How to repoint old stone and brick walls
Mortar doesn't last forever. So you're right that even the most carefully built chimney or masonry wall eventually will need repairs.
You're right, too, about water.
When the mortar between stones or bricks crumbles, it opens a path for water. Cycles of freezing and thawing will speed the wall's destruction.
Repairing old stone walls is an art as well as a practical skill. There are many practitioners of this craft but probably few as affable and knowledgeable as Ian Cramb, a Scotsman now living in eastern Pennsylvania.
Cramb is a fifth generation mason (his son, Ewan, is carrying on the family tradition). Ian learned the trade early in life and went on to work on a number of interesting projects over a long career, including the restoration of the U.S. Capitol, the rebuilding of the Iona Abbey off the coast of Scotland and even a stone house for Eric Clapton in the Caribbean.
His book, "The Art of the Stone Mason," is an excellent source of information.
Strong cement mortar is not the way to go
A big mistake in repairing old stone or brick work is to use portland cement mortar. It's readily available, strong and inexpensive, yes.
But as Ian will tell you, it's too strong.
It doesn't have the flexibility of the lime mortar that was widely used in stone and brick construction well into the 20th century. When it's used to re-point an old wall, it is the brick or stone that will suffer, not the mortar.
A major unwanted result of using portland cement mortar, Ian says, is that hairline cracks open almost immediately between mortar and stone. These cracks allow moisture inside the wall, where it becomes trapped.
Ensuing freeze-thaw cycles can then reduce old bricks to soft, red mush - or reduce stones to a pile of pebbles and sand. Neither of these is very effective at supporting your home.
According to the National Park Services' Heritage Preservation Services, portland cement was first manufactured in the U.S. in 1872 but not widely available until the 20th century.
If your house is older than that, it almost certainly was built with lime mortar or with a mix that incorporated some cement with lime and sand.
And it should be repaired with a compatible mortar.
A two-step process to re-pointing stone walls
Ian's approach involves two separate steps - tamp pointing and then finish pointing.
In the first, he cuts back into the joint at least 3 inches. Then he packs the first 1 1/2 to 2 inches with mortar and a tamper. The mortar is made of 7 parts sharp sand, 1 part lime and 1 part cement (no more! he says).
This initial layer is topped with finish pointing. It should be a uniform depth so it dries evenly but never less than 1/2 inch thick.
Ian mixes his final pointing mortar on a board, by hand with a shovel, not in a mixer. It consists of 6 parts sharp sand, 1 part lime and 3/4 part cement.
Instead of mixing up a day's worth of mortar and adding more water as it dries out, Ian recommends mixing only as much as you'll use in a half-hour.
Get some advice before diving in
I can almost hear Ian say, "Aye, but that's not the half of it," and he'd be right. There is a lot more to it.
If you want to learn more, there's a lot of information out there, even places to take classes. Ian's book is a good start, and there are, of course, many others on the subject.
The National Park Service's Preservation Service offers an excellent summary of how to approach a re-pointing project in an historic home.
The Old House Web has reprinted the entire booklet. You'll find it here.
Supplies and services are available from U.S. Heritage Group, a Chicago based company that also maintains an informative Web site (www.usheritage.com).
But the bottom line is not to reach for that bag of premixed cement mortar at the local big box store.
And if you hire a mason who brings the stuff with him, stop him before he gets to work on your house. Pick up the phone and call someone else.
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.