Cracks, Hollow Plaster and Damp Patches: What to Check Before Repairing
Liverpool School of PlasteringUpdated 7 October 2026.
A crack, hollow-sounding patch or damp mark does not tell you the full cause of a plaster problem. Before filling, skimming or replacing anything, record what you can see, check whether the defect is changing, and look for moisture, movement or loss of adhesion behind the finish.
This guide explains the first checks to make on an internal wall or ceiling. It is a symptom checklist, not a remote diagnosis. If plaster is loose overhead, a crack is widening, or there are signs of active water entry or structural movement, keep people clear of the area and arrange a competent inspection.
Quick answer: should you repair it yet?
| What you can see | First check | Do not rush into |
|---|---|---|
| A fine, stable crack | Mark and photograph it, note the date and check whether it returns or changes. | Skimming the whole surface before understanding why it cracked. |
| A hollow sound when the plaster is lightly tapped | Compare it with a known-sound area and map the approximate boundary without striking hard. | Assuming every hollow area must be removed, or that a skim coat will reattach it. |
| A damp patch, staining or salt deposits | Look for leaks, rainwater defects, condensation patterns, high external ground levels and recent plumbing work. | Sealing or replastering while the moisture source is still active. |
| Loose or bulging ceiling plaster | Keep the area below clear and obtain a competent assessment. | Prodding, drilling or working beneath it. |
| A textured coating in an older property | Consider asbestos before sanding, scraping or drilling. | Dry sanding or disturbing an unidentified coating. |
1. Record the defect before disturbing it
Start with simple evidence. Take a clear photograph with a ruler or coin for scale. Record the date, room, wall or ceiling position, and whether the defect appeared after decoration, heating changes, a leak, building work or a period of bad weather.
- Is the crack straight, stepped, diagonal or following a board joint?
- Does it continue through paint only, the skim coat, the backing plaster or the masonry?
- Is the surface dry, cold, stained, powdery, salty, soft or bulging?
- Does the problem worsen after rain, bathing, cooking or use of the heating?
- Are nearby doors or windows sticking, or are there corresponding cracks outside?
These observations are more useful to a plasterer, damp specialist or surveyor than a photograph of a freshly opened hole with no record of the original pattern.
2. What different crack patterns can tell you
A hairline crack confined to the finish may be associated with drying, local movement or a weak junction, but appearance alone cannot prove the cause. Cracks that repeatedly follow plasterboard joints can point towards movement or weakness in the board-and-joint system. Cracks around openings may reflect stress concentrated at corners, although the underlying wall and support still need checking.
Pause the cosmetic repair and seek competent advice when a crack is widening, repeatedly returns after repair, runs through the background, is accompanied by bulging or displacement, or appears with other movement signs. A structural engineer or appropriately experienced building surveyor may be needed where structural movement is suspected.
Do not describe every diagonal crack as subsidence and do not dismiss every fine crack as harmless. The useful question is whether it is stable and superficial or part of a continuing building problem.
3. What “blown” or hollow plaster means
“Blown plaster” is a common site term for plaster that has lost adhesion to the background or between coats. A hollow sound can help locate a possible detached area, but tapping is only an indicator. Wall construction, voids, chasing and changes in material can also alter the sound.
Use gentle, comparative tapping rather than hard blows. Mark the suspected boundary lightly and check for:
- movement when gentle hand pressure is applied;
- crumbling edges, bulges or visible separation;
- rust staining or failed fixings on boarded backgrounds;
- moisture, salts or contamination that may have weakened the bond;
- a ceiling area that could fall if disturbed.
A small, dry and stable defect may suit a local repair. A large detached area, an unstable ceiling, widespread crumbling or repeated failure needs a wider assessment. A new skim coat is not a reliable way to secure loose plaster underneath.
4. Damp patches: find the moisture source first
Historic England notes that excess moisture can come from occupants’ activities, building defects, services, leaking utilities or drains, groundwater and emergency events. It also warns that remedial work can waste money or make matters worse when the cause has not been properly diagnosed.
Check the pattern rather than relying on one damp-meter reading:
- After rain: inspect gutters, downpipes, roof junctions, pointing, render cracks, window seals and external ground levels.
- Near plumbing: check pipework, waste connections, shower seals, overflows and heating systems.
- Cold corners or behind furniture: consider condensation, ventilation, heating and surface-temperature patterns.
- Low-level staining or salts: record ground levels, drainage, wall construction and previous damp-proofing work; do not assume one cause from the height alone.
- A patch that returns after decoration: investigate the moisture source and the compatibility of the repair and coating system.
A handheld moisture meter can be useful for comparison and monitoring, but a reading on plaster or masonry is not a complete diagnosis by itself. Salts and material differences can affect readings. A competent inspection should combine the building history, visual evidence, moisture patterns and appropriate tests.
5. Damp-resistant plaster is not a cure for an active leak
British Gypsum describes Thistle DriCoat as an undercoat plaster for replastering walls following damp-proof-course treatment, and states that the source of penetrating or rising damp must be identified and eliminated. That is a specific use, not permission to apply it over any unexplained damp patch.
Choose the repair build-up only after the cause, background and moisture conditions are understood. Follow the current manufacturer specification for preparation, thickness, finishing plaster and drying. For the practical repair sequence after diagnosis, read our guide to repairing damaged plaster.
6. Traditional and historic walls need compatible repairs
Older solid walls can behave differently from modern cavity construction. Historic England advises looking at the whole building, including changes to materials, rainwater disposal, drainage, heating, ventilation and impermeable past repairs. A hard or low-permeability patch can move moisture and damage into adjacent traditional fabric.
Before removing lime plaster or covering a traditional wall with dense modern materials, identify the existing construction and obtain advice appropriate to the building. Listed buildings and buildings in conservation areas can also require consent for some work.
7. Check asbestos risk before opening the surface
Textured decorative coatings were used on walls and ceilings and may contain asbestos. The Health and Safety Executive says trained people must use appropriate controls even for some lower-risk work, and higher-risk work requires a licensed contractor. Do not dry-sand, scrape or drill an unidentified textured coating.
If the building or finish could contain asbestos, stop before disturbing it and arrange the right survey, sampling or specialist advice for the planned work. Loose material, debris or damaged insulating board requires particular caution.
8. Decide whether this is a repair or an investigation
A local repair may be reasonable when:
- the cause is understood and has been dealt with;
- the surrounding plaster is dry, sound and well bonded;
- the defect is limited and not changing;
- the proposed repair material suits the background and existing system;
- the work can be completed safely.
Arrange further investigation when:
- cracks are widening, displaced or repeatedly returning;
- damp reappears, the source is unclear or several moisture mechanisms may be involved;
- plaster is bulging, falling or widely detached;
- timber decay, corroded metal, failing masonry or movement may be present;
- the wall is historic and incompatible previous repairs are suspected;
- asbestos-containing material may be disturbed.
What information should you give the person assessing it?
- dated photographs and measurements;
- when the defect appeared and whether it is changing;
- whether it relates to rain, plumbing, heating or occupation;
- previous repair, damp-proofing or decoration work;
- the known wall or ceiling build-up;
- any asbestos survey or test information;
- access limitations and whether loose material is overhead.
Frequently asked questions
Can I skim over cracked plaster?
Only after the crack’s cause and the condition of the background have been assessed. Skimming can hide a crack temporarily, but it does not correct continuing movement or reattach loose plaster.
Does hollow-sounding plaster always need removing?
No. A hollow sound is an indicator, not a complete diagnosis. The extent, stability, location, background and consequences of leaving it in place all matter. Loose overhead plaster needs urgent attention.
Can I plaster over a damp wall?
Not while the moisture source remains active or unexplained. Diagnose and correct the cause first, allow the construction to respond appropriately, then choose a compatible repair system.
Will PVA stop a damp patch?
No. PVA is not a repair for rain penetration, plumbing leaks, condensation or rising moisture. Preparation products should only be selected after the background and defect are understood. See our guide to PVA, bonding grit and suction-control primers.
How long should repaired plaster dry before painting?
There is no safe universal number of days. Drying depends on the repair depth, product, background, ventilation, temperature and whether the original moisture problem is resolved. Read how long plaster takes to dry before painting.
Build practical repair skills
Once a wall is dry, stable and suitable for a plaster repair, hands-on practice helps you learn preparation, tool control and finishing in a controlled training bay. Explore the 2-day hands-on skimming course, or browse the Plastering Guides & Advice hub for the next step. Liverpool School of Plastering provides practical short-course training; these courses are not NVQs or accredited qualifications.
Sources checked
Technical sources checked 7 October 2026.