With so many people who own stucco homes struggling with wall rot, there has been a renewed interest in the subject of waterproofing stucco. Stucco itself is fairly waterproof on its own, but stucco walls and openings in those walls need to be correctly installed in order to achieve true waterproofing. There are three key components to waterproofing a stucco house for the long term: Using a multiple layer approach to stucco installation, including an air gap in those layers, and correct installation of flashing around any openings in the stucco walls.
Multiple Layer Stucco Installation
Knowledgeable stucco installation specialists use a multi-layer approach to installing stucco walls. If the stucco is being applied to some type of sheathing, such as plywood or OSB, this is what a typical stucco wall would be comprised of:
- Single layer of weather resistant barrier (WRB), such as asphalt paper or Grade D paper
- 3D plastic drainage matrix (serving as an air gap)
- Another layer of WRB
- Lath, which is a metal or fiberglass mat
- Multiple layers of stucco (typically 3 layers)
Notice the use of a weather-resistant barrier. This is very important to prevent moisture intrusion. However, as homeowners with wall rot problems have discovered, it’s not enough to prevent moisture intrusion; something must be done to allow moisture to escape if it should get behind the stucco.
The primary purpose of the drainage matrix (also known as a rainscreen gap, drainage mat, or air gap) is included as part of the layers to provide a pathway for any trapped water to drain out from between the walls. It also provides an air gap to allow some moisture to evaporate. This same air gap also discourages moisture from spreading from one layer to another when it is combined with WRB. The draining matrix works with a weep screed at the bottom of the walls to make sure that any trapped moisture is free to drain and run off properly.
Failure to provide some means of drainage is believed to be the primary cause of the outbreak of wall rot in Pennsylvania. Some home inspectors feel strongly that an air gap should be a mandatory part of stucco installation methodologies. Do not use a stucco installation company that does not use some type of air gap or drainage matrix as part of a multi-layer stucco installation.
If stucco is indeed waterproof, and the multiple layer method of installation is being used, then how can moisture still have access behind the stucco? The answer is quite simple. The major points of water intrusion are around openings in the stucco wall and areas where the stucco intersects the roof.
Metal flashing (including kick-out flashing and step flashing) must be installed correctly around doors and windows to form a watertight seal. Flashing must be used with all windows and all doors, as well as with any openings in the stucco wall. Along with the flashing, multiple layers of WRB paper should be placed in the area beneath the bottom window flanges as well as the sides.
The next problem area is where the roof and stucco meet. With sections where the roof does not extend over the stucco wall or where the stucco wall and roof intersect, a special type of metal flashing called kick-out flashing must be installed to prevent water intrusion. In addition, WRB paper should be installed at the soffit line and at the top of the stucco walls. Some experts also recommend that homeowners install rain gutters to direct the flow of rain water away from susceptible areas.
Maintenance and Care
Even the best-quality installations can suffer damage if the stucco is not properly maintained. This includes things like immediately addressing cracks or openings that develop in the walls and not ignoring signs of moisture intrusion such as discoloration around windows. In addition, high-pressure water should never be used to clean stucco walls as it is likely to force water where it does not belong.
The multi-layer stucco method and correct installation of flashing work together to create as watertight a seal for exterior walls as is possible. However, that is not enough to prevent your stucco home from water-intrusion damage. The use of a drainage matrix and weep screed is necessary so that any moisture that makes its way between the walls can either evaporate or quickly drain away before starting a catastrophic cycle of spreading wall rot. Some homeowners will also have their stucco covered with a waterproofing layer as additional protection against moisture intrusion.