Lath and plaster

Lath seen from the back with brown coat oozing through

Lath and plaster is a building process used mainly for interior walls in Canada and the United States until the late 1950s. After the 1950s, drywall began to replace the lath and plaster process in the United States. In the United Kingdom, lath and plaster was used for some interior partition walls, but was mostly used in the construction of ceilings. In the UK, many lath and plaster ceilings are replaced with plasterboard, which became a more common ceiling construction from 1945 onwards.

The process begins with wood laths. These are narrow strips of wood nailed horizontally across the wall studs. Each wall frame is covered in lath, tacked at the studs. The lath is typically about two inches wide by four feet long by 1/4 inch thick. Each horizontal course of lath is spaced about 3/8 inch away from its neighboring courses. In the UK, riven or split hardwood laths were used of random lengths and sizes. Splitting the timber, as opposed to sawing in straight lines, followed the grain of the timber which greatly improved strength and durability.

Next, temporary lath guides are placed vertically to the wall, usually vertically at the studs. Plaster is then applied, typically using a wooden board as the application tool. The applier drags the board upward over the wall, forcing the plaster into the gaps between the lath and leaving a layer on the front the depth of the temporary guides, typically about 1/4 inch. A helper feeds new plaster onto the board, as the plaster is applied in quantity. When the wall is fully covered, the vertical lath "guides" are removed, and their "slots" are filled in, leaving a fairly uniform undercoat.

It is standard to apply a second layer in the same fashion, leaving about a half inch of rough, sandy plaster (called a brown coat). A smooth, white finish coat goes on last. After the plaster is completely dry, the walls are ready to be painted. In the photo, "lath seen from the back..." those curls of plaster are called "keys" and are important to keep the plaster on the lath. Insufficient "keying" and the plaster will fall off the lath. Traditional lime based mortar/plaster often incorporated horsehair which reinforced the plasterwork, helping to prevent the keys breaking away.

Eventually the wood laths were replaced with rock lath (colloquially known as "button board"), which is a type of gypsum wall board with holes spaced regularly across it, available in sheets sized 2 by 4 feet. The purpose of the four-foot length is so that the sheet of lath reaches exactly across three wall studs, which are spaced 16 inches apart on center (United States building code standard measurements). The holes serve the same purpose as the spaces between the wood lath strips, allowing plaster to ooze through the board when the plaster is applied, making the keys to hold the plaster to the wall board.

In addition to rock lath, there were various types of diamond mesh metal lath which is categorized according to weight, type of ribbing, and whether the lath is galvanized or not. Metal lathing was spaced across a 13.5 inch center, attached by tie wires using lathers' nippers. Sometimes, the mesh was dimpled to be self-furring.

Lath and plaster has been mostly replaced with solid drywall or plasterboard (also a type of gypsum wall board, although a bit thicker), since it is faster and less expensive to install. As plastering is a skilled trade, and hence expensive, reducing the amount of plastering required in a new development also reduces the building costs.

An advantage of using lath is for ornamental or unusual shapes. For instance, building a rounded wall would be difficult if drywall were used exclusively, as drywall is not flexible enough to allow tight radii.

Hair reinforcement

Hair is used in plaster as a binding medium, and gives tenacity to the material. Traditionally horsehair was the most commonly-used binder, as it was easily available before the development of the motor-car. Hair functions in much the same way as the strands in fiberglass resin, by controlling and containing any small cracks within the mortar while it dries or when it is subject to flexing.

Ox-hair, which is sold in three qualities, is now the kind usually specified; but horsehair, which is shorter, is sometimes substituted or mixed with the ox-hair in the lower qualities. Good hair should be long (In the UK cow and horse hair of short and long lengths is used), and left greasey (lanolin grease) because this protects against some degradation when introduced in to the very high alkaline plaster[1]. Before use it must be well beaten, or teased, to separate the lumps. In America, goats' hair is frequently used, though it is not so strong as ox-hair. The quantity used in good work is one pound of hair to two or three cubic feet of coarse stuff (in the UK up to 7kg per metric cube). Hair reinforcement in lime plaster is common and many types of hair and other organic fibres can be found in historic plasters [4]. However, organic material in lime will degrade in damp environments particularly on damp external renders.[5] This problem has given rise to the use of polyprolene fibres in new lime renders [6]

Manila hemp fiber has been used as a substitute for hair. Plaster for hair slabs made with manila hemp fiber broke at 195 lb (88 kg), plaster mixed with sisal hemp at 150 lb (68 kg), jute at 145 lb (66 kg), and goats' hair at 144 lb (65 kg).[citation needed] Another test was made in the following manner. Two barrels of mortar were made up of equal proportions of lime and sand, one containing the usual quantity of goats' hair, and the other Manila fiber. After remaining in a dry cellar for nine months the barrels were opened. It was found that the hair had been almost entirely eaten away by the action of the lime, and the mortar consequently broke up and crumbled quite easily. The mortar containing the Manila hemp, on the other hand, showed great cohesion, and required some effort to pull it apart, the hemp fiber being undamaged.[citation needed

See also

• Stucco
• Laths

External links

• Preservation Brief No. 21 by the National Park Service. Contains detailed descriptions of Lath & Plaster construction and advisories on repairing and restoring these walls (especially in historic buildings).
• Repairing Historic Flat Plaster Walls and Ceilings (Introduction)
• Plaster Base Installation, Building Construction & Finishing