Bathroom leaking into the room below: finding the source

A damp patch on the ceiling below a bathroom is usually one of four things: a failed waterproof membrane in the sunken portion, a leaking drain or trap connection, a supply pipe leak, or water tracking through grout and tile joints. Test them in order — supply pipes first because they are cheapest to confirm, membrane last because confirming it means lifting tiles.

Test in order of cost

• Supply pipe. Turn off every tap and check the water meter or watch the overhead tank level for an hour. Movement with nothing running means a pressurised leak, which will also be wet all the time rather than only after use.
• Drain and trap. Block the floor trap, fill the shower area with an inch of water and leave it two hours. If the ceiling below wets without anyone using the bathroom, the leak is in the waste line or trap connection.
• Grout and tile joints. Look for cracked or missing grout, particularly in corners and where the floor meets the wall. Water passing here will show as a slow, diffuse patch that worsens with use.
• Membrane failure. If the above are clear and the patch grows steadily after the bathroom is used, the sunken portion waterproofing has failed. This is the expensive one.

What each repair involves

The four causes differ enormously in disruption, which is why the order of testing matters.

• Supply pipe — usually a short section of wall opened, pipe replaced, re-plastered and re-tiled locally.
• Drain or trap — often accessible from below or through a small opening. Moderate work.
• Grout and joints — re-grouting and sealing corners with a flexible sealant. Cheap, and worth doing anyway.
• Membrane — tiles lifted, sunken portion cleaned, new cementitious or two-component membrane applied and turned up the walls, then re-screeded and re-tiled. Several days of work and the bathroom out of use.

Why re-grouting is worth doing first anyway

Even when the membrane is the eventual culprit, cracked grout and unsealed corners let water reach it faster. Re-grouting and sealing the floor-to-wall junction with a flexible sealant costs very little, takes a day, and occasionally solves the problem outright. It is a reasonable first move while you arrange the bigger job.

If it is a rented or managed property

Photograph everything at each stage and keep the dates. A bathroom leak that damages the flat below becomes a dispute between owners quickly, and a clear record of when it appeared and what was done shortens the argument considerably.