How do I get a flat-pack unit square, and what if it is already built wrong?

Square is not an aesthetic standard, it is what makes doors close, drawers run and the carcass stop moving. And there is no way to eyeball it: a rectangle that is 8mm out of square looks fine and behaves badly. The diagonals are the only honest test, and they are the test almost nobody does.

What you'll need

• Tape measure
• Screwdriver
• A flat floor or two straight battens to build on
• Sash clamp or a strap, for pulling a stubborn carcass (optional)

Step by step

1. Build on a flat surface, not on carpet.A thick carpet lets one corner sink and builds the twist in from the start. Assemble on a hard floor, or on two straight battens laid across the carpet, and move the unit to its final position afterwards.
2. Measure corner to corner, both ways.Inside corner to opposite inside corner, then the other pair. Two numbers. If they are equal the carcass is square; a rectangle with equal diagonals cannot be anything else.
3. Interpret the difference as half the error.If the diagonals differ by 10mm, the top of the carcass is about 5mm out of position. That is more than enough to make a door rub and a drawer bind.
4. Push the corner at the end of the longer diagonal.The longer diagonal needs to shorten, so push the two corners it connects towards each other. A firm shove is usually enough on an unfixed carcass; a strap clamp across the long diagonal does it without effort.
5. Re-measure while it is held, then fix the back.The back panel is what holds squareness. Fix its first two corners, re-measure, then work round the perimeter at 100 to 150mm centres. Never fix all four corners and then check.
6. For a unit already built and wrong: slacken the back rather than the joints.Take out enough of the back fixings that the carcass can move. Leave the cams tight. Push square, re-measure, re-pin. The joints do not need touching.
7. If the back is one-piece hardboard that has been racked, replace it.A hardboard back that has been forced into a leaning carcass and pinned there is now cut and drilled to the wrong shape. A new 3 to 6mm sheet cut square is a few pounds and solves it outright.
8. Check for twist as well as lean.Sight along the front edges of the carcass. If one front corner stands proud of the plane the other three make, the unit is twisted rather than racked, which usually means the floor. Pack the low foot rather than fighting the carcass.
9. Then re-tighten everything and check the doors.Cams a half turn, back fixings firm, doors adjusted last. Doors set to a square carcass stay set; doors set to a leaning one drift.

Tips

• The 3-4-5 method works too if the unit is too big for a diagonal: mark 300mm along one edge and 400mm along the other, and the distance between the marks should be 500mm.
• On a tall unit, build it lying down on its back and stand it up only after the back panel is on. It is far easier to keep square flat, and far easier to lift.
• If a supplied cardboard back is genuinely floppy, replace it with 6mm MDF or ply at the same time. It doubles as the rigidity fix.

Common mistakes

• Judging square by eye or against the wall — Walls are not square either, and a few millimetres of racking is invisible but completely changes how doors and drawers behave.
• Fixing the whole back panel before checking the diagonals — The back is the thing that decides the shape. Once it is fully fixed, the carcass is whatever shape it was in at that moment.
• Trying to correct a racked carcass by adjusting the hinges — It hides the symptom in one door and moves it to the next. The lean is still there and still moving.

If it doesn't work

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