Plaster Cracks and Failures

In the last few months the Association has had numerous queries on plaster cracks. This has been from both contractors and their clients. Our members have been concerned by the increased incidence of shrinkage cracks appearing in the plaster during the curing period. A contractor must be mindful that once dry, plaster has almost no elasticity and is almost always subject to some cracking over time. Any type of structural movement or impact will cause damage to the plaster.

The following factors play a significant role in the quality of the final plaster coat.

1. The quality of the plaster sand is the most important factor. The sand must display the correct grading and have no impurities. Quality plaster sand is becomming extreemly difficult to acquire as the marority of the pits are reaching the end of their lifespan and licences for the mining of sand within the coastal dunes are almost impossible to obtain.
2. Plaster sand must be passed through a sieve with the correct grading to remove all lumps, larger particles and any vegetation.
3. The mix design must be strictly adhered to with the correct volume of water to achieve correct hydration and the desired workability.
4. The apparatus used for transporting of the plaster to the work face must be clean with no old residual plaster in the load bin.
5. Batter boards must be cleaned of old plaster before the new batch is discharged.
6. Weather conditions play a large role and if it is extremely windy and dry the plaster with dry very quickly with shrinkage cracks very evident in the finished product. If these weather conditions are prevalent then moist curing of the plaster is required.Suction into the walls if the bricks are absorbent and have not been dampened.

Plaster is deemed to be acceptable if the cracks and debonding from the superstructure are within the following parameters:

1. The cracks are fine cracks in a hexagonal pattern which measure between 5 and 75mm across each hexagon and the cracks are shallow and do not extend through the whole depth of the plaster.
2. Debonding of plaster is detected when a hollow sound is emitted when the plaster is tapped. Plaster will debond from the superstructure because the outside skin of the plaster that is exposed to the air will shrink at a different rate to the skin in contact with the wall. Small areas of debonding about the size of a side plate are acceptable but larger areas should be removed and replaced.

If the above guidelines are followed the incidence of plaster failure should be reduced.

Ross Stembridge | Building Services Manager