There is just something about a stone wall. Looking at one, you think of its honesty, strength, and permanence. To achieve these characteristics my crew and I have to do a tremendous amount of work that is never seen. If we lay the right groundwork, the wall will withstand the weather as well as the test of time.
A good wall begins with a stable base. For the two-and-a-half-foot-high stone retaining wall we built alongside Dick and Sandy Silva’s driveway, we dug a four-foot-wide trench 18 inches below grade, where we hit a sandy material with good drainage. We were very lucky here—in some locations with lots of loam and poor drainage, we have to excavate down four feet deep, below the frost line, and install piping to drain water out of the footing.
Drainage behind the wall is just as critical as the footing beneath it. The weight of wet soil can topple even the sturdiest wall, so directing water away before it becomes a problem is essential. A good rule of thumb for low walls is to excavate about one foot back for every foot of wall height, replacing native soil with crushed-stone backfill and a perforated drainpipe to channel water away from the structure.
This Old House landscape contractor Roger Cook emphasizes that preparation is everything: “Walls don’t fail because of the way the stone is stacked. They fail because of bad preparation.” All the extensive prep work on this project had one basic goal—to stop the infiltration of soil and water. Wet dirt is the mortal enemy of all stone walls in northern climates, because once it gets into a wall’s nooks and crannies, the heave and drop of freeze-thaw cycles eventually cause the stacked stones to collapse.
Next, we built a firm, frost-proof foundation. For the same reason Tom Silva would never build a well-crafted house on an unstable foundation, I would never build a stone wall on anything but a full base of washed stone. It allows water to drain away from the wall and thus stymies the freezing, thawing and heaving cycle we get during New England winters.
For it to work properly, however, this foundation stone must be kept separate from and uncontaminated by the surrounding soils; if the spaces between the stones get filled, the draining will cease. This is why we line the entire foundation hole with landscape fabric, a porous membrane that lets water drain out of the stones and keeps soil from migrating in. Without this barrier, the wall would eventually fail. It may take a while for this to happen, but the freeze-thaw cycle knows no deadline.
Once the foundation was set, we began to build the wall. The stone we used at the Silva house is a metamorphic quartzite from a quarry in western North Carolina. It is a handsome stone, dark gray when dry and a rich black when wet. Billed as being harder than granite, the rock comes in easy-stacking, flat-sided pieces about two inches thick. As is typical of any flat wall stone, it lays up fairly quickly. But the pieces weren’t big enough to span the width of the base and bear the load of the finished wall. To get the necessary thickness and strength, we laid up a supporting wall of brick and concrete block just behind the face row of rock.
I then set up two yellow mason’s lines, one representing the bottom of the wall, the other representing the top. On top of the base, just below the finished height of the driveway, we set our biggest stones into wet mortar. We built the Silvas’ retaining wall to a width of about 28 inches, tapering to 14 inches at the cap. You always want your base twice as wide as your cap. Building a good cap is like making a fine table: we save the best flat pieces for the top.
Roger and his crew always start a wall the same way: find the biggest stones and use them at the base. At the Silva project, building on a solid footing meant the heaviest pieces went down first, giving the retaining wall a massive, stable foundation to resist the pressure of the earth behind it. On the back side—where stones would be buried beneath the driveway—the crew used leftover fieldstone to fill in quickly, saving the best-looking pieces for the visible face.
The technique used here—hiding mortar behind the joints to create a “dry-look” finish—represents a middle ground in the centuries-old tradition of stone masonry. As This Old House Magazine has noted, dry stone was once the material of choice for homes, bridges, dams, mills, and ice houses before being superseded in the 20th century by concrete, which, although not as long-lasting, was far easier and faster to work with. Skilled masons today distinguish between true dry stone walls, “wet” walls where mortar-filled joints are clearly visible, and “dry-look” walls like this one, in which mortar is recessed and hidden from view.
A traditional New England stone wall is dry-stacked without mortar. Since this was a retaining wall, however, we needed mortar as glue, to hold the wall together and give it strength. To get the best of both worlds, we simulated dry-stacking, carefully troweling mortar behind each joint, away from the wall’s face. With each new layer of stone, we covered the joints of the ones below. Except for a few lines between the large capstones topping the wall, no mortar is visible. The mortar also allowed us to stack the front face of the wall vertically rather than at a slant the way we would without mortar. The strong, straight new wall aligned perfectly with the front wall of the new garage.
There are many different products you can use to build a retaining wall—railroad ties, pressure-treated lumber, telephone poles, concrete stacking blocks—but none can match the feeling of building with stone. As you chisel a stone to fit a certain spot, you feel like one of the builders of the great pyramids. When you set a large boulder for a base stone, you feel like one of the early settlers building a foundation for their barn. Hopefully, some of this romance will be felt by people who view the wall in Billerica. By putting extra work into its construction, we ensure that it will be there in all its beauty for Dick and Sandy’s grandchildren to enjoy.
