GUIDE FOR REMEDIATION AND REPAIR FOR STUCCO AROUND WINDOWS

Abstract:


 

 

Article:

 

GUIDE FOR REMEDIATION AND REPAIR FOR STUCCO AROUND WINDOWS

Portland cement plaster (stucco) is sometimes removed from a building to determine if proper flashing was installed, replace leaky windows, repair water damaged wood framing members or other reasons that would require the removal of the stucco around windows. It is possible to patch and restore the stucco membrane. Removal procedures can vary depending on the substrate, framing and overall remediation plans. The following outlined procedure is for removal and replacement of cement stucco around windows on wood framed walls. This guide is limited to the removal and replacement of the stucco and not specific to window installation and related flashings. There are several types and styles of windows and a variety of methods for flashing. This document is intended to focus on the lath and cement plaster work for remediation.

It is important to determine the expectations of the building owner and inform them of the finishing options. The finishing options to the repairs around windows depending on the degree or desire of finish appearance and should be discussed prior to the start of work. The following will help with some decisions, options and help clear up confusion for all parties involved.

Evaluation:

A comprehensive evaluation of the exterior plaster should be conducted by qualified persons prior to making any decisions for final scope of repairs. An important item needed to determine the full scope of stucco repairs is the type of finish coat (acrylic or cement) on the building and the overall condition of the walls. Has the building been painted and what type of paint was used? Buildings with elastomeric type paints will limit the options of repairs.

Finishing Options Cost ranges from least to most expensive methods #1-#5.

  1. Feather the new finish coat into existing finish coat:
    1. Least expensive method
    2. Matches to existing color and texture may be difficult to impossible
    3. Painting walls will most likely be required. Integral colored cement finish can be fog coated (see NWCB tech document PCP-10)
    4. A hairline crack may appear between the patch and existing wall.

  2. Apply a new finish coat to patch and entire wall:
    1. Eliminates need for painting
    2. Patched area may appear slightly off in color tone or shade (depending on color and type of material)
    3. Will cover existing cracks in field of the wall
    4. A hairline crack may develop over time in some areas between patch and wall.
    5. Existing cracks in field wall may reappear over time
    6. Cost increase may be offset by elimination of paint.
    7. The bond is dependent on the bond of the existing paint. (Test patches)

  3. Skim coat entire wall with a cement polymer-enriched basecoat and an acrylic finish:
    1. Provides a uniform finish color between the patch and walls
    2. Reduces the likelihood of future cracks
    3. Significant cost increase over finish method #2
  4. Fiberglass mesh strips (4" to 8") troweled into skimcoat over the joint area
    1. Provides extra reinforcement against cracks at patched area
    2. Minor cost increase from finish method #3

  5. A fiberglass mesh troweled into skimcoat over entire wall area
    1. Provides the ultimate assurance against any future cracks in a stucco system
    2. Permanently hides any existing cracks in field wall
    3. Provides a vapor permeable strong reinforced coating over stucco
    4. Can be used to level out and repair other existing blemishes
    5. Significant cost increase over finish #3.

General Design Notes for Finish Options:

  1. A test area and owner acceptance is recommended before proceeding with all finish options.
  2. A minor amount of hairline cracks in stucco does not allow moisture behind the stucco and are only a cosmetic concern.
  3. Method #5 is recommended for buildings with existing cracks wider than what would be considered "hairline" or an excessive amount of hairline cracks.
  4. Skim coat refers to a polymer enriched proprietary cement basecoat and may be the basecoat/adhesive used in an Exterior Insulated Finish system (EIFS) for an acrylic finish coat. A cement finish coat may require the proprietary skim coat manufactured by the same stucco manufacturer.
  5. It is important the skim coat material used in methods #3-#5 be compatible with the finish coat, NWCB recommends same manufacturer for skim coat and finish material. Check with the manufacturer for compatibility of the existing finish.
  6. Finish options #2,3,4, & 5 will not be appropriate for stucco that have been treated with an elastomeric coating.
  7. Foam decorative shapes can be added during the repairs and in some instances provide additional functional and/or aesthetic design options. The aesthetic value and addition of decorative foam shapes can sometimes be used offset repair costs.

STUCCO REMOVAL

Stucco removal can be very dusty; protective clothing, eye protection, and a respirator are always recommended. The removal of stucco is best done by cutting the stucco with a power saw using a masonry blade. The blade should be set to just shy of the full thickness of the stucco membrane, usually 3/4 of an inch. This will protect the underlying weather-resistive (WR) membrane from damage. Masonry blades wear down quickly and it will be necessary to adjust the blade depth frequently. The stucco cuts should be approximately 12 to 24 inches away from the window frame. Vertical cuts may be further away from the window as they should reach the next framing member (stud). Once the saw cuts are complete, a cold chisel can be placed into the saw cut and used to break or fracture the last remaining uncut thickness of plaster. The stucco can then be pulled off the wall with minimal damage to the underlying WR membrane. For large areas of removal, it is suggested to cut the stucco into squares that can be easily handled.

In some rare cases the WR membrane may be adhered to the back of the stucco. In these instances it will be necessary to remove the stucco back to point where the stucco is not bonded the WR membrane.

The cut stucco edge will be straight and smooth, it is recommended the edges be jagged and rough. This will produce a better key (bond) between the new patch and existing stucco and can minimize the chance for a separation crack. A hammer can be used to chip away small sections, leaving some of the metal lath exposed if possible.

The WR membrane should be peeled back carefully to minimize tears, holes or other damage to the WR membrane. It is necessary to leave some existing WR membrane for proper lap and integration with flashings. A minimum two-inch over lap on horizontal joints and a six-inch overlap on vertical joints of the WR membrane will be needed when final lathing takes place.

REMEDIATION
(Flashing and WR membrane)

The installation/remediation of the framing, sheathing, window, and all flashings should be complete and inspected by qualified persons prior to installing the WR membrane. There are several options, methods, and products for flashing windows. Many of these options are in the NWCB Stucco Resource Guide. The NWCB recommends the WR membrane and all flashings should be installed in "shingle" fashion if at all possible. This means all upper layers overlap lower layers to direct water down and ultimately out at a "weep" point. This would include all "peel and stick" type flashing membranes, metal and PVC flashings. An asphalt emulsion can be used over the exposed surface of the WR membrane to seal minor holes and tears. Compatibility of products and materials should always checked prior to installation. Some materials can have an adverse reaction to asphalt saturated products or polyolefin spun bonded products.

Around the Window: Another important design consideration is how to finish the stucco around the window. Should there be a casing bead? How wide should the sealant joint be? Should there be a head flashing installed?

Design Options: Casing beads should be installed along the jamb and sill of all windows leaving room for a sealant joint in areas exposed to even moderate wind-driven rain. Nail-flange type windows properly flashed can have the stucco abut the window frame and use a filet bead of sealant after plastering. However, this is only acceptable on flashed nail flange type windows with protective roof overhangs and low to minimal exposure to the rain. The nail flange must be an integral solid and continuous nail flange. Wood frame and commercial style(storefront) windows would always require a designed sealant joint, unless completely protected from the rain. The size (width) of the sealant joint will depend on anticipated building movement, rain exposure and the type of window. The minimum joint design width should be 3/8 inch and the maximum design should be 3/4 inch. A design width of 3/8 inch will produce a 3/4 inch sealant joint. Sealant joints and backer rod should only be installed by qualified persons. The shape of the joint, type of sealant, style of backer rod, and primer are critical components to the success of the sealant installation.

A "weep" type-casing bead should be installed over a head flashing along the top of the window. The head flashing should extend 3/4 inch beyond the window jamb and have end damns. The head flashing can be made of PVC or metal and should be installed to have a slope down and outward. The nailing back flange of the head flashing must be a minimum 2 inches long and return up behind the weather-resistive membrane.

HEAD FLASHINGS IN HIGH WIND DRIVEN RAIN AREAS:
A longer back flange may be required in heavy wind-driven rain areas. The head flashing would come out, over and down the window frame with a hemmed or drip edge. The head flashing should be set in a bed of sealant along the top of the window.

See the NWCB Stucco Resource Guide for complete details and other options.

GUIDE SPECIFICATION FOR PATCHING WITH LATH AND PORTLAND CEMENT PLASTER

PART I: GENERAL

Scope of Work: Work included under this section of the specification for lathing and plastering. The contractor shall provide all labor, materials and equipment necessary to complete patching as shown on the drawings and/or described herein.

General Requirements: Compliance with standards and industry specifications: Northwest Wall & Ceiling Bureau (NWCB) standards (206) 524-4243 The Northwest Wall and Ceiling Bureau Stucco Resource Guide ASTM: C1063 - Installation of Lathing and Furring for Portland Cement Plaster ASTM C926 - Application of Portland Cement Plaster Where products are specified they shall be installed in accordance with local building codes and/or manufacturers recommendations.

Contractor Qualifications: Applicator/contractor must be qualified in the workmanship of lathing and plastering and a member of the NWCB. Shall be approved by architect.

Submittals: Applicator/contractor shall be prepared and may be required to supply samples.

Quality Control: Attend a pre-Installation meeting. Applicator/contractor shall be prepared for third party inspections by qualified persons or agency.

Environmental Conditions: Reference NWCB Stucco Resource Guide. Acrylic and cement finishes have varying requirements for environmental conditions. Protect all material from freezing. Do not apply materials to frozen surfaces. Moist cure cement if required to insure a proper cure. Do not attempt work in unsatisfactory environmental conditions.

PART II: MATERIAL

Weather-Resistive Membrane

  1. Grade D 60-minute asphalt saturated kraft paper
  2. b. 15 lb. felt (repair work only)

Lath: Galvanized (self furring)

  1. Expanded metal lath (2.5 lb/yd or 3.4 lb/yd)
  2. Accessories: casing beads, channel reveals, control joints, expansion joints to match existing in style, material and ground thickness. Asphalt Emulsion (if required)

Plaster:

  1. Portland cement: ASTM C150 Type I/II
  2. Portland cement ASTM C618 Type IP
  3. Masonry cement ASTM C 91
  4. Lime ASTM C206 Type S
  5. Sand: ASTM C144 or C897 "washed" plaster or masonry sand
  6. Fibers ASTM C 1116: glass, nylon or polypropylene -1/2 inch long
  7. Bonding agent: ASTM C932

Mix Proportions: Per the NWCB Stucco Resource Guide or approved by architect

Cement Finish: Portland cement finish stucco pre-blended at factory or site mixed per NWCB Stucco Resource Guide

Skim coat: proprietary polymer enriched cement designed for use as an EIFS basecoat/adhesive. Follow manufacturers instruction for mixing.

Fiberglass mesh: 4-6 oz per square yard

Acrylic Finish: Proprietary acrylic finish approved by NWCB/architect

PART III: EXECUTION

Examination: Prior to starting lath and plasterwork, carefully inspect installed work of other trades. Notify architect or proper authorities in writing of any detrimental conditions before proceeding with work, including all flashings, framing, sheathing or unsatisfactory conditions in existing stucco finish. Owners shall be notified of any plants, trees, furniture, landscaping or other items that may need to be moved or protected.

Flashings: Installed by others

Weather-resistive barrier: Apply weather resistive barrier from bottom to top. Lap weather resistive barrier in shingle fashion with all flashings. Lap weather resistive barrier with existing weather resistive barrier. Seal any holes or damage to weather resistive barrier with a brushed on asphalt solution.

Trim Accessories: Properly install, miter and align trim accessories with existing trim accessories. Fasten all trim accessories to framing members 8 to 12 inches on center.

All intersections, miters, splices and terminations of control joints and channel reveals shall be set in a bed of sealant or back-sealed in a manner approved by architect.

Casing beads shall be set around windows to provide the specified joint design width.

Metal Plaster Base: Cut and fit lath neatly into patch area. An overlap with the existing lath is optimal if possible. The lath shall over lap the nailing flange of all casing beads, channel reveals or expansion joints. Control joints may be placed over the lath. Fasteners should be placed six inches on center along framing members. Fasteners should only be through the sheathing over solid framing members and not into sheathing between studs.

Plastering Protect all areas not to be plastered from damage.

Apply a bonder to edge of existing cement plaster to insure a good bond for new patchwork. Allow to get tacky before plastering.

All plaster work to be applied with hand tools.

Apply a scratch coat of Portland cement plaster approximately 3/8 inch thick. Scarify or score to provide keys for the brown coat. Brown coat may be applied as soon as the scratch coat has attained sufficient rigidity to receive brown coat. The brown coat shall be rodded off to produce a flat even surface with existing plaster. After the surface moisture has left the brown coat, the brown coat shall be hard floated to densify the plaster membrane. A hard neoprene, cork or shingle float shall be used. Moist cure brown coat for 3 days if relative humidity is below 70% and temperature is above 75 degrees F or windy conditions exist.

Follow finish procedure (methods 1-5) as chosen by owner/architect:

Method #1
Finish coat shall be applied no sooner than 7 full days after brown coat has been applied. Apply finish coat and feather into and match existing texture.

Method #2
Clean existing finish coat of dirt, grease, mold, algae or any substance that will prevent a proper bond of the new finish coat (use a bonding agent if needed). Finish coat shall be applied no sooner than 7 full days after brown coat has been applied. Apply finish coat over patch area and existing stucco. Work to architectural breaks and maintain a wet edge with finish. Match approved texture sample.

Method #3
Clean existing finish coat of dirt, grease, mold, algae or any substance that will prevent a proper bond of the new skim coat. Provide a test area if there are any doubts of a proper bond. Allow brown coat to cure 5 days before applying skim coat. Spread skim coat over all surfaces to a uniform thickness of approximately 1/16 inch thick. Work to architectural breaks, no cold joints. Leave no surface irregularities that will not be covered by finish coat. Allow skim coat to dry before applying finish coat. Apply finish coat and maintain a wet edge to architectural breaks. Match approved texture sample.

Method #4
Follow all instructions in method #3. While skim coat is wet, trowel a 4 to 6 inch strip of fiberglass mesh over the joint between the old stucco and new brown coat. Fully embed reinforcing mesh into the wet skim coat. Mesh may be butted or overlapped. Continue to follow the steps of method #3.

Method #5
Clean existing finish coat of dirt, grease, mold, algae or any substance that will prevent a proper bond of the new skim coat. Allow brown coat to cure 5 days before applying the skim coat. Provide a test area if there are any doubts of a proper bond. Spread skim coat over all surfaces to a uniform thickness of approximately 1/16 to1/8 inch thick. Work to architectural breaks, no cold joints. Fully embed a reinforcing mesh into wet skimcoat, troweling from the center to the edge of the mesh, to avoid wrinkles. Mesh shall be continuous over all areas to be plastered. There shall be an overlap of reinforcing mesh of 2 inches. Leave no surface irregularities that will not be covered by finish coat. Allow skimcoat to dry before applying finish coat. Apply finish coat and maintain a wet edge to architectural breaks. Match approved texture sample.

Allow acrylic finish coat to "fully" set before removing protective cover from the elements.

CLEAN UP: All excess materials shall be removed from job site. Surrounding areas shall be clean and free of debris.

 

 

Stucco Guru

Privacy Policy

This site is operated by Imasco Minerals Inc. as a public service. The site is dedicated to providing third party information to the general public and industry professionals.

 

Internet Marketing and Custom Web Design by Think Profits.com in Vancouver