What gap is allowed round a fire door?

Between 2mm and 4mm down the sides and across the top. Our gauge says it right on the blade: 4mm pass, 6mm fail.

Shut the door and slide the gauge in at the top, the middle and the bottom of each side. You'll often get three different readings on the same edge, so go with the biggest.

The bottom gap is a different animal. How much you can have depends on the door's test evidence, and it isn't one number for every door. The certificate or the manufacturer will say.

Big gap on the hinge side? Look at the hinges first. A couple of screws that have pulled out will let the door drop a few millimetres, and putting them back, with a packer behind if needed, often brings it in.

Tight gap, door rubbing? Don't reach for a plane. Take too much off a fire door edge and it can't be put back. Find out why it's binding. Swollen paint, an old strip stuck on top of a new one, or a hinge that's out of line are the usual reasons.