Your question is a good one. Original plaster was put on over wood lath in three coats, about 3/8-inch thick over 3/8-inch thick wood lath. When this plaster and/or wood lath is taken out, the question comes up on what should be used to replace it. There are several options, and I‘ll give you two:
- Replace the damaged wood lath and leave the rest in place. Adding chicken wire or metal lath over the wood lath will give a solid and stable foundation over which to plaster. Of course, if you do go back with 3/8-inch thick plaster in three coats, it's going to be a labor and material intensive project. Pricing for this kind of work is usually considered just too expensive.
- Another option would be to remove the plaster and wood lath and replace it with 1/2-thick gypsum board. The board replaces the 1/2-inch plaster scratch and basecoat. You then put a thin layer of base and a thin layer of finish coat plaster over the board, about 1/8-inch thick (aka veneer coat plastering) and you're done.
I do hope this helps you decide which way to go with this project. If you have further questions on what mixes to use and/or other techniques, e-mail me at firstname.lastname@example.org.
The Plaster Man