with in alignment
applications.

Plywood will sag over those spans without reinforcement.

Walnut ply doesn’t take a finish the same as solid walnut –
the glue in the veneer pores affects the color.

Fixed shelves sag less than floating shelves.
At 1 full inch thickness, and assuming 12 inches deep, a 48 inch shelf may sag noticeably. The 36 inch span shelves should be ok.

There is a shelf sag calculator here

I use about 10 pounds per linear foot for bookshelf loading. Depending on the books, this number might be higher, but in most cases it’s fine.

The solid walnut is much better on the long span than plywood. Even plywood with an edge attached doesn’t come close.

A couple of years ago I did a very large set of display shelving for a tile store. Some of it was built very similar to what you are doing with pin supports for shelves that were around 36” long. The plans were drawn by an architectural design company. They specified two layers of shop grade birch plywood glue laminated with a 3/4 X 1 1/2 hardwood edging. Tile is about as heavy as anything you would want to put on a shelf and they have held up well. Having never used walnut for a large project before I really am not qualified to advise you on whether or not it would work well for your application. As for cutting the plywood buy one of the many circular saw guides or just use a strait edge and a couple of clamps to break it down into manageable pieces before you move it to your basement.

-- A tube of calk and a gallon of paint will make a carpenter what he ain't

Thank you all for the input and advice.

pjones46: I would definitively use dowel for alignment. The width is fixed. I cannot make then larger than 9”.

Charlie: Thank you for the sag calculator. Very useful. I will use 1” thick if we go ahead with this project.

A question I should have asked. If you had to build a similar bookshelf, which wood would you use?