The short answer
Water, in one form or another, does the most harm. Wind lifts coverings so that rain gets under them, frost splits wet slate and tile, moss keeps the overlaps damp, and blocked gutters send water where it should not go. Poor workmanship, such as ridge tiles held by mortar alone, is the other big cause, and it is the one you can still do something about.
Wind loosens the covering, then rain does the damage
A gale rarely strips a sound roof in one go. It works at the edges and high points first: ridge tiles, hip tiles and the verge, the gable edge where the covering meets the wall. Once a ridge tile shifts, the bedding mortar cracks and the gap invites wind-driven rain.
Fixing standards have tightened because of this. Marley points out that BS 5534 changed in 2014: ridges and hips now call for mechanical fixings, with mortar a supplement. An older roof bedded in mortar only is therefore a natural place to start after a storm. On slate and stone roofs the nails and pegs do the holding, and a surveyor looks at the underside of the slates too.
Driven rain and the joints it finds
Rain falling straight down is easy for a roof to shed. Rain pushed sideways by wind travels uphill under laps, along flashings and into gaps that a still day would never test. The usual weak points are junctions: a chimney flashing, a soaker (a folded sheet of lead tucked under the slates where they meet a wall), a valley, a roof window kerb.
Our answer on why a roof can leak only in heavy rain goes through this in detail. The point here is that most rain damage is not a hole in the middle of a slope. It is a small failure at a joint that happens to meet the weather at the wrong angle, and the water may travel unseen for months before a mark reaches the ceiling.
Frost, moss and the slow damage
Slates and tiles take up a little water, and moss and lichen hold more of it against the face. Water that freezes there expands, and over several winters it flakes a tile, splits a slate along its grain or lifts a pointed joint. The 2000 stone slate guide from Cotswold District Council suggests an annual scrape could give a stone roof twice the life, which says a lot about the toll moss takes.
Cleaning carries its own cost and its own risk. In its 2026 cleaning guide, MyBuilder prices hand scraping at about £5 to £10 per m² and flags pressure washing as a risk to fragile tiles. We favour hand work, checking the fixings as we go, because moss often hides the first loose slate.
Gutters, trees and what lands on the roof
Gutters belong to the roof system even though they sit below the covering. A blocked one spills over the front, or backs water under the first course and soaks the felt and the rafter feet. Leaves, moss washed off the slope and granules from ageing tiles all collect in them, and a run set with too little fall holds standing water. Brett Martin's guidance gives a fall of 1 mm for every 600 mm.
Trees add several problems at once: branches that rub or crack the covering in a gale, litter in valleys and gutters, and shade that encourages moss on the slopes beneath. Cutting back overhanging growth before winter is cheaper than repairing what it causes. If a branch has already come down, our guide to the most common roof repairs shows how we deal with the damage.
Condensation and loft moisture
Not every damp roof has a hole in it. Steam from showers, cooking and drying laundry drifts up into the loft, and on cold timber and underlay it turns back into water. Over time that wets rafters and insulation and can look exactly like a leak. BS 5250 is the code of practice for managing moisture in buildings, and Marley applies it to cold roofs through eaves and high-level ventilation chosen to suit the underlay and the pitch.
Blocked eaves vents are a common cause: loft insulation stuffed tight into the eaves can choke the airflow that carries damp air out. A surveyor can tell condensation from a leak by where the dampness sits and whether it follows rain.
Poor workmanship and age
The least visible cause is a roof that was never built or mended to a good standard. Too little headlap, the overlap between courses, lets rain blow under a slate. Cupa Pizarras gives 115 mm of headlap for a 500 x 300 slate at 20 degrees in moderate exposure, and less overlap suits only steeper pitches. Cement fillets in place of lead, sealant smeared over a gap and nails driven through tiles all hold for a season and fail the next.
Age is the slowest cause. Every covering has a typical life, and the fixings, underlay and flashings often wear out sooner than the stone or slate they support. Pegs, battens and underlay usually go first.
If you cannot tell which of these applies, a survey with photographs shows the cause, and the written quote that follows deals with either the repair or the replacement. You can try ground-level checks first; our page on whether to repair your own roof sets out what is safe. Our wider roof repairs page covers the full service.
What a survey records about damage
From the ground we note the ridge line, the verges, slipped or cracked units, flashing condition and the state of the gutters. Inside the loft we watch for daylight, wet timber, torn felt and tide marks on rafter feet. Photographs go in the report, so you see the same things we saw, including the sound areas.
A roof survey is priced at £150 to £500 in MyBuilder's 2026 inspection guide, depending on type. Knowing the cause before you spend is usually worth that much on its own.