Map Cracking and Hollow Plaster Three Days After Finishing
Internal plaster 12 mm thick is being applied in one coat to a newly built 100 mm hollow block wall with a 1:3 cement to sand mix, because the gang believes a stronger mix is better. The wall is in direct sun and there is a steady dry wind. Plastering finishes at 3 p.m. and nobody returns to the site until the next morning. Three days later the face shows a network of fine hairline cracks over most of its area, and several patches about 300 mm across sound hollow when tapped with a knuckle. A 40 mm deep chase for an electrical conduit was cut in the same wall and filled with the same mortar the day before plastering began. The client has seen it and is asking whether the wall is falling down.
1. Separate the two symptoms. What does the fine crack network point to, what do the hollow patches point to, and how would you confirm your diagnosis on site?
Read the worked answer
Two different faults
- Fine map cracking (crazing): the 1:3 mix is very rich, so the surface layer has high cement content and shrinks a lot as it dries. Direct sun and dry wind dried the surface before it had set, and there was no curing to hold moisture in. No single crack is wide, but the network is a defect and a path for moisture.
- Hollow patches: the plaster has lost bond to the blocks. Likely causes here are blocks not dampened so they sucked water out of the mortar, dust on the block face, 12 mm applied in one coat, or the plaster bridging the fresh chase filling, which shrank away from the plaster above it.
Confirming it
- Tap the whole wall with a knuckle or the handle of a trowel; a dull hollow sound means debonded plaster. Mark the edges of every hollow area.
- Press and look for movement at the edges of patches.
- Check whether hollows line up with the chase and with the sunniest part of the wall, and check whether the cracks follow block joints (that would suggest movement, not shrinkage).
- Tell the client plainly which parts are cosmetic and which are bond failures; do not dismiss the whole wall as normal.
2. Set out what should have been done from the start: mix, coats, preparation and curing.
Read the worked answer
- Mix 1:4 or 1:5 cement to sand for internal plaster, not 1:3. Stronger is not better here.
- Two coats: an 8 to 10 mm undercoat scratched for key, then a finishing coat to bring the total to about 12 mm. Never build 12 mm in one pass.
- Prepare the background: brush off dust, dampen the blocks and let the surface come to a touch-dry state so suction is controlled.
- Deal with the chase separately: fill it, let it cure, and use mesh or expanded metal over the chase before plastering if the chase is deep.
- Cure: keep the plaster damp for at least seven days, shade the wall from direct sun, and avoid plastering in the hottest, windiest part of the day.
- Work to a straight edge and check thickness as you go; hand over only after the surface has been checked by tapping.
3. What is your repair plan, and what repair would you refuse to do?
Read the worked answer
Plan
- Mark every hollow area, then cut it back square to sound plaster with a chisel or grinder, undercutting the edges slightly so the patch is keyed in.
- Brush out dust and dampen the exposed background.
- Fill with a matching 1:4 or 1:5 mortar, in layers if deep, keying the surface of the first layer; finish flush with the surrounding plaster.
- Cure the patch for several days, then fill the hairline crack network with a suitable filler once the wall has dried out; if any crack is wider than hairline or follows block joints, investigate movement before filling.
- Record what was removed and what was done, and report to the supervisor.
Refuse
- Skimming a rich coat over hollow, unbonded plaster, or painting straight over it. It hides the defect, fails again within months and leaves you responsible for work that was known to be bad.