Editing Lath and plaster (section)

==Description==
[[File:Lathwork and plaster.jpg|thumb|Partially-exposed wallpapered lath and plaster illustrating the technique.  Example from the [[Winchester Mystery House]], constructed between 1884 and 1922]]
{{unreferenced section|date=March 2012}}
The wall or ceiling finishing process begins with wood or metal [[lath]]s. These are narrow strips of [[wood]], extruded metal, or split boards, nailed horizontally across the [[wall stud]]s or ceiling [[joist]]s. Each wall frame is covered in lath, tacked at the studs.  Wood lath is typically about {{convert|1|in|cm|spell=in}} wide by {{convert|4|ft|m|spell=in}} long by {{convert|1/4|in|mm|0}} thick. Each horizontal course of lath is spaced about {{convert|3/8|in}} away from its neighboring courses. Metal lath is available in {{convert|27|in|cm|adj=on}} by {{convert|8|ft|cm|adj=on}} sheets.

In [[Canada]] and the [[United States]] the laths were generally sawn, but in the [[United Kingdom]] and its colonies, [[Wood splitting|riven or split]] hardwood laths of random lengths and sizes were often used. Early American examples featured split beam construction, as did examples put up in rural areas of the U.S. and Canada well into the second half of the 19th century. Splitting the timber along its grain greatly improved the laths' strength and durability. As Americans and Canadians expanded west, saw mills were not always available to create neatly planed boards and the first crop of buildings in any new western or northern settlement would be put up with split beam lath. In some areas of the U.K. reed mat was also used as a lath.

Temporary lath guides are then placed vertically to the wall, usually at the studs. [[Lime (material)|Lime]] or [[gypsum]] [[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 {{convert|1/4|in}}. 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.

In three coat plastering it is standard to apply a second layer in the same fashion, leaving about {{convert|1/2|in}} of rough, sandy plaster (called a ''brown coat'' or ''browning'' (UK)). A smooth, white finish coat goes on last.  After the plaster is completely dry, the walls are ready to be painted.  In this article's photo ("lath seen from the back...") the curls of plaster are called ''keys'' and are necessary to keep the plaster on the lath. Traditional [[Lime plaster|lime based mortar/plaster]] often incorporates [[horsehair]] which reinforces the plasterwork, thereby helping to prevent the keys from breaking away.