· by Michael Blair, Nashville's Masonry
Brick veneer on a house is not waterproof and was never meant to be. Rain soaks through it, runs down the back of the brick, and is collected by flashing at the bottom of the wall and directed back out through weep holes. Every unfilled gap in the bottom course above a window, above a door and at the base of the wall is part of that drainage system. Filling one turns the wall into a bathtub.
How a brick veneer wall drains
Behind the brick is an air space, typically an inch or so, and behind that a water-resistive barrier over the sheathing. Water that gets through the brick, and in a driving Middle Tennessee rain a great deal does, runs down the back face of the brick through that air space. At the bottom of the wall, and above every opening, a flashing runs from the barrier out to the face of the brick, catching the water and sending it out through the weep holes just above.
The weep holes also let air into the cavity, which is how the back of the brick dries after the rain stops. A wall with drainage but no ventilation dries slowly; a wall with neither stays wet.
What happens when they are blocked
Water still gets through the brick. With the weeps blocked it collects on the flashing, rises until it finds a gap in the flashing lap or a fastener hole, and reaches the sheathing and the framing. The sill plate, the bottom of the wall studs, the subfloor edge, all sit at exactly the level where that water collects. In our humidity it does not dry out between rains.
The damage is invisible from outside. Efflorescence and staining on the lower brick, a musty smell in a room on that wall, soft drywall low on the inside, or a termite infestation that found the wet wood are how it eventually announces itself. By then the framing repair costs many times what the wall would have cost to leave alone.
- Never caulk, mortar or foam a weep hole
- Never let mulch, soil or a patio slab rise above them
- Keep them clear of paint and of the debris pressure washing leaves behind
- Check that there are weeps above windows and doors, not just at the base
Bugs, mice and the honest fix
Weep holes do admit insects, and mice can use the larger ones. The answer is not to seal them but to screen them. Purpose-made weep hole covers and inserts are a stainless or plastic mesh that fits the opening, keeps pests out and lets water and air through. They are cheap and install in seconds from the outside. Some newer walls have them from the start; most older Nashville houses do not.
Copper or stainless mesh pushed into the hole does the same job. Steel wool rusts and stains the brick. Anything that stops water passing, including the foam backer rod people sometimes use, defeats the purpose.
Walls that never had them
Brick veneer built before flashing and weeps were standard practice, which includes many mid-century houses here, has no drainage plane at all. Water that gets through the brick simply sits against the sheathing and dries however it can. Those walls have often survived because the old sheathing was solid lumber and the walls were uninsulated, so they dried through.
Adding weeps to such a wall without adding flashing does little, because there is nothing to direct water to them. It is possible to retrofit flashing and weeps course by course, but it is a masonry job rather than a drill-a-hole job. Where the wall is not showing problems, keeping the grade down, the gutters working and the mortar sound is the sensible course.