Talkin' Plaster: A Glossary of Plaster/Drywall Terminology
There is probably no field in remodeling in which the terminology used is more confused or confusing. To help alleviate some of the confusion, here are the actual names of plaster and drywall components, and materials.
Blueboard or Blue board: A term commonly used in the construction trades to mean plaster board. Another common name is rock lath.
U.S. Gypsum originally colored its plaster board blue to distinguish it from its other gypsum board products. Other manufacturers adopted the color scheme. Also see Plater Board, Rock Lath.
Brown Coat: The second coat of a three coat plaster wall or ceiling system. The brown coat is the leveling coat applied over the first or "scratch" coat of gypsum plaster. It is typically about 3/8" thick.
Any unevenness left in the scratch coat is removed by the brown coat which leaves a flat, level surface for the final or "finish" coat. The brown coat is no longer brown in color, but the name persists from the days when the hydrated lime second coat was darker than the final or "white" coat.
Color Coat: See finish coat.
Drywall: The most common name for gypsum drywall. Also see: Sheetrock.
Finish Coat: The final coat of a thee coat plaster wall or ceiling system. It is applied to the second or "brown" coat and is finished to a smooth surface ready for paint or wallpaper.
The finish coat us usually hydrated lime plaster or a blend of gypsum and lime plaster because lime plaster works to a smoother finish than the stronger, but coarser, gypsum plaster. It is also called the white coat (from the color of lime plaster which is whiter than gray or brown gypsum plaster under it), especially by old time plasterers; and may also be called the color coat because chemical tints were often added to the lime plaster to give the wall its final color.
Finisher A person who tapes and finishes gypsum drywall. Rockers hang the drywall, finishers finish it. Rockers are usually not finishers and vice versa.
Greenboard or Green board: A type of gypsum drywall suited for use in damp (but not wet) areas such as bathrooms. U.S. Gypsum originally colored its water resistant gypsum board green to distinguish it from its other gypsum board products. Other manufacturers adopted the color scheme. The newer purpleboard and paperless gypsum drywall have now replaced greenboard for applications where mold and water resistance is necessary.
Gypsum Board: A panel of gypsum drywall.
Gypsum Drywall: Panels made of a sandwich of gypsum plaster pressed between two sheets of paper, the panels are used to cover interior walls and ceilings, and are the most common materials for that purpose since gypsum drywall replaced true plaster in the 1940s and '50s. Synonyms include: gypsum board, drywall. Also erroneously called plaster board.
Gypsum Plaster: A plaster mixture containing gypsum as the bonding agent. Discovered in the 18th century, gypsum replaced lime as the preferred bonding agent in plaster and cement because it is stronger than lime and cures much more quickly. Commonly used as the first and second coats of a plaster wall, with the smoother lime plaster used as the finish coat.
Horsehair Plaster: An early form of hydrated lime plaster to which horse hair was added for greater strength. Now rarely used except in historic restorations, because fiberglass strands are stronger, and because horses strenuously object.
Lath: Thin wood strips or metal mesh attached to wall studs and ceiling joists to which plaster is applied.
Lather: (Pronounced "LATH er" with a hard "TH" — as in "baTH", not "lath er" with a soft "TH" like in shaving cream.) A person who attaches lath to wall studs and ceiling joists. Once the lather is done, the plasterer applies plaster to the lath. In earlier times, Lathers were essentially apprentice plasterers, and plasterers themselves seldom attached lath — too menial a job for a skilled plasterer. Today both lathing and plastering are probably done by the same person.
Lime Plaster: A plaster mixture containing hydrated lime as the bonding agent rather than the more common gypsum. Until the use of gypsum was developed as a bonding material in the 18th century, this was the only type of plaster available. It is common to use gypsum plaster as the under coats in a plaster wall, because gypsum plaster is stronger than lime plaster; with lime plaster as the final coat because of its smoother finish.
Mudder A slang name for a plasterer or drywall finisher, but only if you are close friends.
Plaster: A composition of hydrated lime or gypsum, sand, water, and sometimes hair or other fiber, applied as a thick paste to walls and ceilings, and allowed to harden through a chemical process. The sand (or sometimes chalk) is the aggregate that gives body to the material, the lime or gypsum is the adhesive that bonds the sand particles together. The water is the carrier that makes the material workable. In both composition and purpose, plaster is similar to cements and stuccos. For plastering a wall, gypsum mixes are commonly used for the first two (scratch and brown) coats of plaster and a lime mix for the final or skimcoat because it finishes to a smooth, hard surface. In earlier times horsehair was used to strengthen the plaster and make it less subject to cracking. Today fiberglass strands may be used for the same purpose. Although we routinely refer to lime plaster and gypsum plaster as distinguishable products, most commercial plaster mixes contain both lime and gypsum. Lime plaster simply has more lime, and gypsum plaster more gypsum.
Plasterboard or Plaster Board; A gypsum board product used as an underlayment for wet plaster. Also known as plaster lath and rock lath. Originally the substrate under wet plaster was wood lath — thin strips of wood attached to studs over which three coats of plaster were applied. In the 1930s this was supplemented with expanded metal lath which was faster to install, stronger and bonded better to the plaster. In the 1940s plaster board began to replace both wood and metal lath. It was immediately popular because it substituted for the first, or "scratch" of plaster. It was not only faster and cheaper, but made a better wall. Plaster board was short-lived, however. It quickly morphed into gypsum drywall that needed no plaster coating at all. The surface of the board was the wall surface.
The term plaster board is commonly used, even by people who should know better, to refer to gypsum drywall, but this is an incorrect use of the term that causes a lot of confusion.
Plaster Lath: See plaster board.
Plasterer A person who applies plaster to a wall or ceiling. The plasterer follows the lather who attaches the lath to the wall studs. May also refer to a person who applies stucco coatings to exterior walls. Today's plasterers are often called on to apply stucco, unlike prior times when stucco-men and plasterers were two different trades.
Purpleboard or Purple Board: A type of gypsum drywall resistant to water and mold for use in damp (but not wet) areas. It has largely replaced green board for use in bathrooms and other damp areas.
Rock Lath: See plaster board.
Rocker: Slang term for a person who hangs gypsum drywall. From the brand name of the original gypsum board, "Sheetrock" by U.S. Gypsum. Hanging gypsum drywall is similarly referred to as rocking.
Scratch Coat: The first coat of a three coat plaster wall or ceiling system. The scratch coat is the structural coat of the wall. It is forced into the gaps in the wall lath so that when it has cured it forms a strong mechanical bond with the wood structure of the wall. It is usually applied about 3/8" of an inch thick, and scored with a scoring tool before it has cured. The scores or scratches help the second or "brown" coat grip the base coat. After 1950 plasterboard or plaster lath replaced the scratch coat in most residential walls and ceilings.
Sheetrock®: A trade name for gypsum board manufactured by United States Gypsum (USG). It has migrated into the vernacular and is used commonly to mean gypsum drywall. Sheetrock was the first widely accepted gypsum drywall product in the U.S., and "sheetrock" has been used to refer to gypsum board for so long that gypsum board installers are commonly referred to in the construction trades as "rockers".
True Plaster: A term used to describe plaster for the purpose of distinguishing it from gypsum drywall.
Wallboard or Wall Board: A widely used slang term for gypsum drywall, but also used to mean plaster board. Unless you know the context, its use is usually ambiguous, and it should be avoided.
Wet Plaster: A term used to describe plaster for the purpose of distinguishing it from gypsum drywall. See plaster.
White Coat: See finish coat.
Gypsum Drywall That Looks Like Plaster
You may not be aware of it, but drywall finishing is described in levels. The Gypsum Association identified levels of finishing in its 1990 standard, GA-214-90: Recommended Specifications: Levels of Gypsum Board Finish. The standard was created to "precisely describe" the desired finish of walls and ceilings. ASTM (formerly known as the American Society for Testing and Materials, and now as ASTM International) has also published a similar standard as ASTM C 840-04.
There are six official levels of drywall finishing, to which we have added a seventh in our standards and specifications. The seventh, Level 6, produces a gypsum drywall finish that resembles a lime plaster wall in both hardness and sheen.
Level 0: Unfinished. No finish of any kind, no taping. Used only for temporary walls.
Level 1: Taped. Paper tape is embedded in joint compound and smoothed with a taping knife or self-adhesive mesh tape is applied without any joint compound. The tape remains exposed and the heads of fasteners are not coated. This finish is used where appearance is unimportant. The surface is left free of excess joint compound but ridges and tool marks are acceptable. This level is often specified in the plenum area above drop ceiling ceilings, in attics, and service corridors, but in residences is rarely used. A level 2 finish is used instead.
Level 2: Embedded Tape. This level of finish is used where appearance is unimportant, but fire protection is required, as in garages; and as backer for tile and wood paneling. Colloquially this is known in the trades as "fire taping" or "tile taping". Joints and interior angles are covered with tape embedded in joint compound and wiped with a joint knife leaving a thin coating of joint compound over all joints and interior angles. Joint compound is applied over all fastener heads and beads. The surface is left free of excess joint compound but ridges and tool marks are acceptable. The usual tool is a 5" or 6" taping knife. A professional is likely to use a taping machine or "bazooka" and a wipedown knife which is essentially a taping knife with a long handle.
Level 3: 1st Finish Coat. Where the wall or ceiling covering to be applied is a bold texture or heavy wallpaper, this level of finish is usually sufficient. This is typically the finish applied to ceilings that are to be textured. It starts with a Level 2 finish. Any ridges or tool marks are knocked down, then one additional coat of joint compound is applied to all joints and fastener heads. The finished compound should be smooth and free from tool marks and ridges. If it is to be the final coat, it is sometimes sanded, but does not have to be and many finishers don't sand because of the risk of exposing the tape. The usual tool is an 8" or 10" drywall knife.
Level 4: 2nd Finish Coat. This is the normal wall finish, but it is also used for ceilings that are to be only lightly textured. An additional coat of joint compound is applied over a Level 3 finish, including all fastener heads, then the entire wall or ceiling is sanded smooth. This is usually an satisfactory finish if the wall is to be painted with flat paint or papered with light-weight wallpaper. If there is strong grazing light, minor dips and bumps in the finish will show, and it is not suitable if semi-gloss or gloss paint is to be used as these imperfections will become visible. The usual tool is an 12" or 14" drywall knife.
Level 5: Skim Coat. If a surface is subjected to grazing light, or semi-glass or glass paint is to be used, then this is the level of drywall finish desired. It starts with a Level 4 finish, then a coat of thinned joint compound is applied over the entire surface. It is commonly applied with a roller, then towelled off. This fills in any dips and leaves a smooth, uniform surface that minimizes the possibility of joints or fasteners showing through. If a ceiling is to be smooth rather than textured, this is the finish level desired because the grazing light of ceiling fixtures can reveal every dip and bump left by a Level 4 finish. The usual tool is an 12" or 14" drywall knife, but some professionals prefer an even larger knife, up to 18". Keep in mind that the larger the knife, the more difficult it is to handle and the longer it takes to become proficient.
Level 6: Burnishing. This is our added level of finish. It starts with a Level 5 finish, then the entire wall is burnished with a drywall knife to compress the finish and give it a plaster-like hardness and sheen. The technique is described in the main article. The usual tool is a 5" or 6" taping knife.
Burnishing as Decorative Wall Treatment
Rather than paint or wallpaper your new wall, you can use burnished plaster as the finish coat by adding tint to the skim coats.
From before the turn of the 20th century, up to the 2nd World War, architects often applied wall color to the final coats of plaster rather than paint the wall. There were several reasons. First, it often took plaster as much as six months to cure enough so paint could be applied. It also helped disguise chips and minor damage to the wall because the white plaster would not show through the finish color.
Today this kind of finish is often called Venetian Plaster, and plasterers and painters charge a lot of money to do it. But
Plaster pigment is toxic. Do not ingest or inhale the powder. Wear a face mask and latex gloves when handling pigment powders.
it actually nothing more than skim coating and burnishing with tinted rather than plain plaster. You can get plaster tints at your local drywall store. Add the tint to the drywall joint compound until you get the color you want. Remember, wet tint looks darker than dry tint, so test it on the wall in an out of the way area, and let it dry to see its final wall color.
For added gloss, and moisture protection, a final coat of wax can be applied to the burnished wall. We use a good quality carnauba wax, but waxes especially formulated for plaster are available. Wait at least 24 hours before applying the wax. Not waiting may cause streaking on the final surface due to residual water in the joint compound. Waxing is recommended in areas exposed to continuous moisture such as bathrooms and kitchens.