Before you pressure wash a brick wall in Mobile

Mobile is warm and damp most of the year, and it rains a lot. A brick wall in the shade of a big oak, one that faces away from the sun, or one that sits close to the house next door may never fully dry. Mildew, algae and moss grow on that damp surface, and the wall goes green or picks up dark streaks, usually from the ground up and under the eaves.

Sprinklers that hit the wall, gutters that run over, and shrubs packed tight against the brick keep it wetter still. That is why one side of a house can be green while the side in the sun stays clean.

The growth itself is mostly about looks. But it marks a wall that stays wet, and that wall is the one to check for soft joints and flaking brick.

A pressure washer is built to blast dirt off hard surfaces like concrete. Brick and mortar are softer than they look. At high pressure the spray cuts into the joints and washes the sand out, and it can carve grooves into mortar in a few passes. Joints that were fine before the wash can look worn after it.

The brick takes damage too. Fired brick has a harder outer skin than its center. Once a strong jet breaks through that skin, the softer inside is open to water, and the face can start to flake or pop off later. You may also see wand marks, light streaks where the nozzle got too close.

A strong spray also drives water deep into the wall. On a brick veneer house it can push water through the weep holes and open joints into the space behind the brick, where it is slow to dry in humid weather. As that water works its way back out, it can carry salts to the face and leave white stains.

The older brick downtown and in the historic districts was often made by hand and fired at lower heat than modern brick. It is softer, and it was laid in soft lime mortar. A pressure washer can strip the face off that brick and empty the joints in an afternoon, and the damage cannot be undone. Spalled old brick has to be replaced, and good matches for it are hard to find.

Newer brick on the houses in West Mobile and Spring Hill is harder and holds up better. But its mortar joints wash out the same way under high pressure, and the veneer wall behind them soaks up water just as easily.

Most green growth and mildew will come off without high pressure. In general terms, the approach looks like this.

• Wet the wall down first with a garden hose so the brick does not soak up the cleaner.
• Use a cleaner made for masonry or for mildew on brick, mixed the way the label says.
• Scrub with a soft or medium nylon brush. Skip wire brushes, which scratch brick and can leave rust marks.
• Give the cleaner time to work, but do not let it dry on the wall.
• Rinse well with low pressure, from the top down.
• Work on a cool or cloudy day, or in the shade. In Mobile's summer sun a cleaner can dry on the brick before you rinse it.

Household bleach gets used on mildew a lot. But it can leave salts in the brick that come out later as white stains, and it can lighten mortar. Many masons steer away from it on old brick.

Strong acid cleaners can burn brick, stain it and eat into lime mortar. Keep them off old brick. Even with a cleaner sold for brick, follow the label and still test it first.

Pick a spot low on the wall and out of sight, behind a shrub or around the side of a step. Clean a small patch the way you plan to clean the whole wall. Then wait a few days and look at it dry. Some brick changes color when it is cleaned, and some cleaners leave a haze or a stain that only shows once the wall dries.

Look closely at the joints in the test patch. If sand comes away on the brush or the mortar looks rougher than before, ease up and use a softer brush, or stop.

If you hire the cleaning out, ask what pressure and what cleaner they plan to use, and ask for a test patch. Someone who mostly washes driveways may not know old brick.

A clean wall only stays clean as long as it dries. Cut the shrubs back off the brick, and thin the limbs that keep the wall in deep shade, so some air and sun reach it. Turn sprinkler heads so they water the bed and not the house. Clean the gutters and fix any that run over onto the brick.

Leave clear sealers on the shelf. A coating that keeps water from getting out of the brick can trap moisture inside in a climate this humid, and that tends to show up later as flaking faces and more white salts.

Look at the joints and the brick faces. Sandy or grooved joints mean the mortar was cut, and the wall will take on more water until it is repointed. Brick with patches where the face is gone, or faces flaking off, may need replacing.

That is tuckpointing and brick repair, which is the work Donald W Carpenter does under Brick Repair and Renovations. The tuckpointing page covers the joints, and the brick repair page covers damaged brick. Text Donald a picture of the wall from the yard and one up close on the joints that took the worst of it, plus your part of Mobile. Call or text (251) 305-5966.