What causes a roof to fail?

A roof is a layer of units held down by nails, clips and mortar, and when the fixings go the covering follows. Iron nails rust, slates slip and the roof starts to leak even though the slates themselves are sound. Tiles fixed with too few nails lift in strong wind. Ridge and hip tiles bedded in mortar alone crack and fall as the mortar weakens; since BS 5534:2014 mortar by itself is not relied on, and they are mechanically fixed as well.

When we survey a roof that is dropping tiles, the question is why the fixings failed: age, corrosion, wrong type, or too few. The answer decides whether a handful of tiles can be refixed or the whole slope needs stripping. Our page on signs a roof needs replacing lists the warning signs.

Under the covering lies the underlay, a thin membrane whose job is to intercept wind-blown rain and snow that slips past the units and steer it into the gutter. Old bitumen felt dries out, tears at the battens and rots at the laps. Where there is no underlay, dust and moisture reach the loft directly. A torn sheet in a valley or along the eaves can send a steady trickle into the wall below.

Battens fail too. Soaked and rotted timber cannot hold tiles at the right spacing, and a sagging ridge often points to rotten rafters or a failing wall plate rather than a covering problem. Because these faults are out of sight, a repair that replaces tiles alone leaves them untouched. Our surveys include a look inside the loft early on, with pictures of what is there for you to see.

A brown stain is not always a leak. Steamy air from kitchens and bathrooms drifts up into the loft, and when it has nowhere to go it condenses on chilled rafters and under the tiles. Given time the wood rots and the insulation goes soggy. Marley's guidance, citing BS 5250, puts eaves ventilation for a cold roof with non-breathable underlay at 10 mm along the eaves where the pitch is over 15 degrees, and 25 mm at 15 degrees or less.

Insulation stuffed hard into the eaves, a bathroom fan venting into the loft and blocked soffit vents all upset that balance. A roof can look perfect from the street and be rotting inside. Treating the symptom is the same mistake in another form. The Energy Saving Trust advises against spraying foam beneath a leaking roof without first curing the fault, since timbers beneath can go on rotting where a leak or condensation persists.

Some roofs fail early because of how they were built. A covering laid at too shallow a pitch for its type will leak in driving rain: For natural slate in moderate exposure, BS 5534 sets the floor at a 20 degree pitch, and every concrete or clay tile has a minimum of its own on the manufacturer's data sheet. Short headlaps, flashings with poor laps, lead too thin for its position and valleys without enough fall all start trouble.

Extensions are common culprits. A new roof meeting an old one, a porch added to a gable, or a rear extension tied into a Victorian slope creates junctions where flashings and soakers must be done carefully. Cheap work done in a hurry often looks fine for a few winters. Asking any roofer for a guarantee in writing is sensible; our workmanship guarantee runs for 10 years and is not insurance-backed.

Weather does its work gradually. Wind lifts loose tiles, frost gets into porous clay and concrete and flakes the surface, and thaw cycles open hairline cracks. Leaf litter and moss clog the gutters and valleys, and the overflow finds its way beneath the covering. Moss itself holds water against the tile and, when scraped off roughly, takes the surface with it.

Age is the quiet cause under all of these. MyBuilder's slate guide, updated 21 August 2026, puts well-laid slate at 80 to 100 years or more, and concrete tiles at 30 to 50. Every covering eventually reaches the end. The useful question is whether a roof is still within its working life and only needs maintenance, or has passed it. Our page on planning permission for a new roof covers the paperwork once replacement is on the table.

Fixing the cause matters more than patching the symptom. A single slipped slate, with everything else in good order, is a repair. Widespread nail sickness, perished underlay and rotten battens call for a strip and relay. We would rather recommend the least work that removes the cause, and the survey photographs show where it sits. If you are wondering whether to attempt it yourself, read whether to do a re-roof yourself, and if you are staying in the house, living in the house during a re-roof sets out the day-to-day. Our main roof replacement page shows the full approach, from the first photographs to the finished slope. To book a survey, ring 01793 626096. If a fault keeps returning, say so when you call; knowing the history helps us look in the right place first.