According to the Stucco Manufacturers Association, if a crack is visible from more than 10 feet away or is a source of leaking, it should be repaired. There are two primary reasons for plaster to crack: shrinkage and structural issues. Shrinkage cracks are synonyms to crazing, mapping and spider web cracking, which may develop as excess water evaporates during the cements’ drying process. Shrinkage cracks typically occur very early and often cannot be seen until after the finish color coat (stucco) is applied. In the finish coat, hairline or eggshell cracking or checking can also be the result of rapid drying which most commonly occurs on hot and windy days. This often demonstrates that the applicator failed to properly hydrate the system during application.
Stress cracks are synonyms to corner, lateral, vertical and reentrant cracks; also larger more pronounced mapping or spider web cracking. They occur when stress is transferred to the plaster system from various internal and external sources. Understand that plaster is a brittle material so it can’t handle tensile forces well however, it does perform well in compression. Stresses are due to many factors including: ground movement due to poor soil conditions, compaction practices or seismic event, foundation problems such as slab or footing creep, sag or curling, framing or structural anomalies (such as offset framing loads), large or long spans, point loads, green lumber, warped or twisted lumber, improper notching of lumber, cantilevers, absence of vibration control in design to deal with mechanical equipment, improper spacing of wood wall sheathing members (failure to provide gaps along the ends and edges), failure to install corner reinforcement at reentrant corners (corner openings of doors, windows, etc.), improper attachment of plaster netting or accessories, and failure to properly engineer and construct a stiff building system that meets L/360 to withstand lateral forces such as wind loads. The latter being one of the biggest causes of plaster failure.
Preventing Premature Plaster Failure
The first step in making sure a plaster system will perform with minimal cracking starts with an experience owner who is properly apprised by his design professional. Proper project planning, design, engineer and clear detailing sets-out the bases for a proper bidding phase. An experienced general contractor, who supervises highly knowledgeable subcontractors, creates yet another line of defense to minimize cracking. Lastly, qualified third-party inspectors should be retained by the owner to critic the design and work performed by the contractors for conformance with the approved plans and specifications in accordance with proper trade and industry practices, complying with all applicable codes, and meets the manufactures’ application requirements.