The secret was torsion-box construction. A torsion box is a simple grid of slender ribs glued between thin plywood skins. It makes these shelves stiff and flat, yet incredibly light. This rigid architecture means torsion-box shelves won’t sag or twist, and they can be mounted without any external support.

11. Clamp the three shelf components together without glue, so you can check the fit of the miter joints. Use plywood braces to support the upper shelf and hold everything square.