NWCB Technical Document 01/99 EXTERIOR SYSTEMS Portland Cement Plaster on Masonry or 600-607 Poured In Place Concrete

Reference Specifications and Recommendations • NWCB Technical Document 600-103 Guide Specifications for Portland Cement Plaster • NWCB Technical Document 600-605 Stress Control for Portland Cement Plaster • NWCB Technical Document 600-608 Preparation Of Concrete or Masonry to Receive Plaster • Lathing: ASTM C1063 • Plastering: ASTM C926

Common Uses: Plaster maintains impact resistance, increases moisture resistance, and decorates masonry or concrete construction. Concrete Masonry Unit construction provides an excellent base for Portland cement plaster. The best application consists of open textured block with flush (untooled) joints, laid to a minimum tolerance of ¼” in ten feet.The mason- ry should be clean, dry, cured, and carrying most of its designed load. Poured in place concrete should be fairly straight and have a slightly rough surface, free of dust or from release agents. Plaster should not be applied to concrete containing additives for damproofing. Plastering over existing block or concrete will require cleaning and the use of a bonding agent, sand blasting or the application of a galvanized self-furring lath material. The option chosen depends on the condition of the existing wall. (See Technical Document 600-608 Preparation of Concrete or Masonry to Receive Exterior Plaster). Lathing over new masonry or concrete construction consists of the application of trim items. Trim items consist of casings or grounds to terminate plaster, control joints to terminate plaster and help relieve expected stresses, and corner beads to align exterior corners. Trim items are set straight and true, and are normally set to a thickness of 1/2” on vertical surfaces.

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© NORTHWEST WALL AND CEILING BUREAU Some specifiers have thought that the addition of weather-resistive barrier between the block and the plaster will give added moisture protection. The addition of a weather-resistive barrier is not recommended over CMU or concrete. The addition of a weather-resistive barrier actually decreases the moisture resistance. The intimate contact between the block and plaster is broken and the possibility of the plaster coming loose is greatly increased. Plaster is applied in two coats not to exceed 5/8” on vertical surfaces and 3/8” on horizontal surfaces. Additional plaster thickness requires the use of a lath and a three coat plaster system. Mixes may be chosen from NWCB Technical Document 600-103 Guide Specifications for Portland Cement Plaster, or ASTM C926, Tables 2 and 3. Factory prepared finishes may be used. Surfaces with high suction (very porous) should be damped before plaster application. Low suction (very dense) masonry may require a dash bond coat to increase suction. Plastering over wet, uncured block may require a dash bond coat to eliminate the possibility of the masonry joint telegraphing through the plaster. The base is applied with sufficient pressure to ensure tight contact with the block. Next the basecoat is rodded to a true even plane; then the surface is floated to pro- mote densification and provide a uniformly rough surface to receive the finish coat. The basecoat shall be cured a minimum of seven (7) days (NWCB Technical Documents 600-103, 600-605, 600-608 and ASTM C926 A2.7 and A2.9). Color (ASTM C926, A2.5.7, A2.5.7.1) can be provided by gray or white cement, integral color (for dash or textures), aggre- gates, factory prepared colored finish, or a breathing stain or paint.A quality, vapor permeable, acrylic finish may be used in lieu of a Portland cement finish. Finish and color should be selected and a sample submitted and approved by the owner or architect before plastering begins.

This technical document is to serve as a guideline and it is not intended for any specific construction projects. NWCB makes no express or implied warranty or guarantee of the techniques, construction methods or materials identified herein. PAGE 2 OF 2

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