When you gut a house, you get to see how it’s been messed up. The carelessly notched joists, outdated wiring, crotchety plumbing, and uselessly soggy insulation are boldly on display, and the necessary fixes are usually pretty evident. If the joist sags, sister it with another. If the plumbing leaks, replace the offending pipe with copper or Pex. If the insulation is bad, seal the wall with foam and jostle some new batts of fibreglass or Roxul between the studs.
With old stone foundations, detecting problems can be hard. Take it apart to get a good look and your house will fall down.
Our foundation is slate, which isn’t a hard stone, but it breaks with nice, flat surfaces and it shapes easily, which makes for tight joints. The foundation wall is also about two feet thick, so it’s unlikely that we will suffer a catastrophic collapse if it’s not in perfect shape. That said, an old stone foundation will probably need tuck pointing, a process of renewing the mortar holding the stones in place.
My journey to masonic enlightenment started with a visit from Myke Hodgins, a talented landscape architect I had met at the home of a friend designing a garden for an international competition. Myke generously came by to take a look at my back yard to advise me about grades and drainage and to answer a few questions about deck-building. Before leaving, he also suggested that I dig a couple of feet down beside the foundation wall before covering it with a deck.
The next day, I took a look at the above-ground foundation, really seeing it for the first time. The rock itself was almost hidden because it had been slathered with different patching materials of many shades of grey. I poked at one of these patches with a cold chisel and a maul and it fell away like a thick fish scale. With the joint exposed, the sand behind it–sand, not mortar–poured out like water, along with some desiccated bug casings.
I banged and dug some more and discovered voids in the foundation between 1 and 15 inches deep. Other patches were adamantly attached to the foundation stones, and the chisel just bounced off them.
After hours of digging a trench in sour-smelling, wet dirt that the neighbourhood feral cats liked to crap in, I had an inspection trench beside the wall.
There, two feet below grade, dirt had replaced the mortar in some of the caverns between stones, and some of the stones themselves had crumbled.
At which point, my neighbour, Tony, came over and made the gnomic observation that “renovation is like dominoes.” Well, yeah, but really, really big dominoes.
Mortar may not be everyone’s thing, so some history may make this lesson a little easier. The different rock-like substances we build with (cement, concrete, stucco, mortar) are made primarily from a binder of some sort and a filler or aggregate of stone and/or sand (e.g., concrete is a binder of Portland cement and an aggregate of stone and sand. Mix with water and serve). One hundred and twenty years ago, the binder most likely used in mortar would have been lime, not concrete. Lime mortar is softer, weaker, and doesn’t seal in water. It’s not even a structural material in the sense concrete is; it just holds the stones in place, it doesn’t hold the building up. During the freeze-thaw cycles of the year, it lets water escape so that it doesn’t freeze inside the very frangible slate and destroy it. Instead the lime mortar itself slowly breaks down. Masons call it “sacrificial.”
I hope I’ve been clear enough for you to see this next bit coming. If an 1890 lime mortar wall is tuck pointed with modern, super-hard materials containing Portland cement, it’s the cement-based materials that endure, not the foundation stone. Why? Because the modern products hold the water in, and the stone, trapped inside this hard material, freezes and crumbles.
I didn’t know what to do. A This Old House video advised the use of modern “Type S” mortar. A mason I spoke with said to use Type S, as well. A restorer in New England mixed “Type O” with sand. Two calls to the Quikcrete company brought one “I don’t know. Our engineers will call you back” and one “Use Type S above grade and Type N below grade. Our engineers will call you back.” Masons in the UK rebuilding 500-year-old churches mix their own mortar from lime and sand. You have to be careful with the lime because it’s corrosive, and it requires a very, very long time to cure. Oh, it can blind you, too, if you’re not careful.
I forgot to say that the multiple promises of call-backs from Quikcrete were just that.
The only satisfaction I gained from this long safari through contradictory information was encountering two factoids: the letters indicating the hardness of mortars–“m” through “n”–are all the letters in the word “mason”; the sand used in mortar should have sharp edges (i.e., not beach sand, which is smooth like beach glass) or it just slides off the trowel.
Then, after following scores of threads in construction fora, I ran across a couple of videos and blog entries that convinced me there wasn’t a solution to be found in Home Depot. This was followed by the discovery of a U.S. government document advising lime-based mortar for historic preservation. At the end of this odyssey, I found a great company, Daubois Inc., in Montreal, that imports lime from France, compounds their own mortars for every use imaginable, and retails them through construction yards specializing in masonry products.
When I phoned Daubois, I was a little self-conscious because I imagined that most of their callers were other companies or from the trade, and not from ordinary home owners. However, Daubois was not Quikcrete. The cheery and competent people I spoke with put me in contact with Alain Jetté, with whom I spent a very instructive and pleasant half-hour on the phone learning everything I needed to know. Alain is an exceptionally patient teacher, but be prepared to use at least some French should you need to contact him.
Daubois has a soft mortar, Restomix (Type O!), recommended for the work I have to do and it ain’t cheap ($25/50 lb. bag).