Repointing Old Masonry

Scott Gibson, Contributing Editor

I've read about using a lime-based cement for repointing the foundations of old homes. Our home was built in 1860. We have not had any luck locating the proper product. Is there a recipe that we could make ourselves, or do you have a specific product that you can recommend?

If your house was built in 1860, its foundation was almost certainly put together with some kind of lime mortar. Portland cement, the all important ingredient of modern mortar and concrete, wasn't produced in this country until the 1870s.

Cement-based mortars are fast-setting, predictable and strong. And that's part of the problem when they're used on historic buildings -- they're too strong. Because cement mortar doesn't have enough give, it can damage old brick and stone. So you're on the right track, and you can make your own lime mortar with one of several recipes. It's not something you're going to find at your local hardware store.

Traditional mixes took time

An excellent description of making lime mortar the old way comes from Ian Cramb, a fifth generation Scottish stone mason now living in eastern Pennsylvania.

It's hard work. Start with quicklime, which is made by heating limestone to high temperatures to drive off the moisture and carbonic acid it contains. Mix that with sand and allow it to stand, or "slake," for up to six weeks, then pass it through a sieve and bank it into a tidy pile with a shovel.

That mixture could be left more or less indefinitely and used as needed. As the Cramb clan prepared for a job, they simply sliced off a section of the pile and took it with them. It was, Ian says, like cutting into butter.

Using hydrated lime is easier

You might have better luck with something called hydrated lime, which has been slaked for you and is sold by the bag. To it, you will add sand and probably some cement.

Ian offers several recipes for lime mortar in his book The Art of the Stonemason (a wonderful book if you can get your hands on a copy). He varies the mix to meet specific requirements but suggests two for general purpose use: 7 parts sharp sand, 1 part lime and 1 part cement or 8 parts sharp sand, 2 parts lime and 1 part cement.

For another point of view, check the excellent description provided by the Technical Preservation Services branch of the National Park Service (these folks do a lot more than lead tours at Old Faithful). You can find the information online by going to www.cr.nps.gov and following the links to the "Preservation Briefs" section.

It would be very good background for the project ahead of you and the discussion will help explain several recipes for mortar that are included. There's even a table that will help you match the mortar type to the kind of masonry you have.

Make a test run first

In the end, there's probably no single answer to your question. After you've done a bit more reading, consider mixing up a batch or two of lime mortar and seeing how they match what you already have

If the process seems inexact, just remember that people built houses 150 years ago without benefit of standardized, packaged materials. Masons no doubt had their own favorite mortar recipes, and ingredients could vary considerably. You'll be in good company.

About the Author
An accomplished woodworker and carpenter, Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He also is former managing editor of the Kennebec Journal, a daily newspaper in Maine.

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