A roof covering shows its age from the street, but the working parts of the roof sit underneath it. The loft is where you see the back of the tiles or slates, the underlay below them, the timbers that carry everything and the insulation laid across the floor. Water that has found a way past the covering leaves a record there long before a ceiling stains, and condensation announces itself in the same place.
You do not need any tools beyond a good torch and a little patience. Ten minutes in the loft on a dry day, followed by a second look after rain, will tell you a surprising amount. What you are hunting for is a pattern, not a single oddity: a dark line that runs up the slope, a patch of insulation that is flatter or darker than the rest, a draught where there should be none.
The notes below follow the order we use ourselves, from light to damp to ventilation, and they say plainly when something calls for a roofer rather than a householder. Our wider work on this is gathered under roof inspections and surveys.
Switch off the torch and look for daylight
Begin with the simplest test. Stand by the hatch on a bright day, let your eyes adjust, and switch the torch off. A sound pitched roof is dark underneath apart from any deliberate ventilation at the eaves. Pinpricks of light mean there is a gap in the covering, and where light can come in, rain driven by a gale can follow.
The gaps tend to fall into a few families. A thin line of light along a slope usually points to a slipped or cracked tile or slate. A small spot at the ridge suggests that bedding mortar has fallen away or a ridge tile has shifted. Light near a chimney often means the flashing, the lead that seals the roof to the stack, has lifted. A bright patch low on the slope can be a missing tile, or a gap where the roof edge meets the wall.
Not every glimmer is a fault
Some roofs are meant to admit air. Eaves ventilators and tile vents show up as small, regular openings and are part of the design. Old roofs laid without underlay show a lot more light, because the backs of the tiles are visible between battens, the timber strips that tiles hang from. That does not make them failing roofs, and our guide to surveying a historic roof covers how they are judged, but it does mean wind and fine snow can get in more easily than they would on a lined roof.
Mark any gap you find. A sticker or a pencil line on the nearest rafter lets you find it again, and you can describe its position to a roofer as being so many rafters from the gable or the chimney.
If you can see daylight at more than one point on the same slope, a closer look from outside is worth arranging.
Tide marks, stains and damp insulation
Water leaves marks even after it dries. On the timber it appears as a pale or dark tide line, like the ring a cup leaves on a table. On the underside of the underlay it shows as stained patches, sometimes with a drip hanging from a nail point. On the floor, insulation that has been wetted goes flat, grey or clumped, and may smell musty.
Read the direction as well as the mark. Moisture that gets under the covering heads downhill along the upper face of the underlay until it finds somewhere to drop. A stain on a rafter or a cross-timber is therefore usually below the opening, not under it. Follow the lines uphill and you will normally reach a chimney, a valley, a ridge or a patch of missing covering. Our guide to telling condensation from a leak explains how to separate the two.
Old marks and new
A dry stain with a hard edge may be old and finished with. A dark, soft patch that darkens further after rain is current. Touch it with a gloved finger, not a bare hand, and check again after the next shower. If you can, photograph each mark with something in shot for scale, such as a tape measure, and note the date.
Look at the places where roofs are joined or interrupted: around a soil pipe, beside a roof window, beneath a chimney breast and along the line of a valley. Those are where a fault is most likely to start.
Compare any ceiling stain with the marks in the loft. They rarely line up exactly, and the offset tells you how far the water has travelled. The chimney stack deserves its own look: damp brickwork, white salts on the brick or loose mortar fallen onto the insulation are worth noting, since the stack, the flashing and the slope all meet there.
Timbers that are soft, dark or flaking
The rafters, purlins and wall plates hold the roof up, and they stay sound only while they stay dry. Damp timber looks darker than its neighbours. Rot, in its early stage, can leave the surface cracked into small cubes or covered in a white or yellowish fungus. A soft spot gives under a thumbnail or a screwdriver tip. Take care not to dig into sound wood unless you mean to.
Look closely at the ends of timbers where they meet masonry, at joints, and below any place where you have already found staining. Water collects at those points. A rafter that bows, a plate that has dropped or a joint that has opened is a structural question, and our page on when a structural engineer is needed says how that is handled.
Tiny holes and fine dust
Small round exit holes in timber, with a pale powder below, suggest an insect infestation of the wood. Some are old and long dead, and some are active, and telling them apart needs experience. We note them in a report and recommend a specialist where the extent is not clear. Do not poke at the powder or treat timber yourself with whatever is on the shelf before you know what you have.
The loft is also where pipes, tanks and wiring run. A damp patch below a water tank, or a pipe with a drip hanging from a joint, may be plumbing that has nothing to do with the roof. If you spot a cable with a damaged cover near wet insulation, leave it alone and ask an electrician to take a look.
Photographs with a torch held at a low angle show the texture of timber far better than a flash straight on. Hold the torch beside the camera lens, take one wide shot and one close shot of each find, and keep them together in a folder.
Ventilation, condensation and the eaves
A cold loft needs air to move through it. Steam from cooking, showers and drying clothes rises through the house, and if the loft cannot breathe, that moisture settles as drops on the underlay and the timbers. Over a winter it can darken timber, grow mould and, on a cold morning, drip like a slow leak. It is among the first things we rule out when someone reports a damp patch with no obvious entry point.
Standards guidance gives numbers. In Marley's summary of BS 5250, with a high-resistance underlay, the eaves need a continuous 25mm opening on pitches of 15° or less, and 10mm on steeper roofs, with a further 5mm continuous at high level where the pitch passes 35°, the span exceeds 10m, or the roof is a single slope leaning against a wall. A low-resistance, breathable underlay on a well-sealed ceiling can need far less, provided the product is certified. The point is that the gap must exist and stay open.
What to look for at the eaves
Shine the torch down towards the wall plate. Loft insulation stuffed tight against the wall plate can close off the gap and starve the roof of air, and that is a frequent reason for condensation. Look for a clear route from the soffit, the board under the overhang, up behind the underlay. Check that no bags, boards or stored boxes are crushing the insulation or sitting against the slope.
Signs that air is not moving include frost on nail points in cold weather, a musty smell, mould spotting on the underside of the felt, and damp patches that are widely spread rather than in one line. Fans that vent into the loft rather than outside add to the load, so check where any extractor duct ends.
Looking safely and knowing when to stop
Treat the loft as a place with hazards. Step only on boards laid across the joists, or on the joists themselves, never on the plasterboard in between, which will not hold a person. Wear a dust mask and gloves, take a head torch if you have one, and watch for nail points that have come through from the roof above. Keep the hatch clear and tell someone you are going up.
Be careful with old materials. Older houses can hide asbestos, and the HSE tells owners to presume it is present in anything built or refurbished before 2000. Do not drill, scrape or sweep anything that looks like old board, lagging or sprayed coating, and do not disturb it. Report it to a specialist.
Wildlife law shields roosting bats and nesting birds, so droppings or a nest mean you stop, leave everything undisturbed and take advice before any work goes ahead.
When to call a roofer
Ask for a survey if you find daylight in several places, a tide mark that is darker after rain, soft timber, a sagging rafter, or wet insulation you cannot explain. Our visit traces the cause first, the photographs are of your roof, not a stock image, and we put forward only the repair the findings justify. A written quote with one total comes after the survey.
A loft check also feeds other decisions. If you are weighing up solar panels, read about a roof survey before solar panels. If an insurer is involved, our guide to roof reports for insurance claims shows how loft photographs are used. For a flat roof with no loft, see flat roof inspection.
