French Cleats for Hanging Bookshelves?

I have a commission to make some wall-to-wall hanging bookshelves roughly 45 inches tall spanning 137 inches. I will make these as face frame cabinets in 5 pieces, with the longest span appx. 37 inches. So far, so good. I had considered hanging these with French cleats but then it occurred to me that after routing a rabbet for a 1/2 inch cleat and a 1/4 inch back, the top of the case might not be stong enough to hold the weight of the cases and the books. The rabbet would be 3/4 inch deep and 1/4 wide, leaving essentially 1/2 inch ply holding the entire weight of the cabinet and books. I suppose it would be possible to rout only a 1/4 inch groove and trap the back, but then I'd still have only 1/2 inches of ply at the location of the groove.

Is this strong enough, in your experience, or should I go back to a stretcher between the sides on the inside of the back, much as most kitchen cabinets are mounted?

I would certainly add the stretcher. Aside from the considerations you mention, all of the weight is being fed through the joint between the top and the side. The added joint between the stretcher and the sides would help feed the loads from the sides to the wall.

(Come to think of it, I've never seen a "French cleat" that didn't have a crosspiece on the back of the unit being hung. Could be wrong, but I thought part of the definition was that the stretcher and the cleat came from the same piece of wood.)

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Originally Posted by Lee DeRaud

(Come to think of it, I've never seen a "French cleat" that didn't have a crosspiece on the back of the unit being hung. Could be wrong, but I thought part of the definition was that the stretcher and the cleat came from the same piece of wood.)

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Thanks for your response, Lee. There would be a crosspiece on the back of the unit being hung. But my concern was that the crosspiece would be attached to the top, which would be reduced to 1/2 inch in thickness. Of course, it could be attached also to the sides.

Dan

I don't know if you're limited to the depth of your cabinets, but can't you just set the French cleat inside the cabinet by increasing the depth of the side panels? Recess the cleat on the cabinet and maintain full depth on your basic structure? Sorry I don't have anything to draw with on the computer, but picture your completed cabinet (without any modification for the cleat) Now picture the cleat on the back, and then the top, sides and bottoms extended to the depth of the cleat. So, once the cleat is attached to the wall you will be able to hang the completed cabinet flush and have the cleat hidden. You should be able to maintain "full strength" with this approach, you haven't removed any wood from the basic structure to install the cleat.

Jay

I know this is a woodworking site but the French Cleat does not have to be made from wood. You can fabricate the same design features of the french cleat from a piece of steel and counter sink it into the sides and top of the Bookcase. This wood give you the strength you need to hold the weight of the cases and books. I don't know how you cases are constructed but is it possible to use a french cleat at the bottom and the top to gain the extra strength for the weight of the cases and books. Just some thoughts off the top of my head.