Property · Insurance Term
Construction Type (ISO Construction Class)
Definition
Construction type is how insurers classify a building by what its walls, floors and roof are made of and how well they resist fire. Common classes run from frame (wood) to joisted masonry, non-combustible, masonry non-combustible and fire resistive. It is one of the biggest drivers of a property rate.
In Plain English
Construction is the "C" in COPE, the four basic factors underwriters review for every building. ISO's commercial property rating uses six classes: frame, joisted masonry, non-combustible, masonry non-combustible, modified fire resistive and fire resistive. In simple terms, the less fuel a building's structure gives a fire, the better its class.
A strip center with block walls and a wood roof is often joisted masonry. A tilt-wall warehouse with a steel deck roof may be masonry non-combustible. A concrete mid-rise office may qualify as fire resistive, while many older motels and garden apartments are frame.
The class affects your rate, the deductibles offered and sometimes whether an insurer will quote at all. It works together with sprinklers and your area's public protection classification. Getting it right on the application matters, because a misclassified building can lead to pricing surprises or questions after a loss.
Not sure how your policy handles this? Have us review it, or get a quote.
Example
Hypothetical and simplified: Two 20,000-square-foot retail buildings sit side by side with the same value. One has wood roof joists on masonry walls (joisted masonry). The other has a steel deck roof on masonry walls (masonry non-combustible). Insurers will usually rate the second building more favorably for fire.
What to Watch For
- Compare the construction class on your application and quote with what the building actually has, especially the roof structure.
- If you renovated or added onto a building, ask whether the class changed for any part of it.
- Buildings with mixed construction are often classed by the weaker type, so ask how yours was rated.
- Keep photos and drawings that show roof and wall materials to support the classification.
Last updated October 14, 2026 · Primesure editorial team · How We Write