Gypsum Wall Board (Drywall) Basics

Gypsum wallboard, commonly known as drywall in the United States, is used extensively in building projects throughout the world. The sections below provide information regarding the design, creation and installation of drywall.

Gypsum Board Composition

Drywall is created by using gypsum, a mineral with a white sandy appearance, blending it with several different types of additives (such as paper pulp and starch) and adding water to create a plaster. This plaster is spread between two layers of paper to a desired thickness, and then baked in an oven to dry out and harden the material. This process produces standard drywall.

In addition to regular drywall there are several different types of specialized products suited to particular applications.

Moisture Resistant Drywall

Moisture resistant drywall, also known as MR board or green board, is commonly used in applications where the drywall is susceptible to contact with water, such as bathrooms and basements. In this product, there is typically no change to the base gypsum plaster mix. In order to provide the moisture resistance, a water resistant green paper is used to prevent moisture from reaching the gypsum.

Some moisture resistant drywall resists mold growth because of another additive to the paper that prevents mold from using the paper as food. For better mold prevention, a fiberglass faced panel is preferred because there is no paper for the mold to live on.

Exterior Gypsum Sheathing

Exterior gypsum sheathing is installed on the outside walls of a structure, directly below siding materials such as wood, vinyl siding, masonry, etc. Designed to withstand potential exposure to moisture, this special type of sheathing contains a core that incorporates wax. It has water repellent paper on the front and back surfaces, as well as the edges of the gypsum board. Other types of exterior gypsum sheathing are made with a thin fiberglass mat facing, which resists water and board deterioration better than treated paper faces. Exterior gypsum sheathing is available with a fire-rated Type X core which is described in the Fire-rated Products section below.

Abuse and Impact Resistant Drywall

Abuse- and impact-resistant drywall products are excellent choices for locations with a high traffic load such as stairways, hallways, lobbies, hospitals, schools, etc. where the potential exists for objects to scrape or poke wall surfaces. Abuse-resistant drywall is designed to resist abrasion resulting from the scraping of objects across the wall surface whereas impact-resistant drywall is built to handle impact from heavy objects that could potentially puncture the surface of the wall.

Four tests are used to determine the classifications of these type of drywalls: surface abrasion resistance, indentation resistance, soft-body impact resistance and hard-body impact resistance.

To achieve the strength required to meet classification requirements for abuse and impact, manufacturers may use several methods, such as addition of fibers to the plaster mix, densification of the plaster formulation, fiberglass mesh facing materials and specially designed paper applications.

Lead-lined Drywall

Lead-lined drywall is a specialty product that is installed in areas susceptible to radiation, such as laboratories and x-ray rooms in hospitals and clinics. To block x-rays, a layer of lead 1/16” in thickness is sandwiched between the outer face of the gypsum and the paper backing, preventing the rays from penetrating through the wall surface.

Installers of lead-lined drywall should be trained in proper installation and disposal techniques due to the health concerns surrounding lead.

Lightweight Gypsum Board Panels

Low sag ceiling boards are usually fabricated in ½ inch thick sheets and utilize a low weight gypsum mix that results in a final board weight of around 1.3 pounds per square foot. The low weight minimizes the amount of gravity sag due to the weight of the board when used in ceiling applications. It is also relatively easy for most homeowners to self-install.

It should be noted that proper attention must be paid to the installation quality of the structural elements to which the boards are fastened. Slight deviations in plane and level of the framing elements may be visible after the boards are installed.

Fire-rated Gypsum Board Products

The following products are designed to offer increased fire resistance in drywall applications, where desired or where required by code. 

Type X Drywall

Type X or Perlite drywall is used in situations where increased fire resistance is desired, particularly in walls and ceilings. To make Type X drywall, fiber strands are added to the plaster mix during creation of the sheets. Fiber strands assist with maintaining integrity of the drywall sheet during fires, providing additional time for evacuation of the building before structural collapse.

Type C Drywall

Type C drywall builds on the fire resistance provided by Type X. Utilizing an enhanced Type X fiber strand core, Type C also includes an additive that compensates for shrinkage during fires in order to provide additional protective stability.

Shaft Liners

Fire-rated shaft liners are 1-inch thick gypsum panels, often fiberglass-faced, which are designed to be placed inside stairwells, elevator shafts and utility raceways to provide fire protection over and above that of the Type X and Type C drywalls. Shaft liner cores are often faced with paper products that provide excellent protection against moisture and mold to enhance the lifespan of these critical products.

The main benefit of shaft liners is that they can be installed from the floor side rather than the shaft side. Since screws or nails do not have to be installed from the shaft side, it saves the contractor from having to install expensive staging inside the shaft. Special studs, called CH Studs, are used with shaft liner panels – for a diagram of a CH stud, see our article about Cold Formed Metal Framing.

Drywall Sizes

Drywall is available in four common thicknesses: 1/4”, 3/8”, 1/2” and 5/8”. As discussed above, shaft liner comes in 1” thick panels. The most commonly used thickness is 1/2”, which forms the walls and ceilings of the majority of residential construction in the US. The 1/4” and 3/8 inch sizes are typically used to cover existing drywall or in areas that require curved walls, and are not used in areas that require strong panels, such as ceilings. 5/8” drywall is normally used in applications where increased fire resistance is desired, or where increased panel strength is a necessity like in high traffic institutional buildings.

The standard sheet size for drywall is 4’ x 8’. However, the 1/2” and 5/8” thicknesses, though remaining at a width of 4’, are also available in lengths of 9, 10, 12 14, and 16 feet. Of course, custom sizes are also available.

Gypsum Board Installation

Drywall is normally installed by fastening it, using drywall screws, directly to wood or metal studs and joists located in the walls or ceilings of a structure. Occasionally, for wood framed buildings, drywall is installed with special drywall nails instead of screws; however, this is becoming less common.

Once the drywall panels are installed, the joints between the individual panels are filled with joint compound and covered with tape to provide a smooth surface. The points of connection (the screw locations) are also filled in with joint compound and smoothed off to provide a flat sheet. Once the joint compound has dried, it is sanded down to ensure that the surface of the walls is flat and ready for painting.

For more information on the various levels of drywall finish, check out our article Gypsum Board Finish Levels.

Article Updated: September 6, 2021

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