The joint
Mortar fills the lower part of each joint and stops about 1 inch below the top.
Once it cures, we fill the top inch with polymeric sand.
The mortar below gives the joint strength and holds the stones. The sand on top gives it room for normal expansion and movement, and finishes cleaner.
From the bottom up: solid subgrade; compacted chipstone in lifts, 3 to 4 passes each; cobble set in mortar; mortar stopped about 1 inch below the top; polymeric sand in the top of the joint. Dug 10 to 12 inches below the finished surface. A bracket on one joint marks the mortar stopping about 1 inch below the top of the stone.
- Solid subgrade
- Compacted chipstone in lifts, 3 to 4 passes each
- Cobble set in mortar
- Mortar stopped about 1 inch below the top
- Polymeric sand in the top of the joint
Dug 10 to 12 inches below the finished surface
Step by step
Dig
We dig 10 to 12 inches below the finished surface.
What goes under the base depends on what we find when we dig.
Base
We lay the chipstone base in lifts and compact each lift with 3 to 4 passes of a plate compactor.
We compact each lift until the base is tight and doesn't move underfoot.
Set
We set the cobbles in mortar, not sand.
Mortar locks the cobbles against shifting, rocking, spreading and sinking under vehicle load, weather and freeze-thaw.
Joint
We finish each joint as the diagram shows: mortar first, then polymeric sand on top.
Polymeric sand may need topping up over time, after weather, washing or normal wear.
Edges
We pack mortar tight against the outside cobbles to lock the edges.
The compacted chipstone base helps hold the edges too.
Sandy Cape lots and inland soil
Sandy Cape lots drain well, but sand shifts and washes out. On those lots we build a well-compacted chipstone base in lifts.
Inland soil, such as clay, loam or wet ground, moves with frost. On those lots we dig deeper, put down more compacted base, and use geotextile fabric where the soil needs separating.
Why cobblestone fails
The common ways cobblestone fails, and the cause of each.
- Sunken stones
- Poor base or not enough compaction.
- Loose or wobbly stones
- The joints failed, or the stones weren't set properly.
- Edges spreading
- No proper edge restraint.
- Weeds in joints
- Failed or missing polymeric sand.
- Washout
- Water gets under the surface and carries the base away.
- Heaving after winter
- Bad drainage, or a base that isn't deep enough.
- Low spots and puddles
- The apron wasn't pitched correctly.
- Cracked mortar
- Joints filled solid to the top, so they couldn't handle movement.
- Uneven cobbles
- Poor installation or settling.
The stone itself
Natural stone doesn't rot, rust or break down in salt air, and it handles New England freeze-thaw.