Portland cement plaster frequently is applied to horizontal or sloped surfaces such as those listed below.
tops of parapet walls
tops of wing walls, and modesty walls at decks and patios
protrusions (so-called "eyebrows") which project over windows and doors
shed-like extensions of buildings
tops of fences
tops of stairs railings, whether horizontal or sloped
arches and canopies over entrances to buildings
plastered cross beams between vertical columns
decorative horizontal runs on vertical walls of buildings, and
recessed window ledges.
Most of the described surfaces will be flat or horizontal, but the tops of some may be convex, sloped to one side, or ridged. Each of these examples is acceptable procedure. Convex, sloped, or ridged conformation is preferred.
Several precautions should be adhered to during planning and construction of those types of surfaces, to prevent intrusion of rain water, irrigation water, or melt from snow or ice. Moisture may pond, even if only slightly, on such surfaces, unless there is sufficient slope or curvature designed into the top surface. A coarse or deeply distinct texture on the upper face is more prone to retention of water on the surface than a float finish.
It is advisable that tops of all plastered surfaces be designed and executed in a manner that will encourage run-off of water, rather than retention on the face.
Some moisture may pass through the plaster membrane over an extended period of time, whether through cracks, fine fissures, at the base of installed iron work or because of some other situation. The important factor is that moisture must be prevented from passing through the weather-barrier membrane installed across the top of the parapet, wing or modesty wall, fence or other horizontal or sloped surface, to wet and expand underlying wooden framing members. As dry wood absorbs moisture, expansion occurs within the section of wood, whether it be framing lumber, plywood sheathing or other wood product. As the wood expands, the tremendous force exerted by swelling wood may fracture a plaster membrane on top of or beside the wood. Cracks formed by expanding wood will allow more water to enter the system, to exacerbate an already unsuitable situation.
Another reason why moisture may pass through a plastered top of any of the features listed in the first paragraph will be described. Some moisture from ponding rain water, irrigation water or melted snow or ice, may reside on top of the wall for an extended period of time, and slowly be absorbed into the surface of the plaster. If freezing occurs while moisture is saturating the surface, the expansion of water as it changes from the liquid phase of water to the solid phase of ice, in the pores of plaster, will rupture the surface slightly. Over a period of time, as additional freezing occurs, the ruptured condition will become exacerbated, to allow somewhat deeper penetration of ponded moisture into the plaster membrane. That freezing action may, over an extended period of time, allow intrusion of water downward to the weather-barrier membrane installed below the plaster.
If that action occurs, it is the opinion of the writer that the plastering contractor should not be considered to be at fault. That gradual deterioration of a surface must be considered to be an act of nature, beyond the normal control of the installer of the cladding.
A fairly common way in which moisture might pass through the weather-barrier membrane is downward along the shafts of nails or staples driven vertically through the membrane to fasten lath, building paper, or metal trim such as cornerite to the top surface of arise of the wood framing member. Nails or staples never should be driven vertically downward through the weather-barrier, to breach the protective layer and provide entrance for water. Fasteners such as nails and staples always should be driven horizontally through the sides, rather than vertically downward through the top, to fasten any material or device, if at all possible.
It is expected, of course, that the architect or designer will specify installation of two layers of good quality asphalt-treated felt or building paper across the top of any of those construction features listed at the opening of this advisory paper. Two layers of 60-minute Grade D building paper, two layers of #15 or heavier asphalt-saturated felt, or a layer of heavy duty mineral-surfaced roll roofing should provide an adequate barrier against passage of water, unless the barrier is breached by nails of staples, or is torn or punctured. If any of those adverse situations should develop during construction, an additional two layers of building paper or felt should be laid across the violated section.
Intruded moisture may create such problems in the future life of the structure as stains, cracks and wood rot, often incorrectly called "dry rot". Note that wood decay is caused by moisture, not dryness.
The attachment of building paper or saturated felt, lath, cornerite or other fixture by nailing or stapling vertically downward through the cap sheet that was installed to protect the top of a parapet wall, stair railing, modesty wall or other construction feature, or ledge, whether horizontal or sloped, is unproper construction practice, in the opinion of the writer. Builders should caution their carpenters about this when installing a protective barrier across the top of any wall. Roofers and lathers also should be made aware of the potential danger that the practice of vertical nailing may introduce into a building.
A preferred procedure is that building paper or saturated felt, lath, paperback stucco mesh, cornerite or other fixture should be nailed horizontally. The weather barrier should not be violated or breached by ornamental iron workers, carpenters, roofers or lathers.
Additionally, when sheet metal coping is installed across the top of a parapet wall or similar feature, nails or other fasteners should not be driven vertically downward through the metal cap and underlying weather-barrier, unless provision is made to seal against intrusion of water downward along the shaft of the fastener.
If some cracks have developed on the top surface of walls, railings or other feature, the following procedure is suggested for remedial work. Cracks may be evident as only very fine fissures on the top, as a more evident crack extending longitudinally along the top of the wall, sometimes in the central one-third width of the top of an arris, or somewhere below and parallel with the horizontal or sloped arris of the wall.
The surface of the top and sides should be cleaned of aerial debris which could act as a form release agent and block bonding of a new coat to existing plaster. I suggest that another substantial coat of stucco, of the same color as was originally applied, be laid over the top surface, and uppermost part of the side walls, to cover any existing fractures. In the event that a fracture has formed roughly parallel in alignment with the arris of the wall or railing, an additional coat of stucco should be applied to the cleaned surface. That new coat of stucco should be fortified with acrylic resin, and should be applied to the uppermost part of side walls as well as the top surface, if a crack has developed parallel with the arris of the wall or railing. The new coat stucco should be applied to an alignment about an inch below any apparent existing parallel fracture. That additional coat should be laid on to a good thickness, of perhaps 3/16 of an inch, for adequate protection of the wall. Additionally, if the top of the original wall, railing or fence had not been curved, sloped or ridged, it would be advisable that the desired slope be developed in the new coat, to facilitate run-off of water, and avoid ponding on top. Also, even though texture on vertical side walls might be deeply imprinted, it would be preferable if texture of the coat of stucco across the top of the wall or railing be reduced in distinctness. Deeper texture on top of a wall or railing will entrap and retain water more than would be the case with reduced texture.
In applying the additional coat of stucco, care should be taken that arisses or shoulders at the top of the wall or railing be thick enough to withstand stress.