DeHaven Construction Company
"Specializing in Stucco, Stucco Repair, Interior Painting, Exterior Painting, Texture Coating (Texcote™) & Elastomeric Coatings"
Application of Stucco to Bases
The scratch-coat plaster applied to metal reinforcement should completely encase the meal reinforcement. After applying plaster to an entire area, the surface should be rodded and scratched to promote mechanical keying (bond) with a bow coat. Scratching is normally done horizontally on vertical surfaces. Horizontal scratches act as water dams and promote curing. Scratching vertically promotes cracking at studs where scratches or score marks are directly over studs, especially if they are too deep. Vertical scathes can promote erosion of plaster during moist curing.
The brown-coat plaster needs special attention after application and rodding. After rodding, the plasterer should pause before starting to float the surface. This time delay is related to cement hydration, loss of mix water through absorption and evaporation. floating with a wood float must be done when the plaster is at the proper moisture content for the desired reconsolidation (densification) of the plaster to be obtained. The correct time to begin floating can be determined by placing the wood float against the plaster surface. The float should not adhere to the fresh plaster. Unless this condition (of the float not sticking to the plaster) exists during the float operation the plaster will not be properly densified.
During application of the scratch coat plaster to a solid base or application of all brown coat plaster to scratch coat plaster, the plasterer can determine the suitability of a plaster mix for developing intimate bond in the following way. Once the brown coat plaster has been applied using the pressure required to develop good contact and when the plaster has the required thickness, the plasterer removes a small section of the freshly applied plaster down to the previous coat or base. This is easily done with the square end of a margin trowel. If the underlying coat has not been complete wetted by the coat just removed, the plasterer must either apply more pressure during plaster application or increase the water content of the plaster. The same test can be used to assess the suitability of machine-applied plasters.
As demonstrated by examples shown , various textures and colors can be achieved in the finish coat plaster. Each texture requires special tools and techniques. The uniform appearance and texture of the finish coat plaster will be influenced by the care taken during the application of the scratch and brown coats. Most color variations in the finish coat are traceable to the basecoat variations. These color variation can be minimized by prewetting the base coat prior to applying the finish coat. During sample panel preparation, and special finishing procedures and their correct timing and sequence should be documented for later use during the actual construction.
Color variation in finish coat plaster can usually be traced to one or more causes including the following:
Color variation in exterior finish coat stucco can be corrected by application of a fog coat or brush coat. For brush coating, a factory prepared brush coat mixture is mixed with water to produce a milky consistency. The wall is dampened lightly and evenly before the brush coat is applied. A fog coat should be applied to a dry surface, using as fine a spray as possible. Curing of the these coat is is necessary under hot, dry, or windy conditions by applying a light fog spray of water the following day.
Delay Between Successive Coats
Within the past 25 years, the time delay before applying the brown coat plaster has been reduced from seven days to one day or less. the application of a coat of plaster should not be interrupted within a pale, and the full thickness of the base coats should be applied as rapidly as the two coats can be put in place. The brown coat plaster should be applied as soon as the scratch coat plaster is sufficient ridge to resist, without cracking, the pressures of the brown coat application. Under certain conditions, application of the both the scratch and brown coats is possible on the same day. The short delay between the scratch and brown coats pockets more intimate contact between coats and more complete curing of the base coat.
Temperature affects the speed of plastering by extending or reducing the time between consecutive operations. Colds weather lengthens the time between rodding and floating; hot weather shortens it. Dry weather has the same effect as hot weather. Dry or hot weather produces dry substrates and causes more rapid water loss from the plaster through both absorption and evaporation. Moderate change in temperature and relative humidity can be overcome by heating materials during cool or cold weather and by pre wetting during hot or dry weather to reduce absorption of the base. Severely changing conditions required similar adjustment but to greater extremes. Heated materials and enclosures are required during cold weather. Regardless of climatic climate conditions, when floating is completely at the proper time, the base coat plaster will perform satisfactorily.
Cracks develop in plaster from a number of causes, drying shrinkage stresses, building movement, foundation settlement, restraints from lighting and plumbing fixtures that penetrate the plaster; intersecting walls, ceiling, plasters and corners; weakened sections in a wall or ceiling from a reduction in surface area or cross section because of fenestration; severe thermal changes; and construction joints. Control joints are used to provide a relief of stresses, thus controlling cracking, The location of control joints should be determined by the designer not the applicator. Their location should be strategically selected to correspond with the lines of weakened structural planes as related to anticipated potential building movement include: corners of openings in the plaster skin such as vents, windows and doors; structural plate line or concentration of large structure elements; where columns or beams join with walls or ceilings; over junctures of dissimilar base; and over existing construction, expansion, or control joints.
Where plaster is applied to a concrete or masonry base with or without metal reinforcement, control joints in the plaster should be installed directly over and aligned with any control joints in the base. Normally cracking will not occur in plaster applied to uncracked concrete or masonry based if the plaster bonds tightly to the base structure. If excessive cracking does occur, the bond or mechanical anchorage is inadequate for the structural movement
Where plaster is applied over metal reinforcement, the metal reinforcement should be fabricated and attached. in a way that allows free movement of panel area between the control joints. Failure to allow free movement will result in cracking in the thinnest portion of the panel. While designing the size of the free moving panel, the surface area should be divided into panels which preferably form a square. AST C 1063 requires that control joints be installed in walls to create panels of not more than 144 sq. ft. area and not more than 100 sq ft. for all ceilings, curves, or angled plaster surfaces. ASTM C 1063 indicates that the distance between control joints shall not exceed 18 ft, in either direction; or a length-to-width ratio of 2 ½ to 1. These joint spacings represent industry recommendation based on typical plaster application. Additional factors such as structure considerations previously notes or even surface texture of the finish coat (cracks are more apparent on smooth textures finishes than coarse textured finishes) may dictate more conservative placement of control joints.
Control joints can be installed with an array of plaster trim accessories. It is very important that metal reinforcement stop at each side of the joint and never continue through the control joint. A weather resistant barrier should continue unbroken behind the control joint. Joints, intersections, and terminations of control joints should be embedded and weather sealed.
The simple notched or grounded control joint is effective where plaster is of uniform thickness.
The accordion-pleat control joint seals out weather and is inserted to isolate adjacent free moving plaster panels. Both legs of this joint must be attached to adjacent but insolated panels. If such a joint is used in open frame construction, the control joint can be affixed to adjacent studs (double studs).
The control joint diaphragm should be closed, not open when it is installed. When cement plaster hardens, it shrinks as it dries and the joint will open. As the weather changes from day to day, the joint will open further when the surface is cooling or drying and close partially when warming or being moistened.
To obtain the best result from the cementatious material in portland cement plaster, some moisture must be maintained in the plaster for the first few days after application. Moist curing, under present plastering practice, usually is applied only to the base coat and continued until the application of the finish coat. Generally, fogging the surface with water at the start and again at the end of the work day will suffice. If the relative humidity is high (more than 70%),. the frequency of moistening the surface may be reduced. If it is hot, dry, and windy, the plaster surface should be moistened and covered with a single sheet of polyethylene plastic (weighted or taped down) to prevent water loss through evaporation.
Immediately before finish coat application, the base coat should be moistened. This moisture along with the water in the finish coat plaster provides the total cuing of the finish coat plaster. No additional water should be applied to finish coat plaster until it has hardened. Addition of water to colored finish coat plaster before hardening is a common cause of color variations as previously noted.
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Application of Stucco to Bases
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