Diagnosing blistering on a newly skimmed ceiling
Three weeks after you finished a 30 m2 plasterboard ceiling with a gypsum skim, the client reports blistering and small areas of hollow sound near the centre. The ceiling was boarded with 12.5 mm boards fixed at 400 mm centres, skimmed in two coats and painted after ten days. The room is a bathroom with an extractor fan that the client says is 'sometimes used'. You have a tap test, a moisture meter, and a straight edge. The client wants the whole ceiling redone at your cost.
1. How would you investigate the cause before accepting liability?
Read the worked answer
Carry out a systematic inspection. Tap test the whole ceiling and mark hollow areas. Use the moisture meter on the plasterboard and skim, comparing the affected centre with the edges. Check board joints, fixings and any signs of movement. Use the straight edge to check for undulation. Record the extractor fan use and the room's ventilation. Look for signs of condensation, such as mould or water staining. Photograph and date all findings. Only after this evidence is gathered can you judge whether the cause is workmanship, background moisture or environmental conditions.
2. What are the likely causes, and how would you distinguish between them?
Read the worked answer
Blistering and hollow areas can come from inadequate key or contamination on the board, from moisture behind the skim, or from condensation in a poorly ventilated bathroom. If the hollow areas follow board joints or fixings, movement or fixing is likely. If moisture readings are high across the centre and the room is poorly ventilated, condensation or background moisture is likely. If hollow areas are scattered and the moisture is normal, poor bonding from dust or suction is likely. The evidence from the tap test pattern and moisture readings is what distinguishes these causes.
3. What rectification and communication would you propose?
Read the worked answer
Rectify only the defective areas if the cause is workmanship, cutting back to sound plaster, sealing and re-skimming to match. If the cause is environmental, explain the evidence and recommend improved ventilation before any repair, because re-skimming without fixing the cause will fail again. Provide a written report with photographs and readings. Offer a repair plan and a realistic timescale. Avoid agreeing to redo the whole ceiling before the cause is established, as that accepts liability you may not owe.