Scratch coat plaster installations are the basis for a good plaster panel and are considered the main structural component for a plaster wall. It connects the plastic material to the structural members by attaching to the lath through the process of bedding the lath.

The lath is attached to framing members, the plaster is attached to the lath by bedding the lath or developing plaster keys which hold the set and cured material in place.

Without keys, plaster falls off walls.

It’s very important that you bed the lath, and provide mechanical and chemical bond for the next coat, the brown coat or leveling coat, by scoring the top of the scratch coat with a scarifier. Some folks use a notched trowel smoothing and scarifying at the same time, but you don’t get the same mechanical bond or as hard a wall with that technique.


3/8″ is the minimum thickness of the scratch coat. Measurement is from the back of the substrate to the crown of the scored material. When you see a fully bedded scratch coat, the wire is covered (as in the picture above.) The furr in the wire measured from the back of the substrate to the top of the wire should be 1/4″. Covered with a layer approximately 1/8″ thick, the furred and embedded wire is 3/8″ thick.

That’s why a scratch coat done well shows no wire.

If you have questions about this condition or other situations, call or email:

Bruce Bell
Bell Construction Consulting

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