Damp and moisture meters
A WC overflow has dripped onto the brick until the mortar is gone. Why is that a plumbing leak and not rising damp from the ground?
Because the water is the WC overflow, not the ground. A WC overflow that has dripped onto the brick until the mortar is gone is a plumbing leak of cistern water, concentrated on one patch of wall. The survey has recorded the drip and the lost mortar under it. It has not diagnosed rising damp, and the pattern does not fit rising damp. Rising damp is moisture from the ground at the base of a wall. This damage follows a pipe that should have discharged elsewhere and instead ran down the face. Read it as plumbing.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
An overflow that drips for months is a slow tap of clean water. Mortar, especially soft mortar, dissolves and leaves the bricks standing with empty joints, often in a streak under the outlet and nowhere else. Groundwater does not choose the brick under a WC pipe and spare the rest of the elevation. The mechanism is local, high or mid-wall, and it stops where the drip stops. Calling it rising damp sends a damp-proofing firm to a cistern fault. The survey can see the pipe and the streak.
Ask the surveyor to trace the lost mortar back to the overflow in the wording. A plumber should stop the drip and say where the overflow ought to discharge. A bricklayer can then point the washed joints, which is a repair of the streak, not a new damp-proof course. Tell the solicitor this is plumbing. Do not notify it as rising damp, and do not let an injection quote attach itself to a pipe the survey has already named.
What should you ask your surveyor?
- Does the lost mortar follow the line of the WC overflow?
- Is the rest of the wall's base free of the same damage?
- Have you called this a plumbing leak rather than rising damp?
What are the typical next steps?
- Read the missing mortar as the result of the WC overflow.
- Do not call it rising damp from the ground.
- Stop the overflow before the joints are repaired.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If the cistern has been replaced, has the mortar repaired itself?
No. A new cistern may stop fresh dripping, and it does not put the mortar back. The survey recorded joints the old drip had already eaten. That damage remains plumbing damage, not rising damp, until someone repoints it. The survey stops at the WC overflow streak and does not add any further conclusion.
Could rain in the same streak be the cause instead?
Only if the survey had not got an overflow sitting on it. Rain is general. This streak is under a drip the report can name. The reason it is not rising damp is the same: the water is not coming up from the ground.