Joisted masonry construction is one of the most common building categories you’ll run across in commercial property. It sits in the ISO Construction Class 2 slot, right above “Frame” and just below “Non-Combustible.” If you’ve ever looked at an office park with block walls and a wood roof deck, you’ve seen joisted masonry in practice.
Definition of Joisted Masonry
Joisted masonry refers to buildings with:
- Exterior walls made of masonry materials such as adobe, brick, concrete, gypsum block, hollow concrete block, stone, tile, or similar materials.
- Floors and roofs built with combustible materials, typically wood joists or decking.
That combination—solid masonry outside, combustible roof/floor inside—is what sets this construction type apart.
Joists: What They Mean in Construction
Since the term joisted masonry revolves around joists, let’s pause here.
- Joist definition: A joist is a horizontal structural member that runs between walls or beams to support a floor or roof.
- What are joists? Think of them as the ribs of a floor or roof—long beams (wood, steel, or concrete) that hold up decking and distribute loads.
- In JM buildings: Joists are usually wood or lightweight steel, supporting combustible roof or floor sheathing.
So when we talk about joisted masonry, we’re literally describing masonry walls with floors and roofs framed by joists of combustible construction.
Joisted Masonry vs Other Construction Types
Frame (Type 1 / ISO Class 1)
- Frame buildings have combustible exterior walls, often wood framing with siding.
- Joisted masonry has masonry exterior walls, but floors/roof still burn.
Non-Combustible (ISO Class 3)
- Exterior walls, floors, and roofs are entirely non-combustible (steel, concrete, or metal decks).
- JM fails this test because of its combustible roof/floor assemblies.
Masonry Non-Combustible (ISO Class 4)
- Same exterior masonry walls as JM.
- Difference is the roof and floors are non-combustible.
- Many commercial retrofits focus on moving from Class 2 (JM) to Class 4 (MNC).
Fire Resistance and Risk Profile
- Walls: Masonry walls provide at least a one-hour fire-resistance rating.
- Floors/Roofs: Combustible joists and decking reduce the overall fire resistance.
- Performance: Fires often spread quickly through concealed joist spaces, leading to roof collapse.
This is why JM construction is seen as moderate fire resistance—stronger than frame, weaker than non-combustible.
Common Uses of Joisted Masonry Buildings
Joisted masonry shows up frequently in commercial building projects where budget and speed matter:
- Shopping centers and strip malls.
- Small office buildings.
- Warehouses and light industrial spaces.
- Older schools and habitational dwellings.
In practice, many JM buildings top out at four stories due to structural limits.
Field Identification Checklist
When you’re trying to classify a building as JM:
- Look at the exterior walls — are they masonry? (CMU, brick, stone, concrete block).
- Peek above ceilings in stairwells or mechanical rooms — wood planks or plywood deck means combustible.
- Check the roof deck — if it’s wood or combustible sheathing on a metal deck, it’s still JM.
- Remember the weakest link rule — if any major floor/roof assembly is combustible, the building is JM.
Insurance Implications
ISO puts joisted masonry in Class 2. Here’s why that matters:
- Compared to Frame (Class 1): Premiums are usually lower, thanks to masonry walls.
- Compared to Non-Combustible or Masonry Non-Combustible: Premiums are often higher, since combustible floors/roofs carry more risk.
- Loss drivers: concealed spaces, roof fires, aging combustible materials.
Your building’s classification directly influences insurance rates, underwriting terms, and required coverage.
Upgrading Joisted Masonry Buildings
Property owners often ask if they can move from Class 2 to Class 4. The answer is yes—if you replace the combustible floors and roofs.
- Install non-combustible roof decks (metal or concrete).
- Replace wood floors with rated non-combustible assemblies.
- Add draftstopping or sprinklers in concealed spaces.
- Document the changes with UL assembly listings and stamped plans.
Once combustible elements are gone, the building may qualify as Masonry Non-Combustible.
Clearing the Confusion: ISO Class 2 vs NCCI 5022
Some articles confuse ISO Class 2 Joisted Masonry with NCCI Code 5022, which is a workers’ compensation class for masonry contractors. They are completely different:
- ISO Class 2: Defines the construction type of a building for property insurance.
- NCCI 5022: Defines the job classification of workers for workers’ comp insurance.
Mixing the two can cause costly mistakes.
Case Examples
- Joisted Masonry Example: CMU exterior walls with a wood truss roof deck.
- Borderline JM Example: Tilt-up concrete with steel joists, metal deck, and combustible coverboard (still JM).
- Masonry Non-Combustible Example: Brick exterior walls with a steel frame and metal deck roof (non-combustible).
FAQ About What Is Joisted Masonry
What does joisted masonry mean?
It means a building with masonry exterior walls and combustible joist-based floors and roofs.
Is joisted masonry non-combustible?
No. The walls are masonry, but the floors/roof are combustible, which defines it as Class 2.
What is masonry non-combustible construction?
It’s similar to JM but has non-combustible floors and roofs, often using steel or concrete decking.
What are joists in construction?
Joists are horizontal structural members that support floors and roofs—think wood planks or steel beams spanning between walls.
Can a JM building be reclassified?
Yes, by upgrading to non-combustible floor/roof assemblies and documenting it properly.
Quick Comparison Table
| Construction Type | Exterior Walls | Floors/Roofs | Fire Resistance | Typical Use |
| Frame (Class 1) | Combustible | Combustible | Low | Houses, small shops |
| Joisted Masonry (Class 2) | Masonry | Combustible | Moderate (~1 hr) | Retail, offices, schools |
| Non-Combustible (Class 3) | Non-combustible | Non-combustible | Higher | Warehouses, industrial |
| Masonry Non-Combustible (Class 4) | Masonry | Non-combustible | Higher | Schools, malls, large offices |
Visual Aid: ISO Class Decision Flow
Key Takeaways
- Joisted masonry = masonry walls + combustible floors/roof.
- Common in commercial buildings like retail, schools, and warehouses.
- Classified as ISO Construction Class 2, with moderate fire resistance.
- Insurance rates hinge on the combustible assemblies.
- Retrofitting to non-combustible floors/roof can move the building to Class 4.
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