THICKNESS OF PLASTER
It would be pleasing to all persons concerned with a construction project if portland cement plaster (stucco) could be applied to uniform thickness over all surfaces of walls and soffits. That would be the ideal situation, but, viewed from a practical aspect, it is unattainable. Thickness of plaster may be expected to vary, on any project, due to one or more of many factors.
Two principal categories exist for determination of caliper of a plaster membrane. One category relates to plaster applied over frame construction, either wood or metal; and the second relates to plaster applied over a solid substrate such as concrete block walls, bricks, stones or cast concrete. One major cause of variation in thickness of portland cement plaster is unevenness or misalignment of the base wall over which plaster is to be applied.
It is hoped that application of plaster over exterior walls will overcome and hide variation in alignment of the framed or solid substrate, to create a more pleasing building. Misalignment of a solid substrate may happen because of problems with the forms that determine straightness (or lack thereof) of concrete walls. Variation in alignment of concrete block walls may occur because of misalignment of blocks by the mason. Lack of plane also occurs in walls of brick or stone.
Wood frame walls suffer from warped and twisted studs, misalignment of studs as nailed to the toe plate and top plate by carpenters, as well as other problems. Metal studs may suffer from misplacement by installers. It is understood that the base wall (the substrate) may bow outward or dip inward because of one or more of the causes listed above, or for some other reason. The architect, owner and builder want application of plaster to obscure those deficiencies in alignment and to develop a reasonably plane and true surface. To achieve an acceptable degree of straightness on the outer surface of a plaster membrane that has been applied over a misaligned substrate necessitates variation in thickness of the plaster membrane. Some areas of plaster will be greater in thickness than required by the appropriate building code and project specifications. Other areas of plaster will be less than the thickness required by code and specifications.
That variation may accompany and be a result of straightening of the outer, finished surface of the wall with plaster. Table 47-D of the Uniform Building Code only addresses thickness of the scratch coat. That is proper, in my opinion. Salient items of Table 47-D and footnotes to that table are provided immediately below. My notes provide the gist of the table and footnotes, not an exact copy of the UBC.
The finished thickness of cement plaster applied over expanded metal lath shall be a minimum of 5/8 inch. When thickness of the plaster membrane is measured from the BACK PLANE of expanded metal lath, exclusive of ribs in the case of ribbed lath, or self-furring, lath, plaster thickness shall be 3/4 inch minimum. In the case of cement plaster applied over wire fabric lath, often called stucco mesh or stucco netting, minimum thickness is described as 3/4 inch minimum for INTERIOR application.
EXTERIOR application of cement plaster over wire fabric lath shall be done with minimum thickness of 7/8 inch, when measured from the FACE of the support or backing. Thickness of cement plaster applied over masonry walls is to be a minimum thickness of 1/2 inch. Thickness of cement plaster applied directly to concrete walls is to be a maximum of 7/8 inch. Thickness of cement plaster applied directly to concrete ceilings is to be a maximum of h inch. Footnote #4 advises that "Because masonry and concrete surfaces may vary in plane, thickness of plaster need not be uniform." Footnote #7 comments that "On concrete ceilings,when the basecoat plaster thickness exceeds the maximum thickness shown, metal lath or wire fabric lath shall be attached to the concrete."
The ICBO does not offer the following comment, but I do, because it is appropriate to construction. Any substrate that is to be covered with cement plaster may vary in plane, and thus, thickness of plaster need not be uniform.
It is my opinion that the average thickness of a plaster membrane should essentially comply with the requirements described in the Uniform Building Code and in job specifications. Table 47-D of the 1985 edition of UBC states that thickness of cement plaster applied over monolithic concrete walls shall be a minimum of 7/8 inch thickness. It is my opinion, if lack of straightness of concrete walls would require a plaster fill of greater thickness, that expanded metal lath be securely attached to the concrete wall to support and hold the plaster directly to the concrete substrate. If approved acoustical plaster is to be applied directly to concrete, or is to be applied over base coat plaster, the thickness of 7/8 inch may be exceeded. The reason for that difference is the reduced density of acoustical plaster as compared with cement plaster.
The UBC requires that a maximum of 1/2 inch thickness of cement plaster be applied to a monolithic concrete ceiling. If it is necessary that plaster applied to a ceiling exceed 1/2 inch in thickness, metal lath or wire fabric lath shall be attached to the concrete.
In reference to thickness of individual coats of cement plaster, various editions of the Uniform Building Code only address minimum thickness of the scratch coat, and minimum thickness of the total of the two base coats. The UBC does not specify a minimum thickness of the brown coat. It is my opinion that the ICBO is correct in writing Table 47-F in that manner. I prefer that the scratch coat be laid on as thickly as possible, and the brown coat applied only as a dressing coat to fill the score marks in the scratch coat, add thickness to the total membrane, and serve other useful functions. Architects NEVER should specify minimum thickness for a brown coat of cement plaster. Instead, architects may specify minimum thickness for the scratch coat, and minimum total thickness of base coats (scratch and brown) for cement plaster, and minimum thickness for the total plaster membrane of scratch, brown and finish coats. Specification of minimum thickness for a brown coat of portland cement plaster is self-defeating for the architect, if the other criteria are observed.
Variation in thickness also may be caused by installation of lath, and by application and dressing of the surface of cement plaster. Outward bowing of lath may result in thinner application of plaster, and inward bowing may cause greater thickness of plaster. If plasterers do not control average thickness of the membrane by considering average thickness over studs, the wall will not meet requirements. It must be recognized that some developers and builders negotiate with, or try to persuade plastering contractors to reduce the cost to the builder for application of cement plaster. That may involve employment of only basic application procedures, generally with verbal but not written knowledge and agreement of the builder. As an example, application might involve non-use of a darby for leveling and dressing purposes. Use of only a trowel and float to dress the surface of the brown coat may result in greater variation in thickness.
A developer or builder may wrongfully request a plasterer to apply cement plaster over frame construction in only two coats, rather than three layers, to reduce the cost for the building. That is a direct violation of Section 4708 (a)of the Uniform Building Code, and also is a frequent cause of complaints about quality of the finished job.
In conclusion, it should be recognized that thickness of cement plaster applied over any substrate may vary in thickness due to unevenness of the substrate, or to application procedure, but frequently is associated with lack of straightness of the base wall behind the plaster.