Joisted masonry (ISO Class 2) has masonry exterior walls with combustible floors and a combustible roof, usually wood.
Masonry non-combustible (ISO Class 4) also has masonry exterior walls, but its floors and roof are metal or other noncombustible materials.
That single difference in floor and roof materials changes how a building burns, how insurers rate it, and how much builders risk coverage a project can get.
This guide compares joisted masonry vs masonry non-combustible on materials, structure, and fire resistance, then explains the effect on commercial property premiums and builders risk policies.
To classify your own building in four clicks, use the interactive class finder further down the page.
Joisted Masonry vs Masonry Non-Combustible at a Glance
| Feature | Joisted Masonry (Class 2) | Masonry Non-Combustible (Class 4) |
| Exterior walls | Masonry, or fire-resistive walls rated at least one hour | Masonry at least 4 inches thick, or fire-resistive walls rated at least one hour |
| Floors and roof | Combustible, most commonly wood | Metal or other noncombustible materials |
| Typical framing | Wood joists, rafters or trusses | Steel members with a metal or concrete deck |
| Main fire weakness | Wood floors and roof burn; concealed spaces hide fire spread | Unprotected steel loses strength in high heat |
| Insurance standing | Generally rated as the higher fire risk | Generally rated more favorably |
What Joisted Masonry Means
Joisted masonry is a building with load-bearing exterior walls of masonry and floors and a roof made of combustible material.
Verisk’s construction brief for Class 2 lists the accepted wall materials as brick, reinforced or nonreinforced concrete, hollow concrete masonry units, tile, and stone.
Noncombustible exterior walls with a fire-resistance rating of at least one hour also qualify. Our joisted masonry basics guide covers the class in more detail.
Industry training guides also call this class ordinary construction or brick joisted.
Because the walls carry the load, tall joisted masonry buildings are uncommon, and an ICAT carrier brochure notes that it is rare to find one above three stories.
Heavy timber (mill) construction sits inside this class. Verisk says it stays joisted masonry because the walls are masonry, even though the wood members are much larger than in ordinary framing.
Verisk credits heavy timber with being harder to ignite, burning more slowly, and having no concealed spaces.
What Masonry Non-Combustible Means
Masonry non-combustible construction has exterior walls of masonry material with floors and roof of metal or other noncombustible materials.
That is the Insurance Services Office (ISO) definition from its Commercial Lines Manual.
Verisk’s brief on Class 4 adds a thickness test: masonry walls at least four inches thick, or fire-resistive walls rated at least one hour. It also allows slow-burning floors and roofs alongside noncombustible ones.
A typical Class 4 building pairs concrete block, reinforced masonry or tilt-up concrete walls with a steel frame and a metal roof deck.
One neighboring class causes frequent mix-ups. Non-Combustible (Class 3) has walls, floors and roof all made of metal, gypsum or similar materials. Masonry non-combustible (Class 4) swaps the walls for masonry.
Fire Resistance Compared
Neither class is fireproof. Each one fails in a different way.
- Joisted masonry often keeps its masonry walls standing, but the floors and roof are combustible and can be damaged or destroyed. Verisk also points to concealed spaces, such as the voids above ceilings and between joists, as a weakness because fire can travel through them unseen.
- Masonry non-combustible removes the wood from floors and roof, so the structure does not feed the fire. Verisk credits the class with stable exterior bearing walls that are less likely to collapse. It also names two drawbacks. Unprotected steel in the floors and roof loses strength and becomes less stable at high temperatures, and slow-burning materials can still add fuel.
Fire Ratings Belong to Higher Classes
Class 4 carries no required hourly fire rating for its floors and roof.
Hourly ratings enter the ISO scale at Class 5, Modified Fire Resistive (one hour or more but less than two), and Class 6, Fire Resistive (not less than two hours), under ISO’s class definitions.
Some web pages attach a one-to-two-hour rating to masonry non-combustible buildings.
That range belongs to Modified Fire Resistive construction, so a Class 4 label alone does not say how many hours a structure will hold.
Real Buildings and Their ISO Classes
The joisted masonry vs masonry non-combustible definitions are easier to remember with a building in mind. These examples apply the ISO wording to common structures. Carriers make the final call.
| Building | Walls | Floors and Roof | Likely Class |
| Strip mall with block walls and wood trusses | Concrete block | Wood trusses and deck | Joisted Masonry (2) |
| Old brick warehouse with heavy timber floors | Brick | Heavy timber | Joisted Masonry (2) |
| Distribution center with tilt-up panels | Tilt-up concrete | Steel joists, metal deck | Masonry Non-Combustible (4) |
| Brick-faced apartment building | Brick veneer on wood studs | Wood | Frame (1) |
| Pre-engineered metal building | Metal panels | Metal deck | Non-Combustible (3) |
| Concrete-frame office with rated floors | Reinforced concrete | Concrete, rated two hours | Fire Resistive (6) |
The apartment building row trips up many applicants. A brick face over wood framing is frame construction, the lowest class. ICAT states that brick veneer does not equal joisted masonry.
How to Classify a Building in Three Checks
The weakest element sets the class. Work through these checks in order:
- Check the walls: Load-bearing masonry qualifies. Brick veneer over a wood frame does not, and one carrier’s builders risk application treating any structure with brick or masonry over wood frame as frame construction. A single layer of brick over metal studs is a noncombustible wall with a brick facade, according to one ISO-based cheat sheet. For a closer look at the lowest class, see our frame construction comparison.
- Check the floors and roof: Wood or other combustible materials point to joisted masonry. Metal, concrete or other noncombustible materials point to masonry non-combustible.
- Check mixed assemblies: The same cheat sheet treats a floor or roof assembly that combines noncombustible and combustible parts as combustible. A masonry building with a steel frame and a wood roof deck therefore lands in joisted masonry.
One wood roof deck can pull an otherwise noncombustible building down a class.
Insurance Implications of Construction Types
ISO built its six construction classes to develop commercial property rates based on susceptibility to fire damage.
Class 1 is the least fire-resistent and Class 6 the most, as IRMI’s definition of masonry non-combustible construction explains. Joisted masonry (Class 2) therefore ranks below masonry non-combustible (Class 4).
Commercial Property Insurance Ratings
A risk management guide from Alexander Schmidt lists construction class among the primary factors in a building’s basic premium rate. With other factors equal, a Class 4 building generally rates better than a Class 2 building.
No source publishes a universal discount between the two classes. Carriers apply their own rates, and occupancy, public protection class, sprinklers, building age and location all move the final premium. Owners should expect Class 4 to help and ask for quotes to see by how much.
Accuracy matters as much as the class. Select Systems warns that misclassification creates material premium discrepancies that surface later in premium accounting.
Builders Risk Policies
Builders risk rating is exposure-based, and protection class and construction type are among its drivers, according to Select Systems. Construction type also shapes the limits some programs will offer.
- Victor Insurance’s builders risk program offers an $8 million limit for frame and joisted masonry construction and $15 million for non-combustible, masonry non-combustible and fire-resistive construction.
- The University of California master builders risk program, in its September 1, 2025 summary, sets a $25 million per-project limit for joisted masonry and $10 million for wood frame.
These figures belong to specific programs and will not match every carrier. They show the pattern: the class on the application can set a limit tier before pricing begins.
Applications ask for the class directly. One carrier’s builders risk application maps joisted masonry to ISO 2 and IBC Types IIIA, IIIB and IV, and masonry non-combustible to ISO 4 and IBC Type IIA.
Matching the structural drawings to those definitions before submission avoids a reclassification after quoting.
What to Send Your Underwriter
An underwriter classifies from documents. A complete submission includes:
- Exterior wall material, thickness and whether the walls are load-bearing
- Floor and roof framing and deck material
- Any mixed or hybrid assemblies, such as a wood roof over a steel frame
- Fire protection details, including sprinklers and the public protection class
- Past renovations that changed the roof or floor system
A renovation that replaces a wood roof with a steel deck can change the class on paper. Carriers apply their own rules, so confirm the new class in writing with the agent.
Which Construction Type Is Better for Insurance?
For insurance alone, masonry non-combustible generally comes out ahead. It ranks higher on the ISO scale, avoids a combustible roof and floor, and sits in the higher builders risk limit tier in the programs above.
In the joisted masonry vs masonry non-combustible comparison, joisted masonry carries the wood-driven fire exposure and concealed-space risk that insurers price into the premium.
The right choice for a project still depends on cost, schedule and local code, which this guide does not compare.
A practical step is to ask the broker to quote both versions of the design before the floor and roof system is locked. That early quote often costs less than a mid-project change.
To see where wood frame and fully noncombustible buildings fit beside these two classes, read our three-way construction comparison.
Frequently Asked Questions
What Is the Difference Between Non-Combustible and Masonry Non-Combustible Construction?
Non-Combustible (Class 3) buildings have exterior walls, floors and roof made of and supported by metal, gypsum or similar noncombustible materials. Masonry non-combustible (Class 4) buildings keep the noncombustible floors and roof but use masonry for the exterior walls. The wall material is the only change between the two classes.
What Does ISO Mean in Construction Classes?
ISO stands for Insurance Services Office. It created the six construction classes, from Frame (Class 1) to Fire Resistive (Class 6), in its Commercial Lines Manual. Insurers use the classes to develop commercial property rates based on how susceptible a building is to fire damage.
Can a Building Change From Joisted Masonry to Masonry Non-Combustible?
A building can change class when its floor or roof materials change, because the class follows those materials. Replacing a wood roof and wood floors with noncombustible assemblies is the usual route. The carrier makes the final call, so requests written confirmation of the new class and any premium effect.
