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How to deal with plaster bubbling up on the inside of an exterior wall

A reader wants to know what to do about bubbling plaster.
A reader wants to know what to do about bubbling plaster. (Reader photo)
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Q: I live in one of the small brick houses built in South Arlington during the 1930s. There are several places on our westward-facing exterior wall where the plaster on the inside wall bubbles up. I've tried scraping, patching and repainting, but it always comes back. I think there may be water coming through the brick or where grout on the exterior has fallen out. What do I do to make the exterior brick wall waterproof? Or how do I patch the grout?

Arlington, Va.

A: Your hunch is probably correct, assuming the “bubbles” crumble into something resembling coarse sand when you scrape them off. From the picture you sent, the bumps look like efflorescence — salt deposits that formed on the surface of the plaster, under the paint, when moisture got into the wall and evaporated, leaving behind minerals picked up on the way. Think of it as a mini salt lake forming on your wall under the cover of paint.

Efflorescence affects masonry materials such as brick, concrete and plaster. It doesn’t damage the underlying surface, but it’s ugly and can make paint peel. Wherever it shows up, the remedy always starts with stopping the moisture, because the deposits are sure to reappear if you just scrape them off or scrape and repaint. If the crust is on a ceiling, a roof leak is often the culprit. On walls, leaks around windows and doors are the most common sources. But your note and the picture you sent suggest the problem areas are in the middle of the wall. Because of this, start by going outside and checking whether you see anything that might explain why those areas harbor moisture. Is there a leaky connection in a gutter overhead? Is soil piled up against the wall? If so, fix the issue, and you should be able to repaint the inside wall without having the coating bubble up again.

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If a window or door aren’t nearby and there isn’t a leak, then deteriorating mortar is probably why moisture is getting through the wall. If it’s possible for you to safely get close to the outside wall, look for missing chunks of mortar and for cracks, even tiny ones. You can also gently poke the mortar with the tip of a screwdriver to see whether it’s loose and crumbly.

To repair deteriorating mortar, hire a mason to repoint the brick, a process that involves chiseling or grinding out the mortar about ¾ of an inch back from the face of the brick, then replacing that with new mortar. If you’re handy and there are only a few small areas with easy access that need patching, you can also do it yourself with a few basic tools. For a homeowner, chipping out the old mortar with a brick hammer and a pointing chisel, rather than tackling the job with a power grinder, minimizes the risk of damaging the brick. This Old House has a good YouTube video about this, titled “How to Repoint Brick Using Only Hand Tools.”

One critical challenge is to use mortar that matches the chemistry of the existing mortar. Around the time your house was built, there was a dramatic change in how brick structures were built. Instead of using traditional lime mortar, which stayed relatively soft and flexible, most builders switched to using mortar that also contained Portland cement. It hardened faster, which allowed bricklayers to build more each day, and it worked fine for walls built with its characteristics in mind. But for repointing walls built with lime mortar, cement mortar is a disaster. Water can get trapped within walls, and pieces of the brick can pop off because the new mortar will be too stiff to accommodate the natural shrinking and swelling of old brick.

An experienced eye can often identify the type of mortar by color: Lime mortar is white, while cement mortar is gray. If you aren’t sure, get advice from a mason who specializes in restoration of historic brick structures. Or ask neighbors with similar houses who have had repointing work done. Sergio Vaca, business manager for Randals Masonry Restoration (703-969-4182; randalsmasonry.com), which works throughout Northern Virginia, said a concrete-based mortar sold today as Type N is almost always the right product to use for repointing brick in homes like yours.

You’ll also want to match the grout color. Mix a tiny test batch and spread a thin layer on a board so it can dry quickly, then compare. If you’re using cement mortar, which contains lime and sand in addition to Portland cement and water, add a little extra lime to make it whiter or a little more Portland cement to make it darker. (Don’t overdo the additions, or the mortar won’t be as strong.) If the mortar is a color other than white or gray, then it was tinted, and your best bet is to buy mortar manufactured to create a similar color. Adding concrete colorants is also an option, but you’ll probably need numerous tests to get a good match.

The type of shaping on the front of the grout also matters. Search online for mortar joint shapes, and you’ll see at least a dozen styles. Concave and inward-pointing V-shape joints shed water best, according to the Brick Industry Association, a trade group, but unless you are redoing all of the grout on your house, stick to the style you have now. Shaping tools, which also help compress the mortar so it can stick well within joints, are sold at stores that carry masonry materials.

But for most homeowners and most repointing jobs, hiring a pro is the best way to ensure a good, safe job. Vaca said Randals Masonry’s prices are based on the size of the job. For spot repointing — when there are just a few places to patch — the fee is $52 to $54 an hour per worker, plus materials. Crews always have at least two people, and jobs that require scaffolding are at the higher end of the hourly rate. For sectional repointing, the most common type of job, there is a square-foot fee of $13 to $16 or a daily rate of $850 to $900. The company also does full repointing, which involves redoing one or more entire walls, but that is rare.

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