Fire-resistance ratings decide how much time occupants have to evacuate and how much of a structure survives a blaze, so the assemblies behind those ratings are central to modern construction. When Underwriters Laboratories certified Design No. V335, a two-hour bearing wall assembly built with fire-retardant-treated wood (FRTW) lumber and plywood, it listed the design under ANSI/UL 263 for the United States and CAN/ULC-S101 for Canada. The listing gives architects and specifiers a code-compliant route to wood-frame walls carrying brick, stucco, fiber cement, and other approved exterior facings. The same layer-by-layer thinking that goes into understanding rainscreens, where drainage, ventilation, and wall assembly performance decide durability, applies to fire-rated walls: every component has a job, and the assembly only performs when all layers work as designed.
What a 2-Hour Fire-Resistance Rating Actually Means
A fire-resistance rating describes how long an assembly contains a fire under standardized test conditions. In the United States, the test method is ANSI/UL 263, the same procedure as ASTM E119: a full-scale wall is built exactly as specified, then exposed to a furnace whose temperature follows a standard time-temperature curve. The wall passes when it meets three criteria.
The Three Acceptance Criteria
- The wall carries its design load without collapse for the full duration of the test.
- Flame and hot gases do not penetrate to the unexposed side of the wall.
- The unexposed surface stays below temperature limits, an average rise of about 250 F with no single point above 325 F.
A 2-hour rating means the assembly holds all three for two full hours of furnace exposure. That window buys two things: time for occupants to reach exits, and time for fire crews to arrive and attack the fire before the structure gives way. The V335 listing covers bearing walls, meaning the wall supports vertical loads from floors and roof above, not just its own weight.
The fire exposure in the test comes from the interior face, which matches how most fires start: inside the building, fueled by contents and finishes. Interior finish quality matters once the building is occupied. Crews that learn how to use a paint roller assembly to avoid lap marks and achieve professional wall finishes protect the appearance of occupied spaces and avoid rework that can disturb fire-rated components.
Wall Assembly Layers: Sheathing, Barriers, and Facings
A listed assembly is a recipe of specific layers, and substituting materials voids the listing. The V335 design pairs FRTW framing with FRTW plywood sheathing, then adds an exterior facing from the approved list: brick veneer, stucco, fiber cement, or another facing tested and published by UL. Each layer has a distinct role in both fire performance and weather performance.
The sheathing ties the framing together and provides the base for the exterior finish. Combining the weather-resistive barrier with the structural sheathing, an approach that streamlines wall assembly and cladding compatibility, reduces trade sequencing conflicts and keeps the cavity drier. On a fire-rated wall, the same sheathing contributes to the char barrier that slows heat transmission, so the grade and thickness called out in the design are not optional.
Building the Assembly in Order
- Erect FRTW studs at the spacing shown in the listed design, typically 16 or 24 inches on center.
- Install FRTW plywood sheathing on the exterior face with the fastener pattern specified.
- Attach the weather-resistive barrier and any drainage plane required by the facing.
- Install the exterior facing from the approved UL list, with anchorage per the design.
- Frame the interior face with the gypsum layers and fasteners called out in the listing.
Fire exposure from the interior face means the interior layers, usually multiple layers of gypsum board, are part of the tested assembly. Changing their thickness, orientation, or fastening schedule changes the rating, so substitutions belong in a revised UL evaluation, not in the field.
How Fire-Retardant-Treated Wood Works
FRTW is lumber or plywood impregnated under pressure with fire-retardant chemicals. When heat reaches the treated surface, the chemicals promote char formation and suppress flaming combustion. The wood still burns, but slowly and without the rapid flame spread of untreated lumber, and the heat release rate drops, which gives the gypsum and other layers more time to do their job.
Pressure Treatment and the Chemistry of Char
The treatment penetrates the wood cell structure, not just the surface, so field cuts expose protected fiber only at the cut face. Treatment levels follow standards such as AWPA U1, and the retention of chemical in the wood is measured and marked by the treating plant. Higher retention generally means better fire performance and a slightly higher cost.
Flame Spread and Smoke Development
Two numbers define FRTW performance: the flame spread index (FSI) and the smoke development index (SDI). Untreated lumber commonly scores well above 100 on flame spread, while fire-retardant-treated wood typically lands in the Class A range of 0 to 25. Smoke development stays low enough that the material qualifies for many interior finish applications. Treatment has trade-offs: some formulations raise the equilibrium moisture content of the wood, so FRTW should not sit in wet conditions on the jobsite, and published design values reflect a strength adjustment for the specific chemical and species.
Because the treatment does not change the insulating character of the wood fiber itself, engineers still calculate wall assembly R-values using ASHRAE and IECC methods. The rated assembly keeps its thermal performance, which means the fire listing and the energy code can be satisfied by the same wall.
Exterior Facings and Cladding Options for Rated Walls
The exterior face of a rated wall does more than shed water; it also participates in the fire test. UL lists each facing combination it has tested, so the choice of brick, stucco, fiber cement, or another approved material is restricted to what the design shows. Comparing wall assembly strategies for a high-performance building envelope shows why the facing choice matters beyond appearance: it changes weight, moisture behavior, thermal bridging, and fire contribution all at once.
Facing Selection at a Glance
| Facing | Fire contribution | Weight | Moisture behavior | Typical use |
|---|---|---|---|---|
| Brick veneer | Non-combustible, adds mass | Heavy, needs foundation support | Vented air space aids drying | Schools, multi-family |
| Stucco | Non-combustible | Moderate | Drainage plane required | Hotels, healthcare |
| Fiber cement | Non-combustible | Moderate | Breathable, low maintenance | Mixed-use, commercial |
| Metal panel | Non-combustible | Light | Detailing controls condensation | Urban infill |
Each facing imposes its own detailing rules. Brick veneer needs shelf angles and weep holes, stucco needs control joints and a drainage plane, and fiber cement needs lapped or flashed joints. Whatever the facing, the anchorage system shown in the UL design, including ties, clips, or furring, must match the drawing exactly.
Applications in Schools, Healthcare, Hotels, and Multi-Family
Occupancies that concentrate people, especially children, patients, and overnight guests, put the highest value on fire performance. The V335 listing targets schools, healthcare facilities, hotels, and multi-family projects, where FRTW walls control the spread of flame and smoke, extend evacuation time, and reduce the damage and repair costs a fire leaves behind.
Code paths matter as much as the chemistry. International Building Code provisions allow fire-retardant-treated wood in specific locations where untreated wood is restricted, including certain exterior walls and roof structures. Teams working through furred continuous insulation wall assembly details for 2021 code compliance still need to verify the fire-resistance rating of the complete assembly, because adding insulation, furring, or cavities changes the heat exposure profile the listing was tested against.
Where Rated Walls Earn Their Keep
- Corridors and stair enclosures that protect egress paths.
- Party walls and unit separation walls in multi-family buildings.
- Exterior walls of Type III construction, where the code limits exposed wood.
- Shaft walls and elevator lobbies where smoke migration is a concern.
Specifying FRTW: What Contractors Need to Know
The listing is the contract. Design No. V335 is published in the UL Online Certifications Directory, and specifiers should reference the design number, the standard (ANSI/UL 263 or CAN/ULC-S101), and the rating on the drawings. The FRTW itself must carry a mark showing the treating plant, the standard it meets, and the retention level.
Documentation and the UL Online Certifications Directory
Request the treating plant’s data sheet in the submittal and keep it with the project file. The data sheet lists the approved species, the chemical system, the retention, and the design value adjustments. Ordering should account for the strength adjustment in the published design values, since the treatment reduces some allowable stresses compared with untreated lumber of the same grade.
Handling, Cutting, and Fastening
- Cut or drill with clean tooling, and treat field cuts with the touch-up product approved by the treating plant.
- Use hot-dipped galvanized or stainless fasteners where the chemical is corrosive to bare steel.
- Store FRTW off the ground and protect it from prolonged wetting, because the increased moisture content of some formulations can corrode connectors and warp framing.
- Photograph the installed assembly at each stage so the rating can be verified after the walls are closed.
As with getting continuous exterior insulation right, where materials and methods decide performance, a fire-rated assembly succeeds only when the specified products arrive, get installed, and get documented exactly as listed. The listing, the material marks, and the as-built photos together prove the wall meets the two-hour rating that the occupancy depends on.