SFRM materials, types, and fire-resistance principles
What SFRM does
SFRM is a spray-applied cementitious, gypsum, or mineral-fiber coating that thermally insulates structural steel. In a fire, bare steel heats quickly and loses strength near 1,000–1,200°F; the SFRM slows heat transfer so the member stays below its critical temperature for the rated time. The rating (1, 2, 3, or 4 hours) comes from an ASTM E119/UL 263 assembly test, not from a field calculation.
Material families and density
Products are grouped as low-, medium-, and high-density. Low-density gypsum-based materials are common on interior beams and columns; cementitious and higher-density products are used where tougher, more moisture-resistant, or exterior-rated performance is needed. Density is a dry property measured per ASTM E605; the manufacturer's listed density range is part of the design.
Section factor drives thickness
The required thickness depends on the steel's heat-absorbing mass, expressed as W/D (weight per foot divided by heated perimeter in inches) for beams or A/P for other shapes. A stocky, heavy member has a higher W/D, heats more slowly, and usually needs less thickness than a light, thin member of the same rating. Contour and box applications also change the heated perimeter.
Worked example
A UL design lists: W/D up to 1.0 → 1½ in.; 1.0–1.5 → 1¼ in.; over 1.5 → 1 in. A W16×40 beam has W/D = 1.2, so the required thickness is 1¼ in., not the 1½ in. used on the adjacent W12×35. The inspector must check each member group, not just 'typical' beams.
Traps
Assuming one thickness fits the whole floor; confusing intumescent coatings with SFRM; using wet density; ignoring that a listed assembly is product-specific.