Masonry Institute sees solution in brick veneer rain screen system

Used with dpermission from:
Masonry Institute of British Columbia
W.C. (Bill) McEwen P.Eng.
Executive Director

Something that could make a difference to British Columbia's leaking condominium projects is the brick veneer rain screen wall system, according to Bill McEwan executive director of the Masonry Institute of British Columbia (B.C.).

McEwan names six key points in condominium construction:

in our climate all exterior wall surfaces should be expected to leak;
wall systems should allow any leakage to escape;
wall surface materials that are mixed on site and trowelled in place are more susceptible to workmanship defects, more difficult to inspect and harder to repair;
complicated details are more vulnerable to leakage;
short term thinking in the competitive condo market has reduced concern for long term quality;
today's tighter, heavily insulated walls are more susceptible to water damage.

McEwan says the brick veneer system can address these problems successfully. But just what is the system?

Brick Veneer: the exposed clay brick, stone or concrete block facing which provides the primary line of defense against the weather. The masonry units are laid in mortar attached to the back-up system by brick ties, and usually treated with a water repellent sealer for added protection.

Rain screen cavity: a minimum one inch air space behind the veneer which allows any water that may penetrate the veneer to migrate to the bottom of the wall and escape. It also allows air to circulate and dry the wall components. Board or sprayed insulation may be included against the inside face of a wider cavity to increase R value, and to minimize thermal bridging losses through studs in wood and metal systems. It can also move the wall dew point out side the back-up system where condensation could otherwise damage framing and insulation.

Flashing and weep holes: components at the bottom of the cavity wall which direct moisture back to the exterior.

Moisture barrier: a special building paper fastened over the sheathing on the inside face of the cavity. It is lapped and taped to provide a third level of protection against moisture penetration.

Back up wall: a wall which provides structural support, as well as air and vapor barrier, interior finishes and some or all of the insulation. The structural component may be concrete block, wood frame, steel studs, or concrete.

McEwan says that while the veneer layer of the system provides a high level of resistance to wind and rain, some leakage, as stated in point one, can be anticipated.

"The strength of the brick rain screen system is its ability to channel moisture to the bottom of the wall and back to the exterior, as called for in point two," McEwan said.

Moisture can move to the bottom of the wall cavity, be collected by a metal or rubber sheet flashing, and pass to the outside of the wall through slots -- called weep holes -- left open between the bricks.

With regard to point three, brick and block are manufactured units, produced to tight specifications and the full 3.5-inch veneer width is automatically provided. The mortar between the units is either site mixed or factory produced.

It is also 3.5 inches thick and is easy to inspect and repair. Mortar joints should be completely filled and given a properly tooled concave finish. Flashing and brick tie type and installation can be confirmed before the veneer is laid. The cavity space and weep holes should be kept clear of excess mortar.

Point four about detail problems is more a concern for face-sealed systems than for brick veneer.

"Interesting brick details can be provided by simple changes to brick color, texture, shape and bond patterns -- rather than by difficult indentations and curves," McEwan said.

Arches, corbels, quoins and soldier courses are a few of brick details which require only slight modifications to the regular brick veneer, he adds, and these do not affect the resistance provided by the cavity and inner moisture barrier.

The fifth point about quality in condo construction, McEwan views as a difficult general problem which cannot be resolved by material choices alone.

"However, the choice to use masonry walls does reflect consideration of long term investment value for the owner, and of quality image for the builder and designer," he said.

Masonry can address the increase in insulation and air-tightness contained in point six in two ways.

To start with, the brick rainscreen veneer wall can keep the inner wall drier.

Then, the use of concrete block rather than wood frame back up, can eliminate the justifiable concerns that have arisen over rotting structural walls.

Like all wall systems McEwan says, masonry walls are not always perfectly constructed or problem free. The basic wall is relatively easy to construct, but penetrations, openings and interfaces with other materials require design and construction consideration.

Contractors, suppliers and tradesmen who are members and supporters of the Masonry Institute of B.C. are concerned that their industry provide the highest possible quality products to the marketplace, McEwan says.