How do you check the loft for a roof leak?

Pick a time when you can see. A bright day lets you spot daylight through the roof, and the hours just after rain show fresh damp. Take a good torch, a phone for photographs and something to mark the spot, such as chalk or a strip of tape. Wear old clothes and a mask if the loft is dusty, since insulation fibres irritate the throat.

Check the access first. The hatch ladder must be sound, and you should be able to see where the joists run. The ceiling between them is only plasterboard, and a foot on it goes straight through. Lay a board across the joists to kneel or stand on, and never step between them. Look for cables and keep clear of any that are wet. If the loft is cramped, full of stored items, or has no boarding, think twice about going in. Our page on roof leak or condensation explains another cause of damp up there.

Start with the underlay, the waterproof sheet that lies beneath the tiles. A leak leaves it dark and damp, and you may see drips falling from a nail or a batten. Timber that is wet turns darker than the dry wood around it and may have a tide mark where the water has dried and left a ring. Insulation that has been soaked looks flattened and matted, and can feel heavy or cold.

Then look for light. On a bright day, a pinprick of daylight through the roof shows a missing or cracked tile, though it also shows a vent, so check the pattern. Smell matters, too. A musty odour often points to water that has been there a while. A dark bloom of mould on the rafters suggests repeated wetting. Take close photographs of each mark, and a wider one that shows where it sits in the roof, so the roofer can see the pattern.

Water rarely drips from the spot where it comes in. It runs along a rafter or the sheet beneath the tiles, then falls where something stops it, so the stain is usually below the true fault. Follow any wet timber or darkened felt uphill. The entry point is often at a feature: around the chimney, beside a roof window, in a valley, where two slopes meet, or just below the ridge.

Compare the loft with the outside. If the stain sits under a chimney, the lead flashing may be lifted. If it follows a line of rafter from the eaves, a slipped tile or a blocked gutter may be to blame. A stain at the foot of the roof could be water running back from a faulty gutter. Mark each place in your notes, then look at the same area from the ground. Our guide to a roof leaking in heavy rain shows the common patterns.

Small, dry stains may be old and need nothing urgent, but they are worth a note and a survey. A fresh drip or a wet patch that spreads needs action. Stand a bowl under it, and lay a plastic sheet beneath to protect the ceiling. If insulation is soaked, move it aside so it can dry, rather than leaving it pressed against the plasterboard.

If water is near a light fitting or cable, do not touch it, and read our answer on water near a light fitting before you do anything else. Then ring a roofer, tell them what you found and where, and send the pictures. The photographs help the roofer plan the repair, and they help an insurer if a storm caused the damage. If you pay for a temporary fix, keep the receipt.

Some lofts are not safe for the householder. No boards, low headroom, old boards of unknown material, wasps or bats, and wiring in poor condition are all good reasons to stay out. Bats and their roosts are protected, so if you see droppings or a bat, stop and ask for advice before disturbing anything. HSE advises assuming asbestos is present in any home built or refurbished before 2000, so do not break up or drill into old panels.

A roofer has the right equipment and knows what the marks mean, and can often read the age of a stain from the colour of the timber. The loft and the outside are looked at together, each fault photographed, and you hear plainly whether a small repair will do. If it is raining in as you read this, tell the roofer on the phone and the visit is prioritised. Start with our emergency roof repairs page for how that works.