Most roof checks start in the street, with someone squinting up at the ridge. A loft inspection starts from the other side, and it often answers questions the street view leaves open. From inside you can see the back of the covering, the underlay, the rafters and battens, the wall plates where the timbers meet the masonry, and any spot where moisture has found a way through. You can also see daylight, which in a loft is rarely good news. This guide covers what a loft roof inspection can tell you about the timbers, the underlay and the condition of the slates or tiles above them. It also covers what the loft cannot show, such as the state of lead flashings on the outside, and how to carry out a basic check without putting a foot through the ceiling. The point is to help you judge whether the roof deserves a closer look, without turning you into a surveyor.
What the loft shows that the street cannot
From the pavement you see the outer face of the covering and little else. You can spot a slipped slate or a gap in the ridge, but you cannot see what the roof is doing underneath. The loft shows the other half of the construction. Water that has got past the covering leaves marks on the timber, and those marks are often easier to read from inside than any wet patch on a bedroom ceiling.
In a typical pitched roof the loft gives you four things to look at. The first is the underlay, the membrane that runs across the rafters beneath the battens, acting as a back-up for rain that wind pushes beneath a slate or tile. The second is the timber frame: rafters, battens, ridge board, purlins where they exist, and the wall plate that carries the foot of each rafter. The third is any daylight coming through the covering. The fourth is the loft itself, meaning the insulation, the ventilation and anything stored up there that could be hiding a problem.
A loft check is especially useful after a leak, because the stain on a ceiling is rarely under the hole in the roof. Moisture tracks along the upper face of the underlay or down a rafter before it drips, so the entry point can be a metre or more from the damp patch. Following the staining uphill from inside is usually quicker than guessing from outside, and it tells you whether to look at a chimney, a valley or a single slate.
It does have limits. You cannot judge a lead flashing from the loft, and you cannot see the top surface of a slate, so cracks and surface wear stay hidden. Think of it as one half of an inspection. Our roof inspection checklist sets the loft check alongside the outside checks, so you can see where each one fits.
Reading the underlay and the sarking
Underlay is the sheet material under the battens. On older roofs it is bitumen felt; on newer ones it is usually a breathable membrane that lets moisture vapour out of the loft. Some older slate roofs have sarking instead, which is timber boarding fixed over the rafters with the slates laid on top, sometimes with felt over the boards. Many roofs have no underlay at all, because the original builders relied on the covering alone and a layer of lime mortar pointing between the slates, known as torching, to hold out wind-driven spray.
What you want to see is a continuous sheet, sagging gently between rafters rather than hanging in tatters. Felt that has gone brittle will crack and drop in pieces when touched. Torn or missing patches are common around the eaves and the ridge, and wherever a previous repair has been done in a hurry. Tears matter because the underlay catches the water that gets past a cracked or slipped covering, and carries it to the gutter. Without it, that water lands on the timber.
On a roof with sarking, look for dark, soft or flaky patches on the boards. Boards that have been repeatedly wet go grey and then black, and eventually lose strength. Pressing a fingertip against a sound board leaves no mark. A board that gives under the same pressure has probably been wet for a long time.
One of the checks we make is to see whether the underlay has been cut and lapped properly where the roof changes direction. At a valley, the underlay should dress into the lead or the valley liner and not simply stop short. At the eaves, it should lead water over the fascia and into the gutter, not behind it. If neither detail looks right, the covering may be doing all the work, and a re-roof is the point at which to put it right.
A covering can look fine from the ground while resting on a membrane that has already given up, because the underlay often fails first.
Rafters, battens and wall plates: what sound timber looks like
Timber that has stayed dry looks plain: pale to mid brown, with clean edges and a surface that a screwdriver tip cannot sink into. Problems show up as colour changes, texture changes and smell.
Signs of wet rot and dry rot
Wet rot appears where timber stays damp for a long period, and it darkens the wood and breaks it into small cubes along the grain. Dry rot is rarer, but it matters more because it can travel through masonry and affect timber some distance from the original dampness. It leaves a mushroom smell and fine grey strands or rust-coloured dust. If you see either, stop probing and ask for a proper assessment of the cause before anything is cut out, because the source of the moisture needs fixing first.
Battens and where they fail
Battens are the small timber laths that carry the slates or tiles. Look for ones that have snapped, sunk or pulled away from the rafter. On a slate roof, a batten that has rotted at the nail line will let slates slip, which then shows up outside as a row of dropped slates. You can often see where this has happened because the slates in that row are slightly out of line.
Wall plates and the foot of the rafters
The wall plate lies on the masonry at eaves level, and every rafter foot rests on it. It is the part most likely to be wet, because the eaves sit near the gutter and water that overflows runs straight down onto it. Check the rafter feet and plates along each side of the roof. On a Bath terrace with a parapet gutter, where the gutter runs behind a stone parapet, damp timber behind the parapet is a common sign that the lead lining has split, and it is worth following to the source.
Do not judge timber on appearance alone. Pale surface staining from old, dry leaks is not a defect, and a dark patch that is dry and hard is usually history. A damp, soft patch is the one that counts.
Daylight, pinholes and nail sickness
On a bright day, switch the loft light off and wait a minute for your eyes to adjust. Any pinprick of light coming through the roof is a route for water. A few gaps at the ridge can be normal where a ventilated ridge is fitted, but daylight along a slope tells you something has opened up: a missing slate, a gap between tiles or an eroded patch of mortar.
Slate roofs have a failure of their own that shows from inside as much as outside. Each slate hangs from a nail, and if the nail rusts through, the slate slips. Heritage guidance on traditional roofs calls this nail sickness, and it describes iron or poorly galvanised nails corroding over time until the slate slides. Seen from the loft, you may find slate fragments and rust-coloured nail heads resting on the underlay or on the ceiling joists below, or a pale trail of rust on the sides of the battens. A single slipped slate is a repair job. Several scattered across the roof are a warning that the fixings everywhere are tired, and that repairs will keep coming.
On a tiled roof, the equivalent signs are broken tile nibs and cracked tiles. Fragments in the loft debris usually mean a tile has already failed above. Check the debris near the eaves, where fragments collect.
Leaks around a chimney show up here too. Staining on the stack's side, a damp line on the timbers where they meet it, or water marks on the chimney breast all point to flashings or flaunching, the mortar cap on the top of the stack. We cover those in our work on lead and chimneys, but the loft tells you where to look.
When you find a stain, mark it. A pencil ring on the timber and a photo with a date let you check later whether it has grown. Stains that are damp one visit and dry the next point to a leak that depends on the weather and wind direction, which is exactly the kind that is hard to find from outside.
Ventilation, insulation and condensation in the roof space
Not every wet patch in a loft is a leak. Humid air leaving the rooms below drifts upward, meets cold surfaces in the loft and condenses, and the result can look just like water coming through the covering. Condensation tends to show as beads or general dampness on the felt, on nail points and on the north-facing slopes where the roof stays cold longest. Leaks tend to leave tracks, with a local stain running down a rafter or collecting at one spot.
A cold loft needs air moving through it. One manufacturer's guidance, based on BS 5250, suggests a continuous eaves ventilation gap of around 25 mm where a non-breathable underlay is used on a shallow pitch, and about 10 mm on steeper pitches. Treat those as typical figures and check against the specification for your roof. What matters on a visit is whether insulation has been pushed hard against the eaves so that it blocks the airflow, which is a frequent fault. Pull it back gently and look for the gap.
Foam sprayed onto the inner face of the roof alters the picture. It hides the timber, so you cannot see rot or leaks behind it, and the RICS consumer guide on spray foam warns that condensation can build up unchecked and harm the building. The same guide advises checking with your mortgage provider whether their policy allows such products. If your loft has been foamed, an inspection from inside may show you almost nothing, and a survey will have to rely on the outside and on moisture readings. Our page on spray foam insulation explains the options, and the loft condensation page shows how to separate the two.
Roofs that have been converted or boarded for storage need a second look. Boxes and boards can block the eaves vents and hide the timbers, so move them before you judge the roof.
Doing it safely, and what to leave to us
A loft is not a room. The ceiling below is usually plasterboard or lath and plaster, held up by joists and not by anything you can stand on. Step only on the joists, or on boards laid across them, and carry a torch even in daylight. Wear a dust mask and gloves if you disturb insulation, and keep your head clear of nails that poke through the underlay. If the access hatch is awkward or the loft has no boarding, our note on roof inspection access explains how we reach different roofs.
Look out for older materials. Asbestos was used in some buildings, in loft insulation, in pipe lagging and in cement sheets. If you find any material you suspect is asbestos, do not touch or disturb it, and ask for advice before anyone works near it. Bat roosts and active nests have legal protection. Should you spot droppings or a roost, leave everything alone and ask Natural England before any work starts.
Take photographs as you go. Wide shots to show where you are, close shots of the problem with a coin or a rule for scale, and a shot of the same area on the outside if you can match it. These help whoever follows up, and they help any insurer who needs to see how damage developed. If the loft check turns up damage after a gale, our guide to roof inspections after a storm explains how the findings are recorded.
When we survey a roof, the loft check is paired with an outside inspection, so the stain you found inside is matched to a cause outside. Our free survey includes photographs of your own roof and ends with a written price that is fixed once we have seen it. If a repair will do, we recommend the repair and price it. Where the timbers need work, we show you the actual timbers in the photographs, and say what we would renew and what can stay.
Roof inspections are one of the quickest ways to find out whether a worry is a small job or a big one.