Cement Plaster Over Concrete Block Walls
This short monograph discusses some of the reasons that portland cement plaster could be applied to concrete block walls. Mr. Geary breaks down the possible projects into two major types: those requiring a metal lath and those that do not. He describes tests used to determine the absorption rates of the concrete blocks and other tests that will help the architect conclude which type of project he is working on. He discusses at length the sequence of plaster layers to apply and the proper procedures of doing so, as well the same information for lath installation. He also stresses the importance of having both minimum thickness requirements and maximum thickness limits, which are not standardized.





Portland cement plaster may be applied over the surface of concrete block walls for a variety of reasons. It may be to enhance the appearance of the building, to level out irregularities of alignment of the blocks, to achieve uniformity and continuity of surface where concrete blocks abut other construction materials, to provide alternative attractive textures or additional resistance to intrusion of moisture. Concrete blocks provide an excellent base for cement plaster because of the solidity and integrity of block walls and freedom from some of the problems associated with wood framing.

Application of cement plaster over concrete block walls contributes to resistance of walls to absorption or intrusion of water, because of greater density and slower rate of absorption of moisture into cement plaster, compared with concrete blocks. In addition, the membrane of plaster is extremely beneficial in deflecting rain.

Masonry walls, like concrete, provide a stable base for plaster because of virtual freedom from shrinkage, expansion, warping or subsidence; any or all of which may occur in wood framing.

Plaster that is bonded directly to blocks generally does not suffer from shrinkage cracking which may occur over open frame construction. Also, plaster applied over a block base generally is not affected by structural stresses which may cause cracks in plaster over wood framing. Stress from a problem in the concrete block base wall could be transferred to an overlying plaster membrane, however.

Various methods of plastering over block walls have proven successful, several of which will be discussed in this article. The omission of a technique from this paper does not indicate that the missing system is not suitable. Undoubtedly, there are excellent procedures that are not included in this article. The best criterion of any system is that it should perform in a satisfactory manner.

Cement plaster may be applied directly to the face of concrete blocks or it may be applied over lath which has been secured to a block wall.

If a block wall has been painted, and the paint exposed to weather, the condition and bond of the paint to the substrate must be checked carefully if plaster is to be applied. If the paint coating is cracked, flaking, curling or otherwise indicates inadequate bond, or the surface has oxidized, all paint must be removed. Removal may be accomplished by sandblasting or hydroblasting. An alternative to removal of the paint coating is installation of metal lath over the wall. If the paint coating is well bonded to the substrate and does not exhibit oxidation or other deterioration of the surface, a bonding agent must be sprayed, rolled or brushed over the entire surface of the painted wall before plaster may be troweled or sprayed onto the substrate, over paint. A bonding agent should be applied over a painted wall ONLY if the paint is bonded tightly to the blocks over the entire surface, and forms a suitably receptive base. If there is any indication of loose bond between the paint and substrate, or if cracking, curling or flaking of paint has occurred, a bonding agent will not benefit application of plaster. If any problem with bond of paint to the wall is observed, lath must be secured to the base wall, or all paint must be removed. If the original block wall leaked, hydroblasting may not be suitable, even though water-blasting sometimes is preferable to sandblasting. Certain precautions must be adhered to whether sandblasting or hydroblasting is done. All windows, doors, vents, metal trim, gutters, downspouts and individual air-conditioning units must be protected against abrasion.

If the block wall has been waterproofed, lath must be secured to the wall before plaster may be applied.

If any caulking is needed, sealant should be inserted into gaps prior to application of the first coat of plaster, so that caulking will be protected from weather.

If mold is present on the surface, the spores of mold must be killed in accordance with my article on mold and mildew, after which all vestiges of mold must be hydroblasted from the walls.

At least two coats of cement plaster are required over block walls to prevent print-through of mortar joints. This requirement is the result of a difference in texture, as well as absorbency or suction, of mortar joints compared with the face of lightweight blocks. Some contractors elect to apply a bonding agent to the face of unpainted concrete blocks to enhance the bond of plaster to the substrate and to achieve uniformity of suction. This treatment may be especially desirable in the case of low-suction or sealed-surface blocks, where a bonding agent would be particularly beneficial.

The first technique to be discussed is direct application of plaster over concrete blocks, since this is the most common procedure. Form release compound may have been sprayed on the forms in which the blocks were cast, and any retained release agent on the face of the blocks may not have altered sufficiently to permit adequate bond of plaster to the blocks. Before the start of the plastering operation, the blocks should be tested for wetability with water. If the blocks wet readily when water is splashed onto the surface, plaster may be expected to bond to the blocks. If wetting is slow or not readily apparent, one should consider the advisability of securing lath to the substrate.

It is good practice to apply a dash bond coat, sometimes called a slush coat or slurry coat, to the blocks to improve the bond of plaster to the base and to overcome suction of the face of the blocks compared with a lower degree of suction over mortar joints. It is beneficial but not necessary to mix a good bonding agent into the dash bond coat. The dash bond coat may consist of one part of portland cement mixed with one to two parts of fine sand, such as masonry sand (not plaster sand), plus sufficient mix water, to which a bonding agent may be added to produce a mix that can be applied with a plaster gun or dash brush. The slurry coat should be dashed firmly against the wall so as to completely cover all mortar joints and faces of blocks to a thickness of approximately 1/16" to 1/8". If the base coat is not doubled back over the slush coat, and severe drying conditions exist, the slurry coat should be fog-sprayed gently with a very fine mist of water to insure proper curing of the Portland cement. Damages from excessive flow of water or strong impact from the stream of water MUST be avoided.

Mortar joints in masonry walls which are to be plastered should be struck flush with masonry units. If the joints are tooled inward and the wait is new, the mortar joints should be treated with a bonding agent and then filled flush with the masonry units before the slush coat is applied. If tooled joints have been exposed sufficiently long to become dirty, with reduction in bonding capability, the mortar joints should be scraped or wire-brushed clean, to expose a bondable surface.

If there is sufficient deviation of plane of the blocks in the wall to necessitate a leveling coat, that leveling coat may be applied over the low spots in the block wall before the slush coat has dried. Apply the dash coat, allow it to take up sufficiently to become somewhat stiff and to have formed some bond to the blocks, then proceed to lay on the leveling coat in low spots by hand or machine. Filling depressions over recessed blocks with layers exceeding 3/8" thickness must be avoided, to prevent delamination. Allow the leveling coat to moist cure in accordance with good industry practice, then permit it to dry to uniformly low moisture content over the entire wall before proceeding with application of the base coat.

After the slush coat and/or leveling coat has cured and dried, the base coat may be applied. The base coat may be laid on to a thickness of approximately 1/4 to 3/8 inch. The base coat may be darbied or straight edged to a reasonably well aligned finish. Some textures, such as Mission texture, do not require as much straightening of the base coat, because part of the reason for their attractive appearance is the deliberate absence of plane in the decorative finish coat. The base coat must be moist cured in accordance with good industry practice, then allowed to dry to uniform moisture content over all of the wall surface before application of the finish coat.

Some contractors elect to build up the base in two coats, a scratch and brown coat, but that may not be necessary over block walls on which a dash bond coat has been applied. If the base is built up in two coats, the scratch coat should be applied on the first work day after the dash bond coat and leveling coat have been applied, to a thickness of about one-quarter inch. It should be scored lightly in one direction only -- horizontally. At the option of the plastering contractor, the brown coat may be doubled back over the scratch coat as soon as all of the scratch coat has been applied on a small job. On a larger project the plasterer may double back before a large wall area is completed, but not until the first coat has taken up and is thoroughly bonded to the substrate. The solid base provided by the concrete blocks facilitates doubling back. It should be noted that a scratch coat is fully and firmly supported without deflection by the blocks, which is a distinct advantage over frame construction. A scratch coat which has been allowed to dry adversely and thus would remain soft does not serve as a suitable base for successive coats, whereas moist curing or doubling back introduces curing water into the scratch coat and enhances hydration of cement paste.

The brown coat should be darbied or straight edged to a true, plane surface. For a superior job, at only slightly greater cost, the brown coat may be allowed to take up sufficiently to permit dry-rodding, then the entire surface of the brown coat may be dry-raked with a straight edge. In any event, the brown coat should be thinner than the scratch and should serve mainly as a leveling and dressing coat.

The brown coat should be allowed to cure naturally for seven days, the first two days of which should provide moist curing, naturally or by wetting, before the third or color coat is applied. The brown coat should have reached uniformity of moisture content overall prior to application of the color coat to achieve the required uniformity of color. Note that niformity of color is much easier to achieve over block walls than over open-frame construction. Slight moistening of the brown coat should be accomplished shortly before the color coat is laid on, except in the case of machine dash integrally-colored texture coat.

Too-early or excessive troweling, or other unneeded manipulation of the brown coat must be avoided, to reduce the possibility of fine cracks forming in the membrane.

The finish coat, sometimes called the color coat, should be applied after the base coat has cured sufficiently and attained uniformly low moisture content or suction over the entire surface of the wall. The color coat should be applied with sufficient thickness to attain uniformity of color over the entire surface of the wall and may be of any desired texture; dash, floated, Spanish texture or other. To insure color integrity and to minimize possible problems which might result from photographing of differences in the substrate through the finish coat, variations in suction or inequalities in characteristics of the surface of the base wall, dash texture should be applied in two coats. If the final surface is to be somewhat smooth, plasterers must avoid manipulation of the surface too early, or with excessive working of the surface, to avoid migration of fines to the surface.

If lath is to be attached to concrete block walls as a base for plaster, paper backing generally is not applied. If not needed for some special reason, I recommend that building paper or saturated felt not be installed over a block wall, behind plaster. Installation of felt or paper, for this type of construction, may induce future buckling, if water should intrude at the top and enter the space between the building paper and the concrete block wall.

In special cases building paper may be applied for a specific reason. Paper backing may be installed to improve the seal of a wall against absorption of moisture, to prevent some unwanted material such as asphalt or oil from bleeding through the plaster later or for additional insulation, which would be minimal. If paper backing is applied to a wall, the wall must be suitably capped or sealed at the top to prevent intrusion of water behind the layer of building paper. If for some specific reason, treated building paper or saturated felt is installed over a block wall prior to application of lath and plaster, the following MUST be accomplished. A continuous bead of sealant must be applied across the top of the block wall, then the TOP EDGE OF THE UPPERMOST SHEET OF SATURATED FELT OR BUILDING PAPER PRESSED INTO THE CONTINUOUS BEAD OF SEALANT.

Total discontinuance between a plaster membrane and the block wall behind the plaster may contribute to a higher incidence of cracking, in spite of the presence of attached lath. Direct contact between plaster and substrate generally is superior, even though lath is installed. I consider it advantageous that a plaster membrane adhere directly to the substrate as fully and securely as possible, in addition to attachment of lath to the base. Bond of the plaster to a concrete block base is improved because of physical and chemical adhesion of plaster, in addition to physical attachment by means of lath.

Additionally, a weep screed should be installed at the base of the wall, if and only if felt or building paper is installed between the block wall and the plaster membrane.

If there is to be paper backing behind the lath, the paper may be applied separately or paperbacked expanded metal lath or paperbacked stucco mesh may be applied. It should be noted that roll-type paperback stucco netting MAY belly on a block wall. If lath is desired the use of expanded metal lath should be considered because it tends to align itself parallel with the base wall. All lath must be furred outward from the base. If paper backing is installed behind metal lath or stucco mesh on a concrete block wall, the lath or netting MUST be furred out from the wall sufficiently well to affect good embedment of the lath. Note that it is not necessary that any type of paper-backed lath be applied. If paper is desired, it is just as suitable to apply treated construction paper weatherboard fashion over the blocks as a separate installation, then attach expanded metal lath or stucco mesh over the building paper or felt.

If control joints have been designed into the concrete block wall, it is necessary that control joints be installed in the plaster membrane OVER the joints in the block wall.

Metal lath may be attached to block walls by means of power-actuated drive pins, by one inch long (minimum length) hand-driven pins carrying a washer for security, or flathead concrete nails that are ATLEAST seven-eighths inch long and which carry washers over the shaft. T-pins and other devices may be used between regularly scheduled drive pins or concrete nails for additional holding of the sheets against the wall. T-pins alone will not and should not be expected to support the weight of lath and plaster because of insufficient embedment into the base wall and relatively small diameter of the shaft of T-pins. Possible failure of attachment must be considered if concrete nails of insufficient length are used. Half-inch long concrete nails NEVER should be considered as attachments to carry the weight of a membrane of lath and piaster, because of insufficient depth of embedment of the shaft of the fastener into firm concrete. Concrete nails must be at least seven-eighths inch in length or longer and must be firmly embedded in the blocks. If concrete nails or galvanized common nails cause serious spalling of the blocks or concrete, to a degree that would adversely affect holding power, the nailing must be supplemented with power-driven attachments spaced at least as close as three feet on center. Consideration should be given to inserting power-driven pins through washers to effectively secure the metal lath or stucco mesh to the wall. The washer should overlap and secure at least one strand of lath or netting to the blocks. This procedure applies whether there be paper backing or not, although felt or paper backing generally will not be installed.

Galvanized common nails (never box nails) have been used as fasteners into lightweight concrete block with good results. Some contractors attach lath initially with power-driven fasteners, then finish nailing with hand-set driven pins, concrete nails of sufficient length, or galvanized common nails. Spacing on the wall of galvanized common nails must be closer than the spacing schedule of concrete nails, which in turn must be closer than the spacing of power-driven pins.

When metal lath or stucco mesh is attached to concrete block walls with concrete nails, a nailing pattern of approximately 12 inches on center vertically and twelve to sixteen inches on center horizontally could be considered. One power-driven pin has more than enough strength to support several square feet of lath and cement plaster. The nails or drive pins must be inserted in the crotch or twisted wire section of stucco netting, or a metal washer must overlap the interwoven strand between two hexagonal openings of stucco netting at least one-half inch, with the nail or pin inserted below the interwoven strand.

If buckling occurs in paper backing behind metal lath or stucco netting, additional nailing with T-nails, galvanized nails or other suitable auxiliary fastener must be accomplished, to flatten buckles to a sufficient degree.

T-nails have been found suitable in helping hold stucco mesh flat against concrete block walls, but only if used in conjunction with metal washers or small tabs of flat metal that overlap the wire, or if two T-nails are driven on each side of a strand and crossed over the wire. T-nails must be at least three-quarters inch in length and must be reinforced by power-activated or hand-set driven pins attached at least every three feet on center vertically and horizontally.

Attachments of all kinds should be driven into the web of concrete blocks, never over the core unless the core is filled with grout. It is satisfactory to drive attachments into mortar joints only if the joints are filled solidly with mortar and the mortar is well-cured and strong. To facilitate locating suitable vertical lines of nails or pins in block walls which might be covered with building paper, establish desired vertical lines before applying the paper, drive nails into the top of the block walls at the desired lines, and after the paper is placed, hang plumb lines from the top nails.

Under no circumstance should a drive pin or nail be struck on the side in an attempt to crimp the pin or nail over the lath for the purpose of locking lath to the block wall. Attempting to bend a pin or concrete nail in a block would result in weakening the block, with reduction in holding power. Drive pins or nails should not be driven into mortar joints unless the lather is assured that mortar joints are full and will accept and hold the fasteners securely without mortar breaking out of the joint.

Architects and contractors should be aware of the possibility that residual form release compound may be on the face of the blocks, or that the block wall may have been treated with silicone or some other waterproofing agent. If such is the case, cement plaster may not wet and bond to the blocks, and thus, metal lath would be required.

If stucco mesh is applied over concrete block walls, the netting must be furred outward from the substrate. Furring nails must be driven straight into the face of blocks. Do not attempt to drive furring nails into mortar joints because the mortar will resist penetration of the nail, and may break out. Inch and one-half x seventeen gauge or one inch x eighteen gauge stucco mesh are two commonly accepted types of stucco netting that are suitable.

If there should be cast concrete sections adjoining the block wall, and the wall is to be plastered, metal reinforcement is required over the juncture of the dissimilar building materials and power-driven fasteners are required over the concrete. Metal reinforcement should be used to bridge the juncture of dissimilar construction materials such as block walls abutting wood frame, or blocks abutting concrete. The metal lath should overlap at least six inches on either side of the juncture. It should be noted that it is easier to drive fasteners into new concrete, immediately after stripping the forms, than is the case with older, more thoroughly cured concrete. In any case, fasteners of any kind should not be driven into concrete blocks until the mortar joints have attained sufficient shear strength to withstand hammering and lateral stress.


It is appropriate that thickness of application of portland cement plaster over concrete block walls be discussed. This information also is applicable, in this respect, to plaster over concrete and brick walls. Table 47-D of the Uniform Building Code requires that the minimum finished thickness of cement plaster, applied over masonry walls, should be no less than one-half inch (1/2"). Naturally, there may be instances where one block projects outward beyond average alignment of the majority of blocks, over which plaster may be thinner than average. There may be instances where a block is somewhat recessed in relation to the balance of the wall, thus greater thickness would be required in that location. For these reasons, thickness of plaster overcoat must vary to accommodate inequalities in alignment of the block, brick or stone wall. The mean average thickness should be one-half inch or slightly greater. In the Uniform Building Code, footnote number 4, relating to Table 47-D, states the matter clearly: "Because masonry and concrete surfaces may vary in plane, thickness of plaster need not be uniform."

The Uniform Building Code describes minimum thickness of cement plaster over masonry walls (concrete blocks, stones or bricks) in Table 47-D, but does not define maximum thickness of application. It is appropriate that I address the subject.

Oftentimes, average thickness of cement plaster over masonry in the range of about five-eighths inch (5/8") is suitable, but no constant figure can or should be stated. Determining factors may vary from one project to another, and even over different areas of the same project. One important criterion is that the mean average thickness be held at or above one-half inch, with some variation probably occurring. Application of an excessively thick membrane of plaster over masonry (or concrete) must be avoided, or delamination of plaster from the substrate may occur. A specific maximum thickness cannot be stated, again because determining factors may and frequently do vary on construction projects. Normally, about three-quarters inch (3/4") to about one inch thickness is a reasonable maximum depth, although greater depth of application may be accomplished if careful control is exercised. If thickness greater than 3/4 inch is needed or desired for a specific and justified reason, a thicker membrane may be built up over a concrete block wall, BUT ONLY IN A SERIES OF APPLICATIONS, --- each coat being no thicker than about three-eighths inch (3/8"). Sufficient time MUST be allowed between each coat, when building what I consider to be excessive thickness, for the cement paste to set and gain strength, and to bond well to the substrate. The surface of lower coats must be left sufficiently open and receptive to a succeeding coat, in order that good bond will be attained. For thickness greater than one inch, normally installation of metal lath should be considered. If this procedure is not observed, delamination may occur between the back of the membrane of plaster and the base wall, or between coats of plaster. An excessively thick layer may tend to fall away from its base while still in the wet, plastic state, because of weight of the mass in relation to low initial bond.

It is necessary, of course, that depressions over recessed blocks or bricks be filled to normal alignment, for esthetic reasons. In cases where this is necessary, all depressions should be built up in relatively thin (1/4" to 3/8") layers, and each application permitted to become firm and well bonded, and having an open, bondable surface, before another coat is applied over the lower layer.

In my opinion, total mean average thickness of a membrane of cement plaster applied over a concrete block wall normally should be approximately five eighths inch (5/8"). This average specified depth would allow for variation of greater or lesser thickness and still provide suitable coverage. Thickness of approximately five-eighths inch (5/8") would include the depth of a dash bond coat, which might be approximately 1/16" to 1/8"; a base coat, which might be approximately 1/4" to 3/8"; plus the finish coat, which might be approximately 1/8" to 1/4" in thickness, if the finish is composed of gray cement plaster. If a color coat of stucco is to be applied, these figures could vary accordingly, because color coat stucco normally is applied in the range of about 1/8" in thickness. The figures provided above are not intended to be considered as constants (which do not vary), but are provided as guidance thicknesses, which may vary, to assist architects in writing specifications for this type of project.

If, for some valid reason such as realignment of base walls which might be seriously out of plane, greater thickness of plaster is needed to correct problems which may have occurred earlier in the construction project, expanded metal lath should be securely attached to concrete block walls. If lath is not attached to the substrate when thickness of application is intended to be greater than one inch in depth, delamination may be expected.



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