Water Penetration Through Exterior Walls

Weep holes provide the best protection against serious problems.

Rainwater penetration through exterior masonry walls to the interior space is causing serious problems around the country, even in some new buildings. Most contemporary walls are designed and built as a cavity system with a masonry veneer (clay brick, concrete brick, stone, etc.), air space, and backup materials.

This wall arrangement is normally designed under the assumption that some moisture could penetrate the exterior cladding, but should not migrate through the recommended 2-inch clear air space located between the masonry veneer and backup system. Any moisture getting through the exterior cladding should migrate down within the air space and drain out form the wall system through the weep holes. Most design guidelines and details focus on the limitation or avoidance of water penetration through the masonry veneer, rather than the functional and safe drainage of the cavity wall system.
I normally make several exterior masonry wall investigations per week, and in many cases, find problems. Many homes have no weep holes and water actually puddles up behind the walls until it is dissipated into the interior structure. These walls are typically on completed homes where weep hole problems are difficult to correct.

Rectangular or circular plastic tube weep holes are installed at base of the brick veneer with a proper spacing, but contractors forget to install, or the designer did not specify, a drainage material behind the masonry veneer. Moisture accumulates above the flashing line, but does not have any passage to the weep holes and out from the wall system. In many cases, mortar dropping block weep holes or the contractor filled the collar joint with mortar that completely plugs the weep holes.
I employ a non-destructive test method to check installed weep holes for drainage. I use a device consisting of a toilet shut-off valve with an adapter for a garden hose at one end, and a 3/8-inch diameter copper pipe on the other side. I attach the device to a garden hose and pour water into one weep hole. Water should drain out from the other weep holes. Otherwise, it is non-functional drainage system.
Unfortunately, far too often there are negative test results because water was not draining out through the weep holes.

Author’s preference
In my opinion, the most effective drainage material is a product from Mortar Net with a dovetail shape. Mortar droppings could accumulate at a lower or upper level of the product, but it eliminates the possibility of a complete horizontal blockage of the cavity.

Many wall systems are designed with insulation inside the cavity and a vapor/air barrier sandwiched between the extruded polystyrene insulation board and an exterior face of the wall backup system. This energy efficient wall system also could improve the wall’s water resistance with the introduction of cavity air space ventilation or a rain screen system.

Rain screen systems
Canadians started using rain screen systems in 1960s. The basic idea behind the pressure equalized rain screen wall system is to provide weep holes at the bottom of the masonry veneer and vents at the top within each floor. Air circulation through the cavity should equalize pressure on each side of the masonry veneer and minimize water penetration into the wall system. This approach should be used on projects located in areas that receive high volumes of wind-driven rain and when resistance to water penetration is a prime concern.
Brick Industry Association Technical Note 27, “Brick Masonry Rain Screen Walls,” contains valuable information for designers of this system. Besides functional weep holes and vents, the air space located behind the masonry veneer should be divided vertically into compartments, specifically on each side of the corner. Designers could follow locations of the clay brick veneer vertical expansion joints to locate vertical dividers inside the cavity.
Drainage material installed at the flashing line to protect weep holes from mortar droppings also should help air circulation through the weep holes. Masonry veneer cavity airflow testing was conducted at the Architectural Testing Labs in York, Pa., in April 2004.

Comparing Shapes
ASTM E 283-91 “Standard Test Method for Determining the Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen” was used to compare the dovetail shape cavity drainage material with horizontally straight versions installed into the cavity wall system with an even amount of mortar droppings inside the cavity. Each test panel was 5-feet wide and 10-feet high, with three open head joint weep holes at the base of the panel. There was 9% airflow reduction at the test panel with the dovetail shape cavity drainage material with the wind speed ranging from 15-50 mph.
The test panel with the horizontally straight cavity drainage material had a significant air-flow reduction ranging from 15% with a wind speed of 15 mph to 47.5% at 50 mph. The horizontally straight cavity drainage material had a continuous accumulation of the mortar droppings, which reduced airflow and circulation through the cavity.
My conclusion is that the best water-proofing of the exterior walls with a masonry cladding is to provide the functional drainage of the wall cavity system through the weep holes. The designer should specify the testing of the weep holes with a minimum of one pour of water (approximately 5 gallons) for each 10-feet length of the masonry veneer located above the flashing line. Water should be pouring out from every weep hole.