PLASTER OVER ADOBE WALLS
Adobe bricks are subject to gradual erosion by rain and sand storms, even though adobe buildings have a long history of good performance in the southwestern United States. In cases where owners desire to protect adobe walls from erosion, it has been customary over the past century and a half or more to shield the adobe bricks and mortar. In the early and mid-1800's, the walls were sometimes shielded with lime plaster. From the late 1800's and early 1900's the custom switched to plaster made of portland cement or a mixture of hydrated lime and portland cement.
The protective membrane of plaster, whether made of lime, cement and lime or only portland cement, served to protect the wall to a reasonable degree. However, if water got behind the plaster membrane for any reason, erosion would occur on the surface of the adobe and eventually the plaster membrane would begin to slough away from the adobe bricks and mortar.
The next progressive step was to attach lath of some kind to the adobe walls to hold the plaster membrane on the bricks in spite of possible slight erosion behind the plaster. Today it is customary to secure metal lath, generally stucco netting, to adobe walls prior to application of the plaster.
The following information represents a survey of opinions of some of the plastering contractors in the western United States who have repaired adobe buildings, as they relate to application of cementitious plaster over adobe brick walls. The writer does not endorse a specific method of procedure in this text. Instead, he offers some variations of procedure for consideration by those who might write specifications for protection of adobe buildings. The writer considers the systems included in this text to be suitable, if good industry practices are complied with.
The cardinal rule is that one never should plaster directly over the unlathed exterior surface of adobe walls. Expanded metal lath may be applied, but the general custom is to secure stucco mesh, also called stucco netting, to the adobe wall. One of two different classifications of stucco mesh usually is considered: 1" x 18 gauge and 1" x 20 gauge. The writer spoke with only one plastering contractor who uses 1 1/2" x 17 gauge stucco mesh over adobe. The 1" x 18 gauge or 1" x 20 gauge mesh may be self-furred or flat, but in any case, the mesh should be furred away from the adobe about one-quarter inch. The purpose of the furring, of course, is to allow embedment of the netting in the plaster membrane. Paper backing would not be applied behind the stucco netting.
Several kinds of fasteners are commonly used to secure stucco mesh to the adobe: a commonly used fastener is a large-headed, galvanized roofing nail at least 2 to 2 1/2 inches in length. Most contractors consider that length of nail shank to be needed to obtain adequate holding power in the adobe wall. Since the galvanized nails have a slightly rough shank, the holding power is improved over smooth-shank nails. The writer is not certain that the feature of roughness of shank is critical, but he recognizes that holding power would be improved by the relative roughness.
If there is any indication that moisture has entered the adobe walls at one time or that the potential for water being carried upward in the walls exists, galvanized or cement-coated nails are preferable. For long term performance, the possible ill effects of moisture should be avoided.
One contractor who was queried uses 6d nails because he is able to bend the shank of the nail over the wire without damaging the adobe significantly. Certainly, if one planned to bend the nails to hold wire netting on the wall, no larger nail shank should be considered. However, most contractors who reported do not bend the nail to hold the netting. If large-headed roofing nails are not used, box nails of 8d to 20d size are used. One contractor consistently uses 20d nails to have the length of nail shank that he considers to be superior. In any case, all box or common nails, if used, should have a washer with proper hole diameter slipped over the shank of the nail to hold the mesh securely on the adobe wall. Those nails never are bent, but always are driven STRAIGHT INTO THE WALL.
Nails greater than 6d in size never should be bent over to secure the netting to the wall. Bending of larger nails will fracture the mortar or brick and reduce holding power. The shank of a 6d nail is not long enough to allow much embedment in the adobe, especially if the nail is to project from the wall significantly to be bent over wire.
Some contractors drive the nails anywhere in the adobe mortar joints between clay bricks, some drive the nails into the juncture of the mortar with the bottom edge of the bricks and some drive the nails into the adobe bricks, per se. Any one of these locations is satisfactory. The only criterion should be that it be possible to drive the nails inward with no or minimal fracturing of the adobe. For that reason, the writer firmly recommends the fasteners be driven straight into the wall. If the bricks are extremely strong and resistant to penetration of nails, it would be preferable to drive fasteners into the line of juncture of mortar and bricks.
There is no need to use wire to tie the mesh to the fasteners, if washers or large-headed nails are employed.
Plastic cement or regular cement may be used as the cementing agent. If plastic cement is used, the mix normally is composed only of plastic cement, sand and water. If regular cement (Type I, II or I-II) is employed. Type S hydrated lime or other suitable mineral plasticizing agent would be added to the mix to develop plasticity or spreadability in the cement mortar and cause the plaster to behave somewhat like mortar made with plastic cement. The mix, in that case, would consist of regular cement, hydrated lime, sand and water.
Suggested mix formulations are tabulated below;
One bag plastic cement - 3 to 4 cubic feet plaster sand. Add only enough water to produce a workable mortar.
One bag plastic cement - 3 1/2 to 5 cubic feet plaster sand. Add only enough water to produce a workable mix.
One bag regular cement - 3 1/2 to 4 cubic feet plaster sand. 20 pounds hydrated lime plus mix water.
One bag regular cement - 3 1/2 to 5 cubic feet plaster sand. 20 pounds hydrated lime plus mix water.
One bag regular cement - 6 to 8 cubic feet plaster sand. One bag Type S or other lime plus mix water.
One bag regular cement - 7 to 10 cubic feet plaster sand. One bag Type S or other lime plus mix water.
The scratch coat should be applied over the stucco mesh to a thickness of three-eighths inch or greater, and may be applied by hand or gun. After the plaster has taken up partially, the scratch coat is to be lightly scored in the horizontal direction only. It is permissible to double back right away with the brown coat over the solid backing, or to permit a delay of several days between applying the scratch and brown coats. If the brown coat is not doubled back, the scratch coat should be moistened during the succeeding day.
The brown coat should be applied over the scratch coat, then darbied to reasonably plane surface or left undarbied, depending upon the desires of the architect or owner. The architect or owner may wish to have texture developed within the brown coat.
The finish coat should be manufactured stucco in the color and texture selected by the architect or owner.
If the scratch coat is not covered the same day by doubling back with the brown coat, the scratch coat should receive several days of moist curing. In any case, the brown coat is to be moist cured for two or more days before being allowed to dry before receiving the finish coat of stucco.
Application, curing, and other features of the finish coat should be accomplished in accordance with the instructions provided by the stucco manufacturer.