What that stain on the ceiling probably means

A brown ring that doesn't grow

An old leak that's been fixed, or one that only happens in very specific weather. Worth checking the loft above it after the next heavy rain; if it stays dry, it's history.

A patch that appears after heavy rain

The classic. Water is getting past the covering somewhere above and running down the underlay or the rafters before dripping. The stain is rarely directly under the leak — trace uphill. Common causes: a slipped tile, a cracked flashing at a chimney or wall, a blocked or split valley.

Damp only in wind-driven rain

Rain being pushed up under the tiles or through a verge. Points at the ridge, the hips, a gable verge, or lead flashing that's lifted. Dry-fix ridge and verge systems fix this permanently.

Damp in a top corner of the room

Usually the gutter. Overflowing or leaking gutters soak the wall top and the wall plate, and the damp shows inside at ceiling level. Check for plants growing in the gutter and for water marks on the outside wall.

Wet insulation but no ceiling stain yet

Get it looked at now — the insulation is holding the water and the ceiling will follow. Often a pinhole in the underlay or a slipped tile above.

Condensation, not a leak

Water droplets on the underside of the underlay across the whole loft, worse in cold weather, no single wet spot: that's condensation from a poorly ventilated roof space, not rain. It still rots timber; the fix is ventilation, not tiles.

What to do

Photograph it, note the weather when it happens, and tell us. A photo of the roof from the ground helps.