How do I build a bookcase that stays square and does not sag?

A bookcase looks like the simplest carcass there is and fails in three predictable ways: it racks into a lozenge, the shelves bow under load, and it tips. All three are design decisions taken before any wood is cut, and the fixes cost almost nothing if you make them at the start.

Safety

What you'll need

• Timber or sheet material for sides, shelves and top
• Plywood or hardboard for the back
• A router or saw for the housings and rebate
• Clamps and glue
• A wall fixing bracket or strap

Step by step

1. Decide the shelf span before anything else.Sag scales sharply with span and is the fault people notice most. Keep the distance between supports within what your material can genuinely carry loaded with books, or plan a vertical divider in the middle of a wide case.
2. Choose the shelf material for stiffness, not appearance.Solid timber and good plywood are stiff; MDF and chipboard sag noticeably over any real span. A front lipping glued on the leading edge of a shelf stiffens it far more than adding thickness.
3. Cut housings for at least some fixed shelves.A shelf housed and glued into both sides ties the carcass together and stops the sides bowing outwards. Two or three fixed shelves in a tall case do most of the structural work.
4. Use adjustable pegs only where you need adjustability.Pegs carry load poorly and contribute nothing to rigidity. Mix them with fixed shelves rather than using them throughout.
5. Rebate the back edges of both sides and the top.A rebate the thickness of the back panel plus a millimetre, right down both sides. It takes minutes with a router and it is what turns a stapled-on back into a structural component.
6. Assemble, glue and check square before the back goes on.Diagonals across the front of the carcass, and check it is not twisted by standing it on a flat floor. Once the back is glued in, the case is fixed in whatever shape it is.
7. Glue and pin the back into its rebate.Glue all round the rebate and pin at intervals. This single component is what stops the case racking. Set it square and pin the corners first.
8. Add a plinth or feet.A recessed plinth stops the bottom shelf sitting in the dust and lets the case stand against a skirting board. It also stiffens the base.
9. Scribe to the wall if the case is fitted.Walls are not flat or plumb. A scribing strip on the back edge of each side, planed to the wall's contour, closes the gap that otherwise shows.
10. Fix it to the wall before you load it.A bracket or strap into a stud or a masonry fixing at the top. This is the step that matters most and the one most often skipped.

Tips

• Design the shelf heights around real books before cutting, allowing for paperbacks on some shelves and larger formats on others, plus clearance to get a book out.
• A vertical divider in a wide bookcase halves every shelf span and is far cheaper than doubling the shelf thickness.
• If the bookcase sits on carpet, it will settle unevenly. Set it on a plinth spanning front to back, and check it for rock before fixing it to the wall.

Common mistakes

• Building the whole case with adjustable shelf pegs — The carcass has nothing tying the sides together and racks, and the shelves themselves carry load on four small pegs. Fixed housed shelves solve both.
• Nailing the back onto the rear edges rather than into a rebate — The back can shear off its fixings, and the case racks. Rebated and glued, it is a structural panel.
• Spanning too far with MDF or chipboard shelving — Both creep under sustained load — they sag slowly over months and never recover, even when unloaded.

Questions people ask