Introduction to Plaster Wall Construction
You may encounter plaster in any one of three different configurations here in the United States; the oldest type of construction is a plaster coat over a masonry surface. In the early days and as late as the 1950's some homes were built with two courses of brick and a small filler section of rubble. Many of the antebellum homes here in Central Kentucky were constructed with both exterior and some interior load bearing walls made in this style. Homes were also constructed in many parts of the South out of a single course of concrete blocks. Most of these interior walls were covered with plaster especially in the homes built from brick.
The second style of construction used small strips of wood called lath. Lath is a rough sawn board about 1 1/4 wide and 3/8" thick. I have never had the pleasure of buying new lath so I am only assuming it must have been sold in bundles of a designated length. I suspect it was sold in more than one length that roughly corresponded to the standard 16 OC wall spacing. Wikipedia states that it was sold in four lengths only. I know I have removed longer sections but 4 foot does seem to be the most common length.
In the book, "Basic Home Carpentry" published by Wm H Wise & Company (1952), it was stated (my words here, not a direct quote):
Wood lath was made from spruce, cedar, white pine, cypress or other woods that were free of any staining tendencies which might cause a bleed through. Lath was sold in two grades #1 and #2. The standard size was 1/3" thick, 1 5/8" wide and usually 4 foot long. The number 2 grade may also be found in 1/4" thick and only 1 1/4" wide. The book also mentions and illustrates something called punched metal lath which was used mostly under ceramic tile and for ceiling work. 
This punched metal lath should not be confused with wire lath, chicken wire look, or standard expanded metal lath which is still sold today, even at the BORG. I am thankful that I have never encountered any of the punched metal laths. It looks like it would be a PITA to deal with and remove.
The wood lath was nailed to the wall studs at right angles leaving small gaps of 1/4" -- 3/8" so that the scratch coat can penetrate the wood in the cracks and form the fingers that help it adhere. Then the intermediate coat was applied and then the final top coat or hard coat as it is often called.
Lath is most commonly found in a 3/8" nominal thickness. Plaster when applied over lath will vary from an absolute minimum of about 1/4" to a maximum of about 5/8" with 3/8' to 1/2" being the most common thicknesses. Where there was a bowed stud or other unusual circumstances I have encountered exceptions where there might be as much as an inch of plaster over the lath.
The third style you are likely to encounter though it may not yet be failing is the plaster board style of plaster construction. Plaster Board completely changed the plastering industry starting after the end of World War II. By the early 1950's lath was rarely used and by the mid 1950's almost never.
Initially it was sold in widths of 16-24 inches and lengths of 32 and 64inches. Both a perforated and a solid type were sold with a 3/8" thickness. Apparently there were two different methods of using it; a single layer topped with 3/8 - 1/2" of plaster and two layers of board with a thin top coat of plaster over that. I have encountered both styles.
 Basic Home Carpentry
By Carl W. Bertsch, published by Wm. H. Wise & Co., INC New York, 1952
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