Stud cracks are essentially straight-line, vertical fractures in plaster which develop over or near the alignment of studs. They are observed most frequently over wood framework, but occasionally may occur over metal studs. Stud cracks may be noted in walls of residential, commercial, educational, industrial and other types of buildings, single-story or multi-story. The following discussion represents some of the factors that I believe may cause plaster to crack in alignment with studs.
Warping or twisting of studs, where deformation of wood framing members imposes greater force against a plaster membrane than the material can resist. Warping or twisting of wood framing members must be considered an act of nature over which mankind has no control. Other factors listed here should be considered as being under the control of some person on the construction project.
Shrinking studs, due to additional loss, after installation, of original moisture; or drying of wood framing members subsequent to wetting by rain, fog, frost or other source of moisture. It is hoped that all wood framing members would have been dried to a reasonable and suitable level of moisture content prior to placement in the walls.
Subsidence of studs upon the toe plate. Carpenters frequently nail the top plate to studs before the toe plate is attached. This sequence causes all studs to be snugly secured to top plates, but may cause a gap beneath the base of some studs, above the toe plate. Some studs may shrink to a shorter length than other 2 x 4s, 2 x 6s or greater dimension of lumber, due to variation in moisture content, susceptibility of some pieces of wood to shrink more than others, etc. I suggest that the toe plate be nailed to studs before the top plate. Plaster that is substandard in thickness will crack more readily than thicker plaster, all other factors being equal. Thinner plaster may not have sufficient mass to resist natural and unnatural forces as well as a thicker membrane of plaster.
It should be recognized that one of the disadvantages of two-coat cement plaster over frame construction, as opposed to the standard three-coat application, is the greater probability of two-coat plaster being applied somewhat thinner than that which is desired.
Insufficiently cured cement plaster may be less able to withstand the stress of warping lumber and other forces than adequately cured plaster.
Vibration from heavy traffic, transmitted from a nearby railroad track, or from truck and bus traffic on a nearby street, may cause excessive stress in buildings. It would be advisable that the building in question be checked for sensitivity to that degree of vibration.
Subsidence of a building.
Heavy rolling doors installed in walls of a building may transmit enough vibration to crack walls, especially if the tracks of the rolling doors are not aligned well.
Nailing or bolting fencing to plaster walls may sometimes induce cracking. When lag bolts, used to secure dimensional lumber to walls for support of a fence, gate, patio cover or other feature, are tightened excessively, the plaster may be fractured. (Note that EVERY opening through exterior walls of wood, plaster, metal or masonry MUST BE CAULKED PRIOR TO INSERTION OF A NAIL OR BOLT, to seal against intrusion of water along the shaft of the fasteners.)
Vibration from machinery, such as air conditioning equipment, sometimes fractures walls, especially at start-up of equipment, when fans and other features may be out-of-balance. Vibration of all equipment must be isolated from walls.
An accidental blow from a vehicle striking a building must be expected to cause damage of this kind.
Severe blows from swinging doors violently blown open or shut by powerful bursts of wind, or by careless people, may fracture the membrane.
Attachment of horizontal ledgers to walls for support of an auxiliary structure such as a patio cover, carport or other attachment, should be accomplished with judgment and care. Extreme tightening of bolts used to fasten pieces of lumber to walls of buildings may impose excessive stress against the substrate.
Earthquakes and minor earth tremors must be expected to cause ill-effect upon buildings, concrete patios, driveways and walks, masonry work and other construction features.
Racking action, imposed against buildings by strong bursts of wind, may be expected to fracture plaster sometimes. This problem is seen most commonly in mechanical rooms and penthouses built on roofs of structures, where they are exposed to greater force than lower portions of the building, and in buildings erected on or near tops of hills, where occasional powerful bursts of wind may strike the walls.
If, during construction of a project, a pattern of vertical, essentially straight-line fractures becomes apparent in walls, over or near the alignment of studs, the project superintendent should periodically check wood framing, thickness of plaster over studs, as well as the other factors that I have enumerated. Note that it could be misleading to measure thickness in the space between studs. The average thickness of plaster over studs should be seven-eighths of an inch, although thinner plaster must be expected in some locations, because of the need to dress finished walls in a manner that will straighten and correct misalignment of framing in the walls. It is expected that a membrane of plaster will provide some esthetic correction for misaligned framing. For that reason, some variation in thickness will occur. Some locations over studs will be greater than seven- eighths inch in thickness, to correct for inward curvature in alignment of framing, while other locations will have less than seven-eighths inch thickness over studs.