This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Exterior plaster requires three-coat work. The first or scratch coat is indispensable when metal or wire lath is used, but almost equally important over wood lath. This first coat should be scratched or roughened while drying, and must be thoroughly dry before the second coat is applied. A greater time ought to elapse between the applications of exterior than of interior plaster coats, inasmuch as it then becomes possible to cut out many of the larger and more important cracks than have had time to appear, and to patch them before the second coat is put upon the house. The second or brown coat is then the less likely to crack; and, if a further extra time is allowed the plastering to dry, it can also be patched at the last moment before the final slap-dash or finishing coat is put upon the walls. This slower progress aids in giving a more permanent job and one that is at the same time less likely to give annoyance from surface cracks afterward making their appearance in the finish plastering.
The question of proportion in mixing the plaster is quite as variable here as in the case of interior plastering, and it is equally impossible to give absolutely definite directions Different plasterers, each being guided by the experience obtained from working in different sections of the country, prefer their individually different ways of proportioning or mixing their materials. In the first coat, cement is added to the lime mortar in proportions varying between ten and forty per cent of the mixture. Some plasterers prefer that the first coat should be less stiffened with cement than the second. With others the reverse is true; while, contrary to the general supposition, the exterior coat appears - in the majority of cases - to contain only that amount of cement necessary to provide the tone or color that is desired for the exterior treatment. Conditions also greatly affect these proportions. When the plaster is added last on a well-seasoned and shrunk frame, for instance, it is worked stiffer than when the building is newer and still far from finished.
The final coat for exterior plaster is generally applied as a slapdash finish, the surface texture being given by the throwing of handfuls of variously sized pebbles or gravel upon the fresh outer coat, thus pitting or marking up its surface. The smaller the size of the particles employed for this purpose, the more likely they are to stick and remain in the fresh putty, slightly tinting the surface with the color - if any -of the gravel employed.
The coloring of exterior plastering is done in much the same way as when it is used inside the dwelling. As a rule, it may be said that not sufficient consideration is bestowed in this country upon the possibilities provided by the use of color for exterior plaster work.
It is agreed that the utmost care to prevent absolutely any leakage is necessary on the part of the workman in the carrying out of this class of work; and it is here that the success or failure of exterior plastering most often hinges. Of course, the joints occasioned by the juxtaposition of the wood finish and plaster around window and door openings offer many opportunities for leakage. The plaster should here be carefully flashed; and, if possible, an outer architrave backhand should afterward be put on so as to cover and protect this joint. Otherwise, a key should be provided for the plastering, by cutting away or hollowing out a space near the inner edge of the wood facure, into which the plaster may be pressed by the workman, and leakage thus prevented even if the wood, as is quite likely, shrinks slightly away from the plaster after it has been put in place.
The problem of making tight this exterior plaster wall is complicated and rendered more difficult when it is divided into panels by a so-called half-timber treatment. In this style of design, a great number of joints between plaster and wood are occasioned where the wide wood boards are almost certain to shrink away from the plastering, and where, too, it is impossible to protect these joints by outer applied battens in any way capable of covering such an opening as may occur. Thorough flashing on all upper exposed surfaces, assisted by protecting overhang of the roof eaves, and broad keys provided for the entrance of the plaster at all perpendicular and lower horizontal joints, must alone be relied upon.
Under no circumstances, so far as the lasting value of the work is concerned, does the mixture play so important a part as the expending of great care upon the thorough surfacing, working, and finishing of the mortar, pressing it into every crevice provided to receive it, flashing thoroughly every exposed or upper surface provided by the finish, and taking every precaution to work out all pinholes or other defects where water could possibly penetrate the surface. Every care and endeavour is directed to providing a solid, evenly worked, and permanent coating which will, in every possible way, throw off and prevent moisture being admitted into the space back of the plaster coating - that vulnerable portion where its attack is most effectually concealed and most to be dreaded.
The exterior plaster treatment of a cement or concrete wall is a problem that from now on will continue to be of rapidly increasing importance. Here, however, it is but necessary to use the cement as nearly neat as possible, adding lime or a make of white cement in case a brighter surface color is desirable. The problem of the aesthetic treatment of concrete construction is one that requires separate and particular consideration. Its solution has, as yet, been hardly attempted. Hollow terra-cotta tile is another material that is being modernly used more and more as a structural base to take an exterior plaster surface finish.
The student desiring to obtain a wider knowledge of the intricate subject of exterior plastering, may be referred to several articles published in the 1907 numbers of The Architectural Review, Boston. For a work treating historically and practically of the entire art and craft of plastering - within and without the dwelling - see Mr. William Millar's treatise "Plaster, Plain and Decorative." It would be as well to remember, in consulting the latter volume, that it was issued in 1897, and that the subject is treated from the point of view of an English workman, accustomed to methods and materials somewhat different from those common in American practice.