Repair of Portland Cement Plaster (Stucco) - tsib.org

1 Portland Cement Plaster ( stucco ) is a building facing materialconsisting of Portland Cement -based materials and sand, mixedwith water to form a workable mixture. Plaster is applied either byhand or machine to exterior and interior wall surfaces in two orthree coats. It may be applied directly to a solid base, such asmasonry or concrete walls, or it can be applied to metal lathattached to frame construction, masonry, or concrete construction. Applied directly to concrete or masonry, Plaster provides atough, 13-mm (1/2-in.) thick facing that is integrally bonded tothe substrate. When applied to metal lath, three coats of plasterform a 22-mm (7/8-in.) total thickness. The first coat is oftenreferred to as the scratch coat, the second is the brown coat, andthe third is the finish coat (Fig.)

2 1). Portland Cement Plaster has many qualities that make it a desirablefacing material, including hardness, fire resistance, and attractiveappearance. Plaster has proved to be a durable wall covering in allclimates and has great appeal as a surface finish because of itsutility, low first cost, and need for minimal like all building materials, Plaster deteriorates with age andexposure to the elements. Although Cement Plaster provides ahard, durable cladding, it is not intended toresist building movements, such as those fromsettlement or the expansion of wood framemembers as a result of moisture intrusion. Thismanual presents recommended procedures forrepairing Portland Cement Plaster . Thispublication does not address synthetic Plaster (EIFS) systems.

3 2001 Portland Cement AssociationIS526 All rights reservedIntegrating patches into the existing Plaster is oneof the many challenges of Plaster Repair . (69912)Fig. 1. Portland Cement Plaster is often applied to metal lath attached to frame of Portland Cement Plaster ( stucco ) Causes of PlasterDamagePortland Cement Plaster can deteri-orate for a variety of reasons,including inadequate designdetails, poor installation, or the useof improper materials. In manycases, damage results from waterinfiltration into the buildingstructure either through cracks,through the roof, aroundchimneys, or through window anddoor openings. Water that getsbehind the Plaster can cause woodlath to rot and metal lath and nailsto rust, and can cause Plaster todelaminate from the in Plaster is usuallyshrinkage-related or the result ofexternal loads, such as groundsettlement or inadequate stiffness of the backing.

4 Common causes of cracking are improperly placed control joints, improper lathinstallation, and inadequate joints are provided in Plaster to relieve drying shrinkagestresses and to provide stress relief in areas subjected tomovement, such as window and door openings. Dryingshrinkage cracks are often caused by violating the rules ofcontrol joint spacing (ASTM C 1063): Each continuous vertical area should be no more than m2(144 ft2). Distance between joints should be no more than m (18 ft)in any direction. No panel should have a length-to-width ratio greater than 2-1/2 to 1. Joints should be positioned at movement joints in the wood-frame Plaster construction, metal lath expands andcontracts with the surrounding Plaster . If lath is installedincorrectly, sufficient tensile stresses can develop within theplaster to cause it to crack.

5 Rules for lath installation include: Lath should not be continuous at control joints. Lath should be lapped 13 mm (1/2 in.) at the sides and 25 mm (1 in.) at the ends (ASTM C 1063), with wire tiesspaced 230 mm (9 in.) apart. If paper-backed lath is used, laps should be installed to allowa paper-paper and lath-lath placed Plaster must be protected from excessive moistureloss in the first few days after application. Otherwise, earlyshrinkage of the Plaster can result in map cracking (Fig. 2).After the cause of deterioration has been identified, anynecessary repairs to the building should be made beforerepairing the Plaster . These may include roof, gutter, downspout,and flashing repairs. Horizontal areas, such as the tops of parapetwalls or chimneys, are particularly vulnerable to waterinfiltration.

6 These areas may require modifications to theiroriginal design, such as the addition of flashing (Grimmer 1990).Identifying Damaged PlasterBefore beginning anyrepair, the Plaster shouldbe evaluated to determinethe extent of damage andhow much must berepaired or areas in need ofrepair are obvious, suchas missing sections orlayers of Plaster (Fig. 3).Water-damaged plasteroften bulges or falls awayfrom the building becauseexcessive moisture causesthe coats of Plaster todelaminate and the metallath and nails to or soft areas thathave delaminated willmake a hollow soundwhen tapped with a examination and chemical analysis (ASTM C 1324)of samples of the Plaster can determine its mix components andcan provide other relevant information, such as the quality of thebond between coats.

7 This information can offer clues as to the2 Portland Cement AssociationFig 2. Improper curing of Plaster can result in map cracking. (69914)Fig. 3. Water-damaged Plaster oftenbulges or falls away from the building.(69906)expected useful life of the Plaster that remains and aid indetermining the appropriate scope of the Repair RemovalPlaster that hasdelaminated from thebase, but is otherwisesound, may require a sawor diamond grinder to beremoved. Soft or crumbledplaster can usually beremoved with a chisel orother hand tool (Fig. 4). If the Plaster is properlykeyed into the lath, thelath may have to be cut aswell. In these cases, soundplaster at the patchperimeter will have to beremoved to make room forlapping the new andexisting patching theplaster, replace it inlayers to the samethickness as the existingplaster.

8 Therefore, each succeeding coat is cut back further thanthe preceding one, the base coat being the smallest area, and thefinish coat being the largest area to be patched (Fig. 5).If matching the color and texture of the existing Plaster is critical,one option is to use abrasive blasting to remove the finish coat ofthe Plaster surrounding the patch. Then the patch andsurrounding areas share the same finish coat upon and Paper RepairIf the existing building paper is torn, it should be replaced withnew paper that meets the requirements of UU-B-790a, FederalSpecifications for Building Paper, Vegetable Fiber (Kraft,Waterproofed, Water Repellent, and Fire Resistant). Slip the newpaper behind the existing paper at the top and sides and allow itto extend over the old paper at an existing lap.

9 Laps should be atleast 100 mm (4 in.). This will help prevent water that getsbehind the Plaster from running down behind the or otherwise damaged lath should be cut out andreplaced with a new section of lath conforming to ASTM C the lath lapping guidelines outlined above. Also, if usinglath that comes pre-attached to a paper backing, you mustseparate the lath from the paper so the existing building paper isin contact with the new paper and the existing lath is in contactwith the new lath. Anchor the new lath to studs at no more than180 mm (7 in.) on center, but avoid nailing the lath to thesubstrate between the studs. Doing so prevents the lath fromexpanding and contracting with the Plaster , which can causecracking. Instead, wire-tie the new and existing PreparationMasonry or concrete substrates usually require preparation toensure that the Plaster establishes a strong bond.

10 These surfacesshould be sandblasted or mechanically abraded to removecontaminants and roughen the surface to maximize the bondingarea of the new Plaster . However, don t rake out masonry mortarjoints to provide a key. Doing so will create Plaster of varyingthickness, which can cause the masonry or concrete is dry, dampen the surface to preventit from absorbing an excessive amount of water from the prepared substrate should be at, or slightly drier than, a sat-urated, surface-dry(SSD) same rules thatapply for a masonry or concrete substratealso apply for theedges of the existingplaster to which the patch material will bond. The edges of the existingplaster should beroughened andproperly plastershould bond well to aproperly prepared sub-strate, bonding agentscan also be used (Fig.)