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There are reasons your contractor may have to rebuild a brick wall — maybe there's been a problem and the exterior brick has had to be removed for the repair, or if you've done an addition and you want to match it to the existing brick house.

Sometimes, if a brick wall is weakening you'll see signs that it is uneven or pulling away from the framing behind it. If that's the case, it will have to be rebuilt.

I can't say it often enough: make sure you check references. I've seen brick masonry go very wrong right at the first course (layer) of brick. And if it's wrong there, every course above will likely be wrong. Be careful: It isn't easy to undo a bad brick job.

As the wall goes up, brick ties are attached to the wall every two square feet or so. These are L-shaped pieces of metal nailed to the wall and bent over double at the brick. When the mortar dries around the tie, it holds the wall in place permanently.

If the mason scrimps on the ties and uses less than are needed, what often happens is the brick facade is wavy. This means the wall sags toward the inside without those supports and the wall is not an even distance from the interior wood surface.

Temperature

It drives me crazy when I see guys laying bricks in bad weather just to avoid wrecking their schedule — they'd rather wreck the house. When laying brick, humidity is not an issue, but temperature is. On hot days, bricklayers will dip the brick in water to slow down the absorption of water from the mortar. If they don't, the mortar sets too fast, meaning a lot of bricks will be seated crooked or out of level. If the mortar hardens too quickly, it dries before it cures, making it brittle and it will crumble in a short time.

If the work is done when it is too cold, the mortar doesn't dry fast enough resulting in mortar squirting out as more weight is added on top. The bricks will slide around on the mortar and the result is a wavy mess.

Building a brick wall is easy to do, and even easier to do badly.

Keep your brick wall dry

I've talked before about brick spalling and crumbling from ice and water damage.

If your brick stays wet, then freezes and thaws several times over the winter, ice begins to shale away the surface of the bricks and damage is inevitable.

Brick walls must be kept away from your house's foundation — if the soil touches the brick it'll keep the brick wet and soft, which is a recipe for disaster.

It's also a great way to grow mould and moss.

Cleaning Brick

If your wall is growing mould or moss, it needs to be dealt with by removing the growth and by changing the conditions so it won't regrow.

Improve the air circulation by removing shrubs that are growing too close to the walls and change the grade so soil isn't touching the brick face.

Scrape the moss or mould off the surface of your brick wall, then wash the wall to kill the spores.

You'll probably have to do this a few times. And make sure you deal with the moisture problem, or you'll be doing this every year.

If you've got graffiti or paint, or just decades of pollution and soot staining your walls, you need to deal with that too, but cleaning brick is tricky. If you use an acid cleaner or harsh chemicals, you may get a clean wall that doesn't last very long.

Clay bricks have a hard outside surface because they are fired, but their centres are softer. If you damage the outer surface, you can allow water and the elements to penetrate the brick.

Some people will sandblast their brick homes to clean them up or for cosmetic reasons. This can be a huge mistake. The blasting — especially if it's done badly — will erode the face of the brick, as well as the mortar joints.

An alternative is soda blasting, which uses sodium bicarbonate instead of sand to strip off paint or stains. It's similar to sand blasting, but uses less pressure and supposedly causes less harm to the environment and to your brick and mortar work.

Pressure washing is something I'd avoid — it too can erode the surface of brick, and it forces water at high pressure in behind the brick wall face where it can eventually lead to serious structural damage and mould growth. And if that water doesn't dry out completely before freezing temperatures hit, you can start a dangerous freeze/thaw cycle that will break down your brick.

Mike Holmes is the host of Holmes on Homes on HGTV. For more information, go to www.holmesonhomes.com