How Deep Do We Dig to Build a Stone Wall?
This Old House landscape contractor Roger Cook explains how to build the proper base for a stone wall
We'd like to build a short stone wall next to our flagstone patio. How far down do we need to dig? —Jonathon Hotz, Reston, VA.
It all depends on how the wall is built.
If it's held together with mortar, it will need a well-tamped base of crushed stone that reaches below the frost line. Without that base, even a slight heave of the soil during freezing weather can crack the mortar. A dry-stack wall, which doesn't use mortar, can move with the soil as it swells and settles with the seasons. For this type of wall, a crushed-stone base about 6 inches deep is sufficient.
For your project, I'd go with a mortared wall. It will hold up better than a dry-stacked one, given all the activity that low walls next to patios are subject to. And mortared walls are generally faster and easier to build than dry-stacked, even with the extra digging.
Start by calling 811 to have the location of the wall surveyed for buried pipes and wires. When you know it's okay to excavate, dig a trench 2 feet wider than the thickness of the wall down to the frost line established by your local building department. Tamp the ground at the bottom of the trench with a plate compactor, then line the sides and bottom with filter fabric to prevent fine soil particles from infiltrating the crushed stone and interfering with water drainage. Now dump in a 3- to 4-inch layer of ¾-inch crushed stone, shovel it level, and settle it into place with the plate compactor. Continue adding and compacting 4-inch layers until the base is about 8 inches below grade. You can now build the wall.
But if either the ground or the patio is pitched toward the wall, hold off on the final layer and lay a 4-inch perforated drain pipe next to the filter fabric on one or both sides. It will intercept surface water and carry it to a dry well. Spread and tamp the last layer of crushed stone next to the pipe(s), then build the wall. For a step-by-step of the building process of a stone wall, go to thisoldhouse.com/sep2015.
When the wall is finished, cover the crushed stone with filter fabric and loam on the ground side and with tamped stone dust and flagstones on the patio side. The base will be hidden, but it's your guarantee that the visible part of the wall will remain intact.
— This Old House landscape contractor Roger Cook