How to get mortar joints just right
Q: I need to repair the mortar joints on my brick home that was built in the late 1800s. I’ve seen some horrible workmanship where the mortar doesn’t match at all, and I want to avoid this. How does one match the original mortar? What would you do to make sure the finished repair is nearly invisible? Is this even a realistic goal, or should I just resign myself to ugly mortar joints that don’t match?
A: Years ago I witnessed a real tuck-pointing travesty. Some mason, handyman or DIYer attempted to tuck-point the mortar joints on a brick home and the resulting mess was almost hurtful to the eyes. Not only was the mortar smeared on the brick, but it was also such a light color that the difference between the old and new was like night and day. It reminded me of a person with a smear of food on their face while eating. You have a tough time looking at them from across the table, to say the least.
I feel this question provides an excellent teaching moment. When I first started building back in the 1970s, I used to go to the library to learn about building. One day I was researching for concrete installation information and I stumbled across some pamphlets published by the Portland Cement Association.
I had never heard about associations before — keep in mind I was a mere 23 years old and had very little life experience. Before long, I discovered there were scads of associations that represented building products. These associations are in the business of not only promoting their products, but more importantly instructing how to install and take care of them the best way.
In short order, I discovered the Brick Industry Association and once there was taken aback by their vast library of technical notes about everything you could ever imagine about brick, mortar, flashings, chimneys and so forth. I reached out to them at the time, and they mailed to me at no cost two massive binders filled with all of the technical notes publications. I thought I had died and gone to heaven.
You would do quite well to read two of the BIA technical notes publications: Nos. 8 and 8B. Both deal with mortar. You can get them as free PDF files from the BIA website, www.gobrick.com/read-research/technical-notes.
Here’s what you’ll discover. There are different types of mortar, and mortar used a hundred years ago is not necessarily like the mortar mix you’d buy today at a building supply business. Many years ago, it was common for bricklayers to just use hydrated lime and sand to make their mortar. Hydrated lime is truly a magical building material.
Modern mortars come in all types depending on the finished design strength of the masonry. It can get very complicated very fast. Many modern mortar mixes are a proprietary blend of Portland cement, hydrated lime and other ingredients blended for the design purpose of the mortar.
One of the places that masons, handymen and DIYers go off the rails is the sand component of the mortar. Look closely at an older weathered mortar joint in between two bricks and you’ll quickly notice that you see more sand than you see the actual cementitious powder that binds the sand together! This is very important when you are trying to match the color and look of the mortar.
You need to do some homework and locate the same sand as you have in your existing mortar if you want to get a close color match once the mortar is installed. Think back where the bricklayers of old would have obtained the sand. A hundred years ago, it was hard to transport heavy things great distances. Is there a nearby gravel pit that’s still in operation? Do old newspaper clippings talk about local gravel pits that are now closed? Do what you have to do to locate sand that’s the same size and color range as your existing mortar.
If I were helping you as a consultant, here’s the first thing you and I would do to start the project. I’d clean the existing masonry. There’s a good chance the existing house is dirty, and so the mortar would be dirty too. I’d want to clean the mortar, at least on the wall that’s being repaired, so I could see exactly what we must match. You can use a pressure washer for this job, but you need to exercise extreme caution that you don’t damage the brick or mortar.
Once the brick and mortar were clean and dry, we would need to judge whether the brick masons colored the mortar. It was often done on jobs so the mortar had a tint close to the color of the brick. I’d then mix up a test batch of the mortar using the guidelines from the BIA technical notes No. 8 and allow it to dry for a week or two. I’d then use a very very mild acid wash to dissolve the lime from the sand on the surface to see if we have a match. We need to expose the actual sand particles to see the true finished color of the mortar. With trial and error, we could come close to matching the original color.