Magnesium Oxide board "MgO board" is a factory-made, non-insulating sheathing board product. It can be used for a number of applications including wall and ceiling linings, fascias, soffits, tile backing and underlayments.

It is made of magnesium oxide, a type of mineral cement, and is commonly called simply "MgO" (pronounced emm-gee-oh) due to its chemical composition of magnesium (chemical symbol Mg) and oxygen (chemical symbol O).

Thinks of MgO as a type of sheating board-sort of like drywall or cement board-but with much-improved characteristics such as fire resistance, weatherability, strenght, resistance to mold and mildew, and so on.

MgO is available in many forms, and for building construction comes in various thickness and sheet sizes. It is also comes in various grades, such as smooth finishes, rough textures, and utility grades.

It is white, beige or light gray in color, and has a "hard" sound when rapped with your knuckle