One of the most popular posts I’ve written for Preservation in Mississippi has to be a post about rock lath. But what is rock lath? According to the Dictionary of Architecture and Construction 4th Edition by Cyril M. Harris rock lath, also known as gypsum lath, is defined as follows;
Gypsum lath, board lath, gypsum plaster-board, rock lath: (ˈlat͟h )(rhymes with “math”) A base for plaster; a sheet having a gypsum core, faced with paper, which provides a good bond for plaster; usually manufactured in 16-in. by 48-in. or 24-in. by 96-in. panels, 3/8 or 1/2 in. thick with round or square edges.
These are some samples of rock lath:
If you came here on a search engine query for Rock Lathe, here is the definition of Lathe according to the Dictionary of Architecture and Construction 4th Edition by Cyril M. Harris.
Lathe: (‘lāt͟h)(rhymes with “bathe”) A machine for shaping circular pieces of wood, metal, etc.[ie stone], by rotating the material about a horizontal axes while a stationary tool cuts away the excess material.
Rock Lath and Rock Lathe are two very different things that have no association with one another. Below is a Rock Lathe:
Have you ever seen a material attached to a wall that looked like drywall but maybe had some holes in it and possibly some plaster stuck to one side? It’s possible you never have. The material is called Rock Lath and much like our past MissPres Architectural Word of the Week “Grounds,” if it does it job correctly and without interference, it will never be seen.
While similar to modern Drywall (Sheetrock) as a gypsum material, rock lath was treated chemically so it would be capable of accepting a wet veneer of plaster. (This might also speak to why even though submerged during Katrina for several hours the rock lath did not mold the same way Drywall does.) The most obvious difference between today’s Drywall and Rock Lath is that in Rock Lath a 5/8″ hole is cut to allow the brown coat of plaster to spill through and create a key. These keys harden to hold the plaster veneer to the Rock Lath.
The Rock Lath itself comes in sheets 16″ x 48″ and 1/2′ thick. These sheets were applied to the wall in the same fashion as Drywall. When all the Rock Lath is installed on the wall a brown coat (or undercoat) of plaster approximately 1/2′ thick is applied to the Rock Lath. The finish coat of plaster is applied after the brown coat has dried. Both coats must be of gypsum plaster base, and should not be made from lime, Portland cement, or any other base. The time saver with this process is that a third coat, the scratch coat–normally required in a wood or metal lath and plaster wall–and its drying time are eliminated from the process. Unfortunately, good mechanical keys were not often formed in Rock Lath and the chemical bond between the board and plaster must keep the wall section together. Thankfully the strength of the bond of gypsum plaster to Rock Lath is great, requiring a pull of 864 lbs. per-square-foot to separate gypsum plaster from the Rock Lath.
In the 1942 portion of the Old Pascagoula High School building, to save time on construction and possibly due to rationing, this new material was used for the walls rather than the plaster that was used in the 1938-1939 section of the building. Rock Lath, at this point almost exclusive used for residential construction, was coming into use in non-residential construction, and by the 1950’s would be used almost as frequently as expanded metal lath.
Another building in Jackson County that utilizes Rock-Lath is the Ocean Springs Community Center. This building might also contain the world’s most valuable rock lath wall. On the interior of this c. 1950 concrete block and asbestos-clad community center is a mural painted by Ocean Spring’s best known artist, Walter “Bob” Anderson. In perhaps the best investment offer ever, Mr. Anderson offered to paint a mural on the walls of the community center for $1. Today the mural is valued in the double-digit millions of dollars. While most of the mural is painted on plaster walls over concrete-block, one small section over an alcove is painted on Rock Lath. The wood framework over the alcove has settled differently than the rest of the block building, causing some cracking in the mural. The city has just finished a Save America’s Treasures grant from the National Park Service to stabilize the mural, and the space looks great.
I hope you’ve enjoyed our little lesson on rock lath. It just goes to show, you never know what’s lurking in our walls.
This post is a throwback to April, 2012. You can read the post as it originally appeared here. There have been several other MissPres posts about historic sheetrock and other lath types found in historic buildings that you might find interesting…