NWCB Technical Document 01/05 EXTERIOR SYSTEMS Stucco Over Concrete or Masonry Walls 600-609 The NWCB believes it is best to install a cement plaster over CMU or concrete walls without a traditional secondary Weather-Resistive Barrier (WRB) common on framed walls. Stucco can be applied to a concrete or masonry surface with or without a reinforcing metal base (lath). The bond and/or thickness of the cement plaster will determine the need for a lath. However, the WRB (building paper and house wraps) prevents the bond between the plaster (stuc- co) and the concrete. This coupled with the fact that attaching a lath and WRB to concrete usually result in tears or holes in the barrier and ultimately has proven not to provide the desired additional moisture protection. In fact, the addition of the WRB has led to some specific problems. With CMU walls it is difficult to attach the fasteners into the grout joint when they are covered with the WRB. In freeze-thaw conditions, moisture can get behind the WRB and freeze. The expansion of the freezing water can push the plaster off the concrete or CMU wall. This method of stucco over concrete and/or masonry without a secondary WRB has been used successfully for many years in the United States and has a longer record of success in Europe. Since this is not designed as a drain- age system, there is no need for a “weep screed” at the base of the wall. If a second layer moisture defense is desired, there are trowel, spray and roller applied secondary coatings that will work. The product should be vapor permeable and compatible with Portland cement adhesion. Most of these products come from the EIFS (Exterior Insulation and Finish System) industry. When these products are used, there should be a metal lath attached to the CMU or concrete. Another option is to have the wall furred out with “Z” channels or framing members, sheathing and attach the stucco system as a conventional system over framing. This system would be considered a drainage system and require a weep screed at the base of the framing. 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. www.nwcb.org HEADQUARTERS OREGON 2825 Eastlake Ave E Ste 350 | Seattle, WA 98102 tel 503-295-0333 tel 206-524-4243 | email [email protected] email [email protected] © NORTHWEST WALL AND CEILING BUREAU.