Scratch Coat

What a scratch coat is

A scratch coat is a base coat of cement mortar applied over lath or a prepared substrate. While it is still workable, the surface is combed with horizontal grooves — 'scratched' — and then left to cure.

Those grooves are the point of the coat. They increase surface area and give the next layer of mortar a mechanical key to grip into, rather than relying on adhesion to a smooth face alone.

Where it is used

Scratch coats belong to cement-based wall and floor assemblies. In shower and bathroom construction they commonly appear as the first mortar layer over metal lath on a wall that will later receive a mortar bed, a waterproofing layer or tile, depending on the method being used.

• Over metal lath on framed walls in wet areas
• Over concrete or block where a flatter, keyed face is needed
• As the first stage of a multi-coat mortar wall build-up

Why substrate preparation matters

Everything above a scratch coat inherits its geometry. A wall that leaves this stage bowed, hollow or out of plumb carries those errors into the mortar bed and then into the tile, where they show up as lippage, uneven grout lines and awkward transitions at corners and niches.

Preparation is also the last easy opportunity to correct a substrate. Once later layers are on, adjustments mean removal rather than refinement.

What the process generally involves

The specifics change with the assembly, but the sequence is consistent: confirm the substrate and its fixings, establish reference points for plane and plumb, mix and apply the mortar, comb the surface, and let it cure before the next stage.

• Check the substrate, framing and any lath or fixings already installed
• Set out reference points so the finished plane is deliberate, not approximate
• Apply the mortar coat and comb the surface while it is workable
• Allow the coat to cure before the following layer is built on it