Does the flimsy back panel on flat-pack furniture actually matter?

Nothing in a flat-pack kit looks less important than the back: a floppy sheet of hardboard, a bag of pins and a fiddly ten minutes at the end of a long build. It is also the component that turns four boards and some pivoting corners into a rigid box. Skipping it or half-doing it is the single commonest reason flat-pack furniture ends up wobbling.

What you'll need

• Panel pins or 15mm screws
• Small hammer, or a driver on low torque
• Tape measure, for the diagonals
• Replacement 6mm MDF or plywood sheet, if upgrading (optional)
• Straight edge and a sharp knife or a saw (optional)

Step by step

1. Understand what it does before deciding how much to care.A rectangle with pivoting corners has no resistance to leaning. A diagonal brace would stop it; a sheet fixed round the perimeter does the same thing in every direction at once. That is the back panel's entire purpose.
2. Square the carcass before the back goes anywhere near it.Measure both diagonals across the back. Equal within about 3mm. The back panel does not make a carcass square — it makes it permanent, and whatever shape it is in is the shape you keep.
3. Fix it at close centres, all the way round.Every 100 to 150mm along every edge, and into any mid rail or fixed shelf as well. Between widely-spaced fixings the thin sheet simply flexes and the carcass carries on racking.
4. Seat it in the grooves if there are grooves.Better units have a rebate or groove for the back. A back sitting proud of its groove at one corner is a carcass that is not square, and forcing pins into it locks that in.
5. Use screws rather than pins where you can.15mm screws into the panel edges hold far better than pins, do not work loose, and can be removed later if the unit needs squaring again. Pilot the holes at the edges of chipboard to avoid splitting it.
6. Replace a cardboard back on anything tall or loaded.Some cheap units use corrugated board. Cut a 6mm MDF or plywood sheet to the same size, square, and fix it all round. It costs a few pounds and transforms the unit — this is the highest-value upgrade available to flat-pack furniture.
7. Cut a replacement square, not to the old one.If the old back was fitted into a racked carcass, copying it copies the error. Push the carcass square, measure the opening, cut the new sheet to that.
8. Deal with the cut-outs deliberately.Where a unit needs a hole for a socket or a cable, cut the smallest opening that works and keep it away from the perimeter. A large hole in the middle costs little rigidity; a slot cut in from an edge costs a lot.
9. Anchor the unit to the wall regardless.A properly fixed back stops racking. It does not stop tipping, and it is not something to anchor into — the anchor goes into the solid frame, top or side.

Tips

• If a wall-fixing screw has to pass through the back panel, use a large washer, or better, fix through the solid top rail instead.
• Pins bent over during fitting are worse than useless — they hold nothing and split the hardboard. Pull them and start the next one straight.
• On a unit that will be moved often, screws through the back are worth every extra minute. Pinned backs loosen a little with each move.

Common mistakes

• Leaving the back off because the unit stands against a wall anyway — The wall stops it falling backwards. It does nothing about the sideways lean, which is what actually happens.
• Fixing the back with four or six pins — The thin sheet flexes between fixings. Rigidity comes from close centres, not from the sheet being present.
• Forcing a back panel into a carcass that will not accept it — If it does not fit, the carcass is out of square. Forcing it locks the error in and the doors will never sit right.

If it doesn't work

Questions people ask