The short answer
Because the fault is usually small, hidden or a long way from where the water appears. Water can run along a rafter or the underlay for a metre or more before it drips. Common hidden causes are a failed flashing, a cracked valley lining, perished underlay, a nail hole, wind-driven rain forced under the laps, a blocked gutter, and condensation that only looks like a leak. A survey from the roof and the loft finds the source.
Water does not drip where it comes in
The first thing to understand is that a stain on a ceiling is rarely under the hole. Water that gets past the covering lands on the underlay, the layer of felt or membrane under the tiles, and runs down the slope until it meets a rafter, a batten or a joint. It then follows the timber, and finally drips from the lowest or most convenient point. That can be a long way from the entry. So a roof that looks fine over the stain can still be the source, and the fault may be higher up the slope or around the corner.
The usual hidden causes
Flashings top the list. These are the strips of lead or other metal that seal the joints where a roof meets a chimney, wall or roof window. A flashing that has lifted, split or lost its pointing lets water in at the junction, and from the ground it looks like a neat line. The same goes for the lining in a valley, where two slopes meet and carry a great deal of water. Our page on signs of failing lead flashing shows what to look for.
Underlay is next. Older roofs have bitumen felt that can crack and perish with age, so the tiles above look sound while the lining beneath them lets water through at every nail and lap. Slipped or cracked tiles are sometimes hard to see from the ground, since a slate that has slid down by a centimetre exposes a gap that a binocular view may not show. Cracked ridge mortar, a blocked gutter that overflows behind the fascia, and a crumbling chimney pot or flaunching (the mortar cap that holds the pot) are other common sources.
Weather plays its part. Wind-driven rain can be blown up under the overlaps of tiles and slates and wet the underlay only in a storm from one direction, which is why some leaks come and go; see our page on a roof that only leaks in wind. Moss can hold water against the covering, and our page on whether moss causes a roof leak explains when it is a problem.
Is it actually condensation?
Sometimes the water is not coming from outside. Warm, damp air from the rooms below meets cold roof timber and turns to droplets on the felt, the nails and the rafters, which then drip and stain the ceiling. That tends to happen in cold weather, over a wide area, and with no relation to rain. Our guide to condensation in the loft explains how to tell the two apart and what fixes it. Plumbing leaks, such as a weeping tank or a dripping pipe joint, can look the same, so check the loft tanks and pipes too.
How a roofer finds it
We work from the inside and the outside. In the loft, we look at the underside of the roof for staining, damp timber and light coming through, and we check the insulation for wet patches. On the roof, we inspect the flashings, valleys, ridge, gutters, tiles and chimney, and photograph anything we find. Sometimes the answer is obvious. Sometimes it needs a hose test, where one section of roof is wetted at a time while someone watches the loft, which isolates the fault methodically. Thermal imaging and moisture meters can also help; see our guide on finding and fixing a roof leak.
Resist the urge to smear sealant over everything. Sealant can hide a symptom for a season and leave the cause to get worse. Our page on whether roof leak sealants work says when they help.
Age plays a part in all of these. Lead that has fatigued, mortar that has crumbled and underlay that has dried out do not fail overnight, they fail slowly, and the first sign is often a faint stain that dries off and returns a month later. Treat a recurring mark as an early warning. The longer a small leak runs, the more likely it is to soak insulation, rot a batten or the end of a rafter, and turn a modest repair into a larger one. Photograph the stain each time it appears and note the date and weather.
What to do now
Put a bucket under the drip, move belongings, take dated photographs for any insurance claim, and call a roofer. If the water is near electrics, switch off the circuit and ring an electrician. We prioritise emergency calls and will make a leaking roof safe first. The survey is free, and you get photographs of your own roof, so you can see the fault for yourself. You can start from the homepage, or call 01256 213956.
Every roof is different. A free survey with photos settles it for yours.
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