Stress cracks in plaster, while common in older structures, can be just as prevalent in modern plaster applications. Understanding the leading causes of stress cracks can help inform possible remedies. We have compiled some of the leading causes of stress cracks visible on the surface of historic plaster finishes and some of their possible remedies.
High humidity environments can be detrimental for wood-framed structures. When wood is repeatedly exposed to high humidity environments, it often causes the areas around the studs to swell, pushing the plaster surface outward or deflect. Similarly, expansion can cause the wood lath to separate from the wood studs within the wall. To help remedy this, a moisture-poof barrier should be installed between the exterior surface and interior framing, however, that was not always the case for historic buildings. This will not only help protect the plaster application, but also will increase the longevity of the framing, insulation, and other structural elements within the wall. Seasonal weather cycles always play a role in the expansion and contraction of building materials. This movement over time causes stress and failures at the plaster layers. This concept basically describes the coefficient of thermal expansion and how the size of an object changes with a change in temperature.
Separation From Lath:
Plaster can separate from the lath for a variety of reasons, including expansion of the wood lath/wood framing, deterioration of metal nails used to secure the lath to the framing, nails backing out of their position, seismic activity, and improper plaster application/inadequate curing methods. Separation due to wood expansion, lath movement, and metal nail deterioration can often be solved by re-securing the lath to the framing; this may require some plaster to be removed. Seismic damage restoration is often unique to each case and dependent on the extent of damage. Small damage can often be fixed simply be re-securing the plaster mechanically or with acrylic consolidants. Unfortunately, improper plaster application, or inadequate curing, often results in all of the plaster having to be removed and reapplied. It is imperative that all plaster work is conducted by an experienced professional.
Especially common in older structures, building settlement is to be expected. Often, capable architects and designers can anticipate a certain level of settlement, and construct finishes and structural elements to help compensate for this natural shift/displacement of load. In spite of their best efforts, large cracks can still form. If lath movement is not to blame and there is no physical damage to the structure, then they are simply patched. Large cracks in corners, around stairways, and on load-bearing walls are usually due to issues with the structural integrity of the building. To ensure the physical structure is not compromised, an expert should be consulted; this usually results in some portion of the wall having to be opened. These types of cracks are often a visible indicator of larger structural problems in the plaster and behind the wall. Structural cracks can fall away from the surfaces, which can be a safety risk. This cracks should be monitored on a regular basis for expansion.
Uneven Load Distribution:
Multilevel buildings are at a greater risk for uneven load distribution. The weight of a second and third story, plus the roof, can cause the building’s load to be focused in select areas. Moreover, this can be exacerbated by heavy furniture and crowded placement. If rearrangement of furniture does not help, then it is important to seek the advice of a certified building engineer. Often, the solution will be specific to each project. It is important to share any information you have about renovations or expansions made over the history of the building. Often, floor joists were cut to allow plumbing and electrical conduit to pass from room to room—this commonly results in sagging ceilings and floors. Depending on the extent of work need to dissipate the load evenly, entire plaster sections might have to be replaced.
Crumbling brick, or soft soil can cause a portion of the structure to sag or sink. Foundation repair should always be left to the experts. Once soil samples have been taken, water runoff paths identified, and the foundation fully inspected, a plan can be drawn up to help fix the structural problem. After the work has been complete, the plaster can be re-inspected for any additional damage the work might have caused. If the cracks are large, it is best to remove the section of plaster and replace it; smaller cracks might be reparable.
Plaster Materials & Applications:
The correct selection of plaster materials, aggregate, binders and ratios often play a role in system failures and cracks. If there insufficient aggregate or binder, such as, animal hair thoroughly mixed into the plaster this will weaken the plaster and will crack or fail otherwise down the road. Also, if the plaster is to thinly applied it can also fail or crack.
Stress cracks visible in plaster are often indicative of larger structural issue. It is important to always consult an expert to help identify if the problem is severe and immediate action should be taken. Never repair plaster without first reviewing possible contributing factors and identifying a possible treatment plan.