To Point or Not to Point? The Wrong Mortar Can Harm Historic Masonry
We’ve all heard of home inspectors recommending that a chimney be repointed before the sale of a house, but what does that mean? The average Joe on the street knows what it means: “Putting the cement back in between the bricks, right?” Basically, yes; that’s right. But did you know there are different types of mortar? Did you know some have little or no cement at all? And were you aware that if you repoint with too much cement in your mortar, you might damage the masonry itself?
Did you know that “repoint” isn’t even, technically, a word? Even though it’s used by architects and practitioners in professional documents, dictionaries don’t recognize it. But that’s a topic for another blog. (Don’t get me started.)
First, a little Mortar 101 is in order. Mortar is typically made up of three dry components: a binder, an aggregate and lime. This is usually Portland cement, sand and hydrated lime. It’s the ratio that determines the strength, or ASTM classification, of the mortar. The pre-mixed bags found at home centers are usually ASTM type “S” mortars, similar to those used on commercial construction sites to lay modern brick and block walls. They contain lots and lots of Portland cement and probably differ wildly from the mortar found in the average historic home.
Before 1872 in the United States, there was no such thing as Portland cement. Mortar was generally lime and sand mixed or lime, sand and natural cement (discovered in the 1820s during construction of the Erie Canal in upstate New York).
The paradox of a masonry structure is that its strength comes from its ability to fail. Well, what the heck does that mean? you’re wondering. And rightly so.
Here’s how an old friend best explained it to a class of preservation students. Masonry units, be they brick, stone or block, are laid in mortar. That mortar absorbs and expels moisture. Moisture is water, and water freezes. When it freezes it expands, increasing volume by as much as 12%. So, in a sense, the mortar expands, even minutely. Something has to give: the brick or the mortar.
If the mortar is “harder” (meaning a high cement content) than the bricks laid in it, the bricks will spall and pop, their faces crumbling and falling off. But if the bricks are “harder,” the mortar will give, often without cracking or falling apart or leaving any visible record of the strength through failure. And if the mortar joints do fail, it’s far less expensive to repoint masonry than it is to rebuild it.
A good mason will be able to mix up a repointing mortar that will not jeopardize the historic masonry fabric of your home or building. If the color or texture are more challenging, there are firms available online that will custom match mortar samples for under $200. That’s right folks. You can cry “Foul!” the next time a mason says, “I can repoint your brick wall, but I can’t match the old joints where they meet.” Or, “it’ll take a couple years for it to blend in, if ever.” Or, worst, “It’ll never match.”This is the same guy who buys bags of pre-mix mortar at Home Depot and repoints old, soft-brick chimneys. After a couple of winters, the chimney is crumbling and falling apart. If he’s really slick, the owners unwittingly call him back to rebuild the chimney that he destroyed!
A good repointing job should last at least 30 years. But, like most things in life, you get what you pay for. The cheapest guy, or the one who says “I can’t match it,” will look like a deer in the headlights if you start using terms like compressive strength, Portland cement or lime putty mortar. If you start to think, “Maybe I know more about this than he does,” you probably do.
You should ask for three references – specifically, for recent repointing jobs – and then go look at his work. A good masonry contractor will not spend his time forever going back and forth with you providing endless references and answering questions ad nauseam. He’s busy, in demand and doesn’t desperately need your job. But he’ll give you a comprehensive consultation and estimate, and he’s got a hal dozen references ready for a potential customer. Once you find him, hire him.