Why is my roof leaking around a vent pipe?

A soil and vent pipe carries drain gases up and out of the house, so it has to poke through the roof. Every penetration is a possible leak, and the roof relies on a purpose-made fitting to close it. On many houses that fitting is a flashing, a sheet of metal or moulded plastic that spreads the joint onto the surrounding tiles so water runs over it and not into the gap.

Nothing about this joint is permanent. The pipe expands and contracts, the sun works on plastic and rubber, and the tiles around it settle slightly over the years. Leaks around the pipe are therefore among the commoner repairs, and they often look worse than they are. A drip down the pipe inside the loft can run a long way before it shows, so any mark on the plasterwork may be well away from the pipe itself, and the first clue can be a damp bedroom corner.

There are four usual suspects, and a survey checks them in order. The first is the rubber or neoprene collar on a push-fit fitting, which cracks and splits as it ages and leaves a gap around the pipe. The second is the flashing skirt, which can lift, buckle or crack where it has been walked on or struck by a branch. The third is the tiling disturbed around the flashing, which leaves a path for water to track under the skirt. The fourth is old sealant, which hardens, shrinks and pulls away from the pipe.

With a lead flashing, the trouble is more often in the dressing. Lead is a soft metal that is shaped into the tile profile, and a poorly dressed edge lets water creep up beneath it in driving rain. Our page on lead roofing explains the material, and the answer on causes of roof leaks places this fault among the others.

A tube of sealant is the quick option, and it will sometimes hold for a season or two. It is rarely the end of the matter. Sealant grips a clean dry surface, and a roof is neither for long. When it fails, it often does so invisibly, because the skin stays whole while the grip underneath lets go and water tracks behind it.

That is the main objection to leaving a sealed repair alone. If the underlying flashing or collar has failed, the sealant is holding a worn part in place and hiding the fault from the next inspection. We cover the limits of the product in our answer on whether roof sealant stops leaks. A tube can serve as a temporary measure in an emergency, though the permanent repair replaces the part, and a new collar or flashing is not an expensive item.

The work starts with finding which part has failed. We check the collar, the flashing, the surrounding tiles and the pipe itself, and photograph each so you can see the fault for yourself. The repair then depends on the finding. A split collar is replaced with a new fitting of the right size. A lifted or cracked flashing is renewed, with the neighbouring tiles raised and laid back over its upper edge and under its lower edge, the arrangement that leads water away from the hole.

On some roofs the best answer is a lead flashing, dressed over the tile profile and tucked into the course above. If the pipe itself is damaged or has moved, that is a drainage question and may need a plumber. After the survey the price is set down in writing as one sum. The roof repairs hub lists the other leaks we deal with, and explains how a visit is arranged from the first phone call.

Not every wet pipe is a leak. Cold pipes in a loft collect condensation, and a drip from a pipe on a cold morning that stops by afternoon is more likely to be moisture from the house than rain from outside. A leak from the roof tends to follow the weather and gets worse in driving rain from one direction.

A simple check helps. Look at the pipe and the area around it in the loft after heavy rain, with a torch, and note whether the timbers near the penetration are wet or marked. If you can see daylight round the pipe, or a trail of staining below the flashing, the roof is the likely cause. Our guide to finding a roof leak sets out the order of checks, and our survey pictures of the roof side let you set the inside view against the outside one.