Damp and moisture meters
Can a cold-water cistern in the loft sweat onto the ceiling even though the tank itself is not leaking?
Yes, a cistern can sweat on its outside when moist air meets the cold surface, and those drips can stain the ceiling below while the seams stay tight and the water level stays steady. The survey can note beads on the tank and a stain under it. It does not pressure-test the cistern.
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?
People hear 'tank in the loft' and assume a split seam or a failed ballvalve flooding the ceiling. Sweating is different. The outside of a cold metal or plastic cistern beads in warm moist loft air, the beads run to the underside, and the ceiling stains in a patch that matches the tank. The water line does not drop, and the loft is not flooded. An overflow that drips is a third mechanism. A home survey should say whether the cistern was wet on the outside, overflowing, or actually leaking at a joint.
The cistern may be lagged, boarded round, or out of reach. The inspection does not dismantle the lagging and does not guarantee the tank. If the ceiling stain is under the cistern and the felt around the tiles is dry, do not start by stripping the roof. Improve the lagging and the loft moisture, and only then look at a seam if the stain does not match sweating. This is not a finding that the roof covering has failed, and it is not rising damp in the room below.
What should you ask your surveyor?
- Were there beads on the outside of the cistern, or a drip from a seam?
- Does the ceiling stain sit under the tank rather than under a gap in the tiles?
- Was the cistern lagged, or bare and cold?
What are the typical next steps?
- Do not reroof because a sweating cistern has marked the ceiling.
- Do not treat outside beads as a split seam unless a joint was seen to leak.
- If the tank was not reached, the cause of the stain under it is still open.
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
Is an overflowing warning pipe the same as sweating?
No. An overflow is water from inside the tank running out of a pipe. Sweating is moisture from the air on the outside face.
Does new lagging hide a leak you still need to know about?
It can. Lagging is right for sweating and useless if a seam is split. The survey should not pretend lagging was removed.