When it comes to maintaining the durability of brick masonry wall systems, moisture control is perhaps the most crucial design consideration.
Unwanted moisture can cause masonry to experience dimensional changes, metals to corrode, and insulation to lose its effectiveness. Moisture can deteriorate interior finishes and lead to the appearance of efflorescence on exterior surfaces. Freezing and thawing from unwanted moisture can cause cracking, crazing, spalling, and deterioration over time. Water penetration also provides the moisture that mold needs to grow on susceptible wall elements.
"Over time, even the most watertight brick walls are going to leak, so let's assume they will and design for it," says Brian E. Trimble, PE, director of engineering services and architectural outreach with the Brick Industry Association. Thus, brick wall systems should be designed to expel unwanted moisture that does penetrate the wall.
Of the three types of brick walls (drainage, barrier, and single wythe), drainage walls—which are used in the vast majority of brick-clad commercial and institutional buildings—expel unwanted moisture the best, says Trimble. Drainage walls feature a cavity between the brick and the concrete block or stud back-up through which water can drain downward to flashing and out of the wall through the weeps.
Trimble offers the following tips for maintaining watertight brick drainage walls:
1 Maintain sufficient cavity space. The BIA recommends a two-inch air space between the brick and the block or stud backup. Anything less, says Trimble, and you'll run into problems.
The two-inch gap allows tolerances to occur within the wall without compromising the wall system. "Framing systems often have greater allowable tolerances than the wall, and will encroach on the air space," says Trimble. "Masons have nowhere to make up the difference, since the outside face of the wall has to be in a certain location." That's why it's best to make up tolerances in the cavity.
But what if the cavity is too small? In that case, mortar can potentially "bridge" across the cavity, providing a direct path for water to enter the interior wall.
2 Keep the cavity clean, but not pristine. A common mistake in brick construction is allowing excess mortar to drop into the cavity area, which can impede drainage. "During construction, mortar squeezes out the back face of the brick and drops down on top of the flashing," says Trimble. These obstructions can render flashing and weeps ineffective.
Ideally, the cavity should remain clear of all mortar droppings, but that's not realistic, says Trimble. He recommends a more sensible approach: "Clean, but not pristine."
"The cavity has to be clean enough to allow the water to drain, but you have to make some allowances for mortar droppings," says Trimble.
One approach for maintaining drainage (albeit one that is being used less frequently these days) consists of pouring a two- to three-inch layer of pea gravel on top of flashing. BIA recommends that the gravel size be larger than the weep hole opening (at least 3/8 inch), and that a bed of mortar be placed under the flashing for added support of the gravel.
More recently, the BIA has been recommending the use of devices that collect mortar droppings to maintain drainage. There are a number of products on the market that can help maintain a clear cavity and ensure proper drainage without worrying about near-flawless craftsmanship. Mortar-dropping collection devices, such as those offered by Mortar Net (www.mortarnet.com) and Advanced Building Products (www.advancedflashing.com), are one- to two-inch-thick plastic mesh "nets" that are placed above the flashing to prevent mortar droppings from blocking the weeps.
3 Add a second layer of defense. Various barriers can be introduced into the cavity wall to assist in moisture management. The systems generally fall into three categories: weather-resistive barriers, air barriers, and vapor retarders.
A weather-resistive barrier, such as building felt or Tyvek, can redirect moisture that happens to get across the cavity. Air barriers block random air movement through the cavity and allow water vapor that does enter to diffuse back out again. Vapor retarders reduce vapor moving through the wall system.
4 Go high and far with through-wall flashing. To ensure a proper seal, the BIA recommends through-wall flashing extend a minimum of eight inches up the concrete-block or stud backup. When using a mortar-dropping collection device, the flashing may need to extend higher than eight inches.
In addition, the flashing should extend to the outside face of the wall to form a drip edge, which will help redirect and shed water. The flashing should never be held back from the face of the brick, says Trimble. Doing so will allow water to re-enter the wall.
5 Make sure your weeps let air in and water out. "The best types of weeps are those that get water out of the cavity the quickest," says Trimble. Open-vent systems are preferred over rope wicks and small diameter plastic tubes because they expel water quickly and let air into the cavity, which helps the walls dry out faster.
Open-head joints should be spaced 24 inches on center. Also, consider placing vents or mesh over the open head joints to keep bugs out.
6 Cap off walls and parapets correctly. The tops of walls and parapets should have an adequate cap or coping. Caps should slope downward away from the face of the wall above. Copings may slope in one or both directions. In all cases, the slope should be a minimum of 15 degrees from horizontal. Also, both systems should overhang so that the inner lip of the drip is at least one inch from the face of the wall.
Metal copings are preferred to brick, concrete, or cast stone materials. Metal systems should extend at least eight inches down the face of the wall, with the bottom edge sealed against the masonry to prevent wind-driven rain from entering the wall.
7 Inspect the work as often as possible. Once an industry mainstay, the practice of design teams inspecting the brick construction work has fallen by the wayside, says Trimble. There are three levels of inspection in building codes: A, B, and C, with C being the most stringent. Specify the appropriate level of quality assurance and urge the owner to include these extra services in the contract early on in the project.
8 Demand good workmanship. Brick cavity walls are only as good as you build them. That's why it's important to choose a masonry firm that knows what it's doing. You probably already request sample panels to verify brick color: Use them to assess the standard of workmanship. This also puts the masons on notice as to the standards that you expect in the job.
9 Ask yourself: Are water repellents necessary? While not a vital component of maintaining a water-resistant brick wall, some clear coatings can minimize the absorption and penetration of water. BIA does not recommend them for new brick construction, but some of these coatings—specifically materials that "breathe," like silanes and siloxanes—can be useful for existing buildings that leak.
Coatings that form a film, such as acrylic or stearates, can trap water in the wall, which can lead to spalling or disintegration of the brick.
For more on brick masonry design, see "Quilted Beauty" atwww.bdcnetwork.com/article/CA633896.html.
