Moisture reading before skimming a refurbished wall
You are asked to skim a 24 m2 internal masonry wall in a 1930s house that has just been replastered in sand-cement after damp-proof course injection. The builder wants the wall skimmed tomorrow so the decorator can start. A protimeter gives readings of 22% in the centre of the wall and 14% near the top corners, taken at 15 mm depth. The wall is cold to the touch and the room is unheated with no windows open. The specification states gypsum skim must not be applied to backgrounds with a moisture content above 5% WME (wood moisture equivalent) unless a suitable barrier system is used. The client is pressing for completion because they are moving in within ten days.
1. What should you do before any skim coat is applied, and why is the reading pattern significant?
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Do not skim. The readings of 22% WME against a 5% limit mean the background is far too wet, and the pattern (high centre, lower at corners) suggests moisture is still migrating from the injected zone and has not had time to dry evenly. The cold, unventilated room will slow drying further. The correct action is to record the readings with locations and dates, explain the specification limit to the builder and client in writing, and recommend ventilation and/or controlled drying with a dehumidifier, then re-test. The centre reading is the controlling figure because it is the highest and represents the last area to dry. Skimming now would trap moisture, risking blistering, efflorescence and debonding of the skim.
2. The builder proposes a gypsum-based bonding agent to 'seal it in'. Evaluate this proposal.
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A gypsum bonding agent does not form a vapour barrier and will not solve a moisture problem; it may even reduce the ability of the background to dry. The specification allows a barrier system only where suitable, which normally means a moisture-tolerant system such as a renovating plaster or a liquid-applied damp-proof membrane with a compatible finishing coat, not a standard gypsum skim over a wet background. Adding a barrier over 22% WME could trap water and cause the finish to fail. The safer route is to let the wall dry to within specification, or agree a documented alternative system with the specifier. Any change should be confirmed in writing before work proceeds.
3. How would you communicate the delay without damaging the client relationship?
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Explain the technical reason plainly: the wall is holding too much water and a skim now would fail, costing more time later. Give a realistic drying plan with re-test dates, and offer a written record of the readings. Offer to sequence other work so the decorator is not idle, and confirm the programme in writing. This shows you are protecting the client's interest rather than simply refusing to work.