A Variety of Factors Can Cause Plaster Cracks
How can a property owner be sure that the plaster provided by the building contractor is acceptable, and if not, what can be done about it? It’s not a simple question to answer. Here we discuss perhaps the most obvious defects in portland cement plastering: cracks – those that are non-structural and the more challenging and nasty type: structural cracks.
Non-structural cracking is a network of fine cracks, usually in a hexagonal pattern, which typically measures between 7/32″ and 5/16″ across each hexagon. Craze cracks are usually very fine and shallow and do not extend through the whole depth of the plaster. They are usually the result of over-trowelling a rich mix – one with a high cement content – or using sand containing an excessive amount of dust: more than 15% by mass passing a 0.075 mm sieve. Crazing often occurs within a few hours of the plaster being applied to the wall and cracks may hardly be visible until dust or moisture makes them noticeable. However, the good news is that craze cracks are of relatively little importance, they do not open and close with time, and can be covered using a reasonable quality paint. If necessary, glass fibre tissue can be applied during the painting operation.
Map cracking is similar to crazing except that it is usually deeper – sometimes going right through the plaster – and the hexagons of the pattern may measure are more than an 1/2 inch across. These cracks normally occur when a plaster mix with a high cement content is used or the plaster is allowed to dry too quickly.
Causes of such excessive early moisture loss may include:
- Evaporation – if the wall is not protected from sun and wind.
- Suction into the walls, if the substrate is absorbent and has not been isolated.
- Use of a sand that is badly graded and lacks fine material (less than 5% by mass passing the 0.075 mm sieve).
- Not using building lime or a masonry cement when the sand lacks fine material.
When the cracks are noticed while the plaster is still plastic, they are often floated closed, only to reappear some time later. These cracks can be filled with a proprietary filler and be painted over. Glass fiber tissue can also be applied during painting. Cracking which results when an excessive amount of water is lost from the plaster in the first hours after application is known as plastic shrinkage cracking. Map cracking can be due to plastic shrinkage as can the horizontal cracks which form at corners and between windows.
Drying Shrinkage Cracks
Drying shrinkage cracks are the result of moisture loss after the plaster has hardened. Plaster will always shrink and crack so it is desirable that is should develop a large number of fine, unnoticeable cracks at close spacing. Plasters with very high cement contents and those made with poor quality sand having a high water requirement will tend to develop a few, widely-spaced cracks. Plaster applied in layers that are too thick will also tend to crack in this way. These cracks are said to be normally stable and able to be filled with a proprietary filler and painted over.
Finally, you could encounter cracks that are clearly structural. These may have resulted from cracking of the wall caused by differential movement of the foundations, moisture expansion or drying shrinkage of the substrate, or physical movement of the wall. This type of crack often forms in straight vertical or horizontal lines, or in stepped diagonal lines, and may be quite unsightly. Because these cracks originate in the wall and not in the plaster, repairing the plaster is ineffectual. A specialist should be called in to establish the cause of the cracking and to recommend remedial measures that may include structural alterations which change cracks into movement joints.
For Further Reading:
Resources for further research into exterior cement plaster cracking include:
• “How to Keep Plaster From Cracking”, Professional Builder.com: https://www.probuilder.com/how-keep-stucco-cracking
• “Avoid Cracked Plaster By Using Stucco Control Joints”: https://www.thebalancesmb.com/stucco-control-joints-844422
• Exterior Cement Plaster installation: https://www.cement.org/learn/materials-applications/stucco/stucco-frequently-asked-questions