Before cultured stone can be applied to a wall, a scratch coat must be applied evenly over the surface. As the mortar is applied, you’ll want to texturize, or “scratch,” the surface so the brown coat is easier to apply. Once your scratch coat is applied, it’s best to allow the mortar to solidify for 10 to 12 hours.

The scratch coat base is essentially the same as a stucco scratch coat and it supports the stone attachment to the wall.  Since the weight of the stone is carried by the scratch coat we had to add extra screws at less than 6 in. on center to hold the furring securely to the framing beneath the rigid foam insulation.

We installed two layers of housewrap — the first sheet is the drain layer and the second sheet is a sacrificial bond-break. I probably could have gotten away with a single layer since we have a fully vented rainscreen space but following the manufacturer’s instructions we installed two layers. The bond-break layer protects the drain layer from being contaminated by the mortar so its water-resistance properties aren’t compromised.

Over the two housewrap layers we stapled on galvanized expanded wire mesh. The mesh reinforces and holds the mortar on the wall. We used medium crown 16 ga. galvanized staples spaced 2 in. to 3 in. apart along each furring strip.

The mortar layer screeds on with a trowel and is pressed through the wire mesh. The finish thickness is between 1/2-in. and 3/4-in. thick. After mortar is evenly spread across the mesh a notched trowel is used to ‘scratch’ the surface. This leaves a rough surface for the stone mortar to key into for a good bond.

More info on the foam layer and the base build-out.

More info on installing the cultured stone.