What Is a Fire-Rated Wall?

Fire Rating Standards

• Several organizations provide fire ratings. The basic standard is the International Building Code, which sets rules for all types of construction. The Underwriters Laboratory, operated by major insurance companies, also sets standards and rules for fire-rated walls. These govern such things as doors and windows, which also must be fire-rated; wall coverings; insulation, and other construction components. These vary by use, with hospitals and schools usually requiring the highest ratings.

Rating Considerations

• The thickness of a wall and its components figure into the fire rating. Masonry blocks, for instance, are rated based on both thickness and content, with those made of concrete with limestone aggregate rated higher than those using cinder. Concrete walls covered with plaster or stucco also rate higher than those with less fire-resistant materials.

Wood Standards

• Wood is a combustible material used in houses and many other building walls, but it also can be fire-rated. The American Society for Testing and Materials (ASTM) measures "flame spread" and "smoke developed" to determine the fire safety of wood and other materials. Materials with low flame and smoke numbers are rated Class 1, for use in areas of greatest risk, like exits and public areas. Plywood, untreated lumber and similar materials used in most homes are Class III fire-rated

Other Elements

• Fire ratings also cover such things as wallboard, with gypsum wallboard at the highest level; paints; and insulation materials within or on walls and ceilings. The American Wood Council provides tables showing fire ratings for various types of wood wall construction, broken down by one-hour and two-hour fire resistance and resistance to fire from just one side or from both.