Also known as angular cracks, corner cracks, door and window cracks, etc.
Reentrant cracks are fissures in vertical Portland cement plaster walls or horizontal soffits as well as in concrete slabs on-grade or elevated, placed in horizontal alignment. Reentrant cracks start at the apex of inside ninety-degree corners, typically. They may start, also, at inside acute or obtuse angles in Portland cement concrete or in plaster. Reentrant cracks are widest in gap at the apex of the inside corner where the fracture starts, and reduces to no gap (the end of the fracture) somewhere a few inches to sometimes, a foot or two from the edge of the opening. Long cracks are the result of significant structural force applied against the plaster membrane within the wall system, while short cracks develop because of a lower level of stress.
The reason that the widest gap of the fissure is located at the apex of the inside corner, and ceases to exist a short distance from the edge of the opening, is described in the following text.
The stress which has caused the angular fracture apparently concentrated its effect at the apex and edge of the opening, and did not produce an adverse effect more than a short distance from the edge. The force of stresses normally is insufficient to cause a long fissure, and oftentimes is not sufficient to crack the plaster or concrete to any extent.
Reentrant cracks normally are aligned at an angle from the apex of the corner into the field of plaster or concrete, but sometimes, may be nearly vertical or horizontal in direction. When wood trim has been buried in plaster around the perimeter of windows, or by doors, angular cracks may emanate from the corner formed by the juncture of two pieces of lumber.
In the design of concrete slabs, if interior openings are to be created, it is desirable that corners of inside openings be rounded, or that control joints or relief joints be placed to intersect inside corners of openings.
In an attempt to prevent angular cracks at corners of windows, doors, vents, meter boxes and other openings in walls, it may be helpful to consider the following procedure.
Strips of 2.5 Ib. galvanized expanded metal lath may be cut to form rectangles about three to four inches wide and about ten to twelve inches long. Those Strips, called butterflies, may be pressed lightly into the surface of the scratch coat, immediately after the scratch coat has been scored. Alternatively, the strip of lath may be embedded within the brown coat, and covered with cement plaster as additional brown coat is applied. It should be noted that installation of butterflies should be specified by the architect or builder, with recognition of the additional cost, or butterflies will not be installed. Normally, owners of buildings will grant the additional expenditure for installation of butterflies only on custom homes and commercial construction, and sometimes not even in those instances.
If internal stress develops anywhere within the wall system, from warpage, shrinkage or expansion of framing; buckling or curling of shear panels of plywood, chipboard or other; subsidence of the structure; violent slamming of doors or other movement of the building, is sufficient to overcome the crack-resistance of the plaster membrane, development of a reentrant crack may be anticipated.
Corner cracks, per se, are not physically harmful because of the asphalt-treated weather barrier membrane of building paper or felt installed behind the plaster membrane. In many of the instances in which I have observed angular cracks at inside, ninety-degree corners, the fissures have been fine and tight, and have not been an esthetic problem, either.
All concerned persons should be aware that plaster will not fracture of its own volition in that manner, and that some physical force within the wall system must initiate the crack.