Guide
Scratch coat over metal lath: when a floor needs one
Wood moves with the seasons and bends under weight. Tile and stone do not. A scratch coat over metal lath puts a thin, solid layer of mortar between the two, so the finish sits on a stable base.
What it is
Metal lath is an expanded steel mesh. It is fastened over the subfloor or the wall, usually on a sheet of building paper or a cleavage membrane that keeps the mortar from bonding to the wood.
The scratch coat is a layer of sand and cement mortar troweled into the lath and raked with a notched tool before it sets. The grooves give the next layer, a mortar bed or the thin-set for the tile, something to grip.
When a floor or wall needs one
Natural stone, marble or large tiles on a wood-framed floor, where the finish needs a stiff, flat base.
A mortar bed going over wood: the lath holds the bed together and the membrane lets the wood move under it.
Shower walls and other walls that will get tile or stone in a mortar bed instead of on board.
Older houses where the framing is stiff enough but the subfloor is too uneven for the finish, or needs a solid mortar base over the wood.
What it does not fix
A scratch coat is not a structural repair. If the joists are too small, too far apart or damaged, the floor will still bend. Tile needs a floor stiff enough for tile, and natural stone needs a stiffer one still; a contractor or engineer checks the framing first.
Questions
Is a scratch coat the same as a mortar bed?
No. The scratch coat is a thin first layer worked into the lath. A mortar bed can be built on top of it when a floor also needs to be levelled or sloped.
Can tile go straight on the scratch coat?
Only where the tile or stone maker allows it. Over wood, the standard method is a mortar bed on top of the lath and scratch coat.