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A door stop does not hold the door. It parks it at an angle.

The door reaches the stop and springs back · It holds for a second then swings open · It is stuck to the magnet but a nudge frees it · The door hits the wall before the magnet · The stop is on the floor and the door never reaches it · It was replaced and it still does not hold · The plate on the door is rusty · The whole stop pulls out of the floor
FreeMeasure the gap with a stack of business cards
FreeRaise the magnet two millimetres, or re-drill the position
BoughtA plate or a magnet, only when the part itself is gone
Measure the gap Why two millimetres matters
Easy · 20 min · Two of the three answers are free, and neither of them is a part

An elevation of a door edge in section with a steel plate screwed to it low down, and a floor-mounted magnet body sitting on the floor beside it, with a dashed line showing how the two faces line up and a red dimension where they do not; below left, a plan view of the wall, the hinge, the door swung open and the magnet sitting on the arc, with the arm from the hinge marked; below right, three bars showing the force left at two, four and eight millimetres of gap

diagnose
What failed
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Where
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what to touch fault correct Click the door, the plate, the magnet or the floor

Force falls off with the gap, and the gap is invisible

Two facts, and the second is the one nobody measures. A magnetic door stop is a permanent magnet and a plain steel plate, and the force between them collapses as they separate. On a small neodymium stop it is roughly three kilograms at half a millimetre, about one kilogram at two millimetres, a quarter of that at four, and effectively nothing by eight. It is not a gentle slope. That is why a stop can feel as though it is touching — you can even see the two faces apparently together — and still let the door swing.

The second fact is what makes this page short. The stop does not hold the door; it parks it at an angle. The magnet only ever has to beat the small component of the door's weight trying to swing it, so almost any intact magnet is strong enough. What decides where the door stops, and whether it can reach the magnet at all, is geometry: how far the magnet sits from the hinge, and how high its face sits above the plate. Ask yourself which of those three is a strength problem, and the answer is almost always none of them.

So the test is a measurement, not a judgement. Stack business cards — about 0.3 mm each — under the magnet's face until the stack just touches the plate, and count them. Three cards is a millimetre. Ten is three millimetres, which is already marginal. If you can get fifteen in, you have found the fault and it is not the magnet. Then, with a tape, measure from the centre of the hinge to the centre of the magnet: it should be between two thirds and three quarters of the door's width. Closer and the arm is short so the same force gives less turning resistance; further and the door runs out of swing before it reaches the magnet.

Three faults, three measurements

A visible gap you can slide cards into is the commonest and the cheapest. Floor-mounted stops usually have two or three millimetres of adjustment — a threaded body, a locknut, or a stack of shims under the base. Wind it up until the faces just kiss with the door at rest against it, then lock it. If the magnet is already as high as it goes, the plate on the door can usually be lowered by slotting its screw holes. Do not shim the magnet so high that the door has to be lifted over it.

The door stops in the wrong place, or hits the wall first is position, and more force cannot fix it. Swing the door slowly by hand and find the angle you actually want it to hold at — usually a little short of the wall, so the handle clears. Mark that point on the floor with chalk, with the door held there, then check the arm: two thirds to three quarters of the door width. Move the magnet to the mark, drill fresh holes rather than reusing slotted ones, and plug them properly.

The gap is tiny and it still lets go is the one case where a part is involved, and there is a five-second test that separates the two halves of it. Touch a steel paperclip to the magnet's face: if it snaps on and hangs, the magnet is fine and the plate is the problem — rust, paint, or a stainless plate that was never magnetic. If the clip will not stick, the magnet has lost its strength, usually from being dropped or from heat, and only then do you buy a magnet.

Three rules, and the first two both make it worse while feeling like an upgrade. Do not buy a stronger magnet. The gap is unchanged, so the door now arrives at the magnet faster and hits harder — which is how plates get torn off door skins and magnets get cracked. Force was never the missing thing. Do not peel off the rubber pad on the magnet face. It looks like a shim stealing your magnetism; it is the buffer. Without it every closing is a metal-on-metal impact, which is loud, and which eventually cracks the magnet or pulls the plate's screws out of a hollow door. If the stop pulls out of the floor, stop and look at what came with it. A clean disc of plaster or screed means the fixing had nothing to grip, and the same screw will fail again; you need a longer anchor or a different position. On a floor with underfloor heating, do not drill at all — go to a wall-mounted stop.

Faults next to this one

Peephole blurry or loose A door viewer is two lenses in a tube, clamped by the ring on the inside. Window not closing tight A window seals because the lock points pull the sash onto the frame and squash the gasket. Sofa sagging A seat is three layers in series: foam, webbing, frame.