What is a scratch coat?
A scratch coat is the first, coarse coat of mortar applied to rough masonry or concrete, levelling the background and providing a sound, well-keyed base for the finish coat. It is also called the base coat or first coat of render. The name comes from the practice of scratching or combing the surface while it is still green, so that the next coat has something to grip.
What the coat has to do
- Fill hollows and open joints, so the finish coat can stay an even thickness throughout.
- Even out suction. Bricks and joints draw water at different rates. A finish coat that loses water too quickly in places dries out, discolours or cracks.
- Provide a key. A scratched or combed surface gives the next coat mechanical grip.
- Bring the wall flat and plumb. The thin finish coat cannot correct a wall that is out of true.
Choosing the mix
| Background | Suitable mortar | Reason |
|---|---|---|
| Old, vapour-open solid masonry | Lime mortar | Stays soft and breathable, moves with the wall |
| Modern masonry and concrete | Cement-based or cement-lime mortar | Stronger, less sensitive to damp |
| Damp-loaded areas, cellars, plinths | Cement-based or specialist renovating mortar | Resists sustained moisture and salt |
A hard cement render on soft historic brickwork is a classic source of damage. The render is stronger and stiffer than the brick, traps vapour and drives moisture and salts back into the wall, leaving crumbling joints and spalled faces.
How it is applied
- Clean the background and take off anything loose or dusty.
- Control the suction, by damping down or by applying a bonding agent or spatterdash, depending on how strongly the background draws.
- Set out beads or screeds to establish plumb and plane.
- Lay on the mortar by hand or by machine.
- Rule off with a straightedge between the screeds.
- Float the surface, then scratch or comb it to form the key.
- Let it cure properly before the finish coat goes on.
Thick build-ups are applied in two passes rather than one, because an over-thick single coat shrinks and cracks. Do not work in frost: mortar that freezes loses strength and adhesion permanently.
Related terms
- Render
- Finish coat
- Lime mortar
- Straightedge
- Bonding agent
- Plaster key
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