The short answer
A vent pipe is a hole through the roof, so the seal around it is the weak point. A leak there usually comes from a split rubber collar, a lead slate that has lifted or cracked, or condensation running down the inside of the pipe. The cause can be told apart from the loft, and the repair is small if the tiles around it are sound.
How the pipe passes through the tiles
A soil vent pipe carries drain air up and out of the house, so it has to poke through the roof covering. Where it does, the tiles are cut or a dedicated pipe tile is used, and something has to close the gap between the pipe and the roof. That closure is the part that fails.
On a modern roof the seal is usually a collar, a flexible ring that grips the pipe and sits on a flat plate beneath the tiles. On an older roof it is more often a lead slate, a sheet of soft lead dressed over the tiles with a tube formed up around the pipe. Both are simply flashings, the strips of waterproof material that carry rain over a joint.
The collar and the slate are exposed to sun and frost, and the flexible ring is the first to age. A pipe also moves slightly as it warms and cools, which works on the seal every day. So a roof that is otherwise sound can still leak at the one place a pipe goes through it, and that makes the pipe a sensible first suspect when a stain appears under it.
Split collars and lifted lead slates
A collar fails by hardening. Over the years the ring stiffens, a split opens at the pipe or a gap forms where the ring meets the plate, and rain runs straight down the outside of the pipe into the loft. From a ladder it can look fine, because the split sits on the sheltered side.
A lead slate fails differently. The lead can lift at its upper edge where it was never tucked under the course of tiles above, crack where it was bent too tightly around the pipe, or slip down the slope. Lead is long lived: In MyJobQuote's words, lead flashing "can last for over 80 years", so a leak usually points to how it was fitted or disturbed rather than to age. A wrongly cut tile beside it can break the seal too.
Do not seal the gap with mastic from a ladder. It gives a few weeks of relief and hides the real defect, and a new collar or lead slate cannot be fitted neatly over a lump of old sealant. Our guide to the parts of a pitched roof shows where flashings sit.
Condensation inside the pipe
Not every drip is rain. A soil pipe carries warm, damp air from the drains, and where it runs through a cold loft the pipe wall is cool, so moisture condenses on it and runs down to the ceiling or pools at a bracket. The ceiling stain looks like a leak but appears in cold spells, not in rain.
There is a simple test. Look at the pipe in the loft on a dry, cold morning. If it is beaded with water along its length, or the insulation around it is damp, condensation is the likely cause. If it is dry until it rains and the wet starts at the roof end, the seal has failed. The same moisture mechanism is behind the loft problem described in our answer on condensation in the loft.
BS 5250 sets out how moisture is managed in buildings, and the idea is to keep warm, damp air out of the cold loft and give the loft a way to breathe. For a pipe that usually means a snug lagging jacket and sealing the ceiling around it, not roof work at all.
What the repair involves
We start in the loft with a torch, look at the underside of the tiles around the pipe, and then go up to see the collar or lead slate itself. You see the photographs of what we find, so the choice of repair is shared.
If a collar has perished, the fix is to lift the tiles around the pipe, fit a new collar and plate, and relay the tiles. If a lead slate has lifted, we can sometimes tuck it back under the course above and re-dress it. Where the lead is cracked, a new slate is cut and fitted. A replacement collar in a conservation area or on a listed home should match what was there, and listed homes may need listed building consent, so ask first.
This is normally a short repair on our pitched roofing list. The price turns on access, because a pipe on a high slope needs a tower or scaffold, and on whether the tiles beside it are cracked. If condensation is the cause, we tell you so and send you off with advice to lag the pipe rather than selling a roof repair.
