DOUBLE BACK APPLICATION OF PLASTER
Conditions may exist on certain plaster jobs which cause it to be suitable or even advisable to double back with the brown coat over the scratch coat.
Conversely, there are situations where conditions on the job site dictate that the brown coat NOT be doubled back over the first or scratch coat of cement plaster.
The International Conference of Building Officials, in the 1976 edition of the Uniform Building Code, approved doubling back as an alternate method of application, in Section 4708(e). Thus, in those areas where the Uniform Building Code is the basic construction code, doubling the brown coat over the scratch coat is in compliance with code. Doubling back should be done ONLY when conditions are favorable. A description of suitable conditions will be provided later in this treatise.
Advantages and disadvantages of the double-back method of application should be considered in each case where the system has been proposed, to determine guidelines which would be applicable for each project.
An advantage of doubling back exists during periods of adverse drying conditions, in which mix water evaporates rapidly from the scratch coat, to the detriment of applied plaster. An example of conditions, which would conform to that category, and could lead to consideration of doubling back, would be a hot or windy day, which would induce rapid loss of moisture from the scratch coat.
Another advantage could be possible improvement of conditions for curing during application of cement plaster, because of the following benefit. Excess mix water from the brown coat is reintroduced into the scratch coat by absorption and thus, desired hydration of cement paste in the lower coat of plaster becomes a continuing reaction, thus making a stronger, better membrane of the base coat of plaster.
Additionally, laying another coat of plaster over the scratch coat during the early portion of the period of potential hydration creates a thicker layer of cement plaster to serve as a reservoir to retain moisture within the plaster membrane, and assist in additional curing of the cement.
The benefit that may accrue from application of the brown coat over the first coat, by doubling back, is obvious from the factors described above.
It should be noted that there is no need for doubling back when moisture is retained in the scratch coat for a relatively long period of time, as may be the case under certain job conditions. Under such circumstances, it would not be normal for the double-back system of application to be used. Typical conditions which would be conducive to retention of moisture in cement plaster are:
- application of cement plaster inside a room or building;
- where a drying wind, or higher temperature, is not a problem at job site; or
- where a high level of relative humidity exists at the site, as in the presence of an overcast sky or fog at the site.
Situations sometimes exist on a job site which require additional dwell time, or residence time, of the scratch coat on walls of a building before a brown coat should be applied. Some potential conditions, which oftentimes suggest extended dwell time on a scratch coat, prior to application of the brown coat, are:
- Where potential warping or subsidence of framing studs may be a problem which would contribute to delayed cracking of plaster;
- Where plywood sheathing might buckle or curl, and thus impose excessive force against a plaster membrane;
- Where the scratch coat has not been applied thickly enough over frame construction; or
- Where low temperature would retard development of strength within the scratch coat to withstand additional pressure.
Firm backing behind the plaster membrane must be in place, or the double-back system of application should not be considered. Inflexible backing must provide rigid support for a scratch coat, if doubling back with a brown coat is to be considered. Typical forms of rigid backing, properly protected from moisture, are: exterior gypsum sheathing, plywood or chipboard panels, sheets of expanded foam plastic and concrete block walls. All are acceptable to provide the required stiffness behind the scratch coat, in preparation for doubling back with the brown coat.