Fire blocking is solid material installed inside concealed framing cavities to cut off the drafts that let fire travel up and across a building unseen. Under 2021 IRC Section R302.11, stud walls need it at every ceiling and floor level and at horizontal intervals not exceeding 10 feet, plus at soffits, stair stringers and around pipe and wire penetrations.
This article walks through where fire blocking goes, which materials the code accepts, and how it differs from draft stopping. All figures come from the 2021 International Residential Code. Your jurisdiction's adopted code edition and its local amendments govern, and any listed firestop sealant or system must be installed per its listing.
What fire blocking does
A wood-framed wall is a row of hollow chimneys. Without a barrier, smoke and flame that get into one stud bay can rise straight to the attic or spread sideways through a dropped soffit, all behind finished drywall. Section R302.11 states the purpose plainly: in combustible construction, fireblocking cuts off both vertical and horizontal concealed draft openings and forms an effective fire barrier between stories, and between a top story and the roof space.
Section R602.8 (walls) and Section R502.13 (floors) both point back to R302.11, so one section controls fireblocking throughout the house. For how studs, plates and blocking fit together generally, see wall framing basics.
Where is fire blocking required?
Section R302.11 lists the locations for wood-framed construction:
- Concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs:
- vertically at the ceiling and floor levels;
- horizontally at intervals not exceeding 10 feet.
- Interconnections between concealed vertical and horizontal spaces, such as occur at soffits, drop ceilings and cove ceilings.
- Concealed spaces between stair stringers at the top and bottom of the run. Enclosed spaces under stairs fall under Section R302.7.
- Openings around vents, pipes, ducts, cables and wires at ceiling and floor level, filled with an approved material that resists the free passage of flame and products of combustion. The material filling this annular space does not have to meet ASTM E136.
- Chimneys and fireplaces, covered separately in Section R1003.19.
- Cornices of a two-family dwelling, at the line of dwelling unit separation.
Fire blocking at floor and ceiling levels
In typical platform framing, the plates already do much of the work. The double top plate closes the top of each stud bay at the ceiling, and the bottom plate sitting on the subfloor closes it at the floor. Where the framing leaves a gap, for example a balloon-framed wall running past a floor, a chase, or a furred wall standing off a foundation, a block has to be added. At the floor line, the rim joist and floor sheathing separate one story's wall cavities from the next.
Fire blocking in walls over 10 feet
Item 1.2 is the rule behind the "walls over 10 feet" question. If a concealed stud cavity runs more than 10 feet without being closed by a plate or other fireblock, it needs a horizontal fireblock so that no run exceeds 10 feet. This comes up in two-story foyers, great rooms with tall walls, and gable-end walls. For walls built with parallel rows of studs or staggered studs, Section R302.11.1.1 allows batts or blankets of mineral or glass fiber, or other approved nonrigid materials, to provide that horizontal fireblocking.
Soffits, drop ceilings and stairs
A kitchen soffit or a dropped ceiling creates a horizontal void that connects to the vertical stud bays behind it. Item 2 requires that connection to be blocked so fire cannot move from the wall into the soffit and along the room. Stair stringers create a sloped concealed space, and Item 3 requires it to be closed at the top and bottom of the run. The stair opening guide covers the framing around the stairwell itself.
Pipe, duct and wire penetrations
Every hole drilled through a top or bottom plate for a drain, vent, duct or cable reopens the path the plate was closing. Item 4 requires the annular space around the penetration at ceiling and floor level to be filled with an approved material. In practice this is often a fire-rated sealant or foam, which has to be one the building official accepts and installed per its listing.
Fire blocking materials
Section R302.11.1 sets out the materials that count as fireblocking, except for the annular-space fill in Item 4 above.
2021 IRC Section R302.11.1: fireblocking materials.
| Material | Requirement |
|---|---|
| Lumber | Two-inch nominal lumber |
| Lumber, two layers | Two thicknesses of 1-inch nominal lumber with broken lap joints |
| Wood structural panels | One thickness of 23/32-inch panels with joints backed by 23/32-inch panels |
| Particleboard | One thickness of 3/4-inch particleboard with joints backed by 3/4-inch particleboard |
| Gypsum board | One-half-inch gypsum board |
| Cement-based millboard | One-quarter-inch cement-based millboard |
| Mineral wool or glass fiber | Batts or blankets, or other approved materials, installed so they are securely retained in place |
| Cellulose insulation | Installed as tested in accordance with ASTM E119 or UL 263 for the specific application |
Section R502.2.2 adds that fireblocking shall be of any grade lumber, so offcuts that would never pass as joists are fine as blocks. Two-inch nominal lumber cut to fit tight between studs is the most common fireblock on site. A row of solid blocks also stiffens the wall, the same way joist blocking and bridging does in a floor.
Using insulation as fire blocking
Three subsections deal with insulation:
- Batts or blankets (R302.11.1.1). Mineral or glass fiber batts, or other approved nonrigid materials, may be used for the 10-foot horizontal fireblocking in walls with parallel rows of studs or staggered studs.
- Unfaced fiberglass (R302.11.1.2). Unfaced fiberglass batt used as fireblocking must fill the entire cross section of the wall cavity to a height of not less than 16 inches, measured vertically. Where piping, conduit or similar obstructions are in the way, the insulation must be packed tightly around them.
- Loose fill (R302.11.1.3). Loose-fill insulation cannot be used as a fireblock unless it has been specifically tested in the form and manner intended, to show that it stays in place and retards the spread of fire and hot gases.
Keeping fire blocking intact
Section R302.11.2 is one sentence: the integrity of fireblocks shall be maintained. A block notched for a pipe, or a plate bored for wiring and left open, is no longer a fireblock. The trades that come after framing are where most fireblocking fails inspection, so check plates and blocks again before insulation and drywall.
Fire blocking vs draft stopping
The two terms get mixed up because both close off concealed spaces. They cover different places and use different rules.
2021 IRC Sections R302.11 and R302.12: fireblocking compared with draftstopping.
| Fireblocking (R302.11) | Draftstopping (R302.12) | |
|---|---|---|
| Purpose | Cut off vertical and horizontal concealed draft openings; fire barrier between stories and between top story and roof space | Limit the area of the concealed space in a floor-ceiling assembly |
| Where | Stud walls, soffits, drop and cove ceilings, stair stringers, penetrations, chimneys, two-family cornices | Floor-ceiling assemblies with usable space above and below |
| Key figure | Horizontal intervals not exceeding 10 feet in stud walls | Concealed area not exceeding 1,000 square feet |
| Materials | Section R302.11.1 list (2-inch lumber, 23/32-inch panels, 1/2-inch gypsum and others) | Not less than 1/2-inch gypsum board, 3/8-inch wood structural panels or other approved materials, adequately supported |
When is draft stopping required?
Section R302.12 applies in combustible construction where there is usable space both above and below the concealed space of a floor-ceiling assembly. Draftstops must divide that concealed space so its area does not exceed 1,000 square feet, in approximately equal areas.
Where the assembly is enclosed by a floor membrane above and a ceiling membrane below, draftstopping is required when either:
- the ceiling is suspended under the floor framing; or
- the floor framing is built from truss-type open-web or perforated members.
The second case is the one framers meet most. Open-web floor trusses leave a continuous void that runs in every direction, unlike solid sawn joists that form closed bays. Section R502.12 points floor framing back to R302.12 for this reason.
Draft stopping materials
Under Section R302.12.1, draftstops are not less than 1/2-inch gypsum board, 3/8-inch wood structural panels, or other approved materials, adequately supported. They are installed parallel to the floor framing members unless the building official approves otherwise, and their integrity must be maintained.
Showing fire blocking on a framing plan
Fireblocking is rarely drawn member by member. Most plan sets carry a general note citing Section R302.11 and, where open-web trusses are used, a draftstop line on the floor framing plan dividing the floor into areas within the 1,000 square feet limit. On a second floor framing plan over living space, that draftstop line and its note are what the plan reviewer looks for.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check fireblocking, draftstopping or any other code requirement, and it does not size members: its sample profile uses demo span figures and metric sizes, and the editor works in millimeters. Add the code notes and draftstop lines to the exported drawing yourself or have your designer add them.
When the framing layout is ready to sketch, open the framing studio to place joists, beams and openings for a preliminary drawing, then confirm fireblocking and draftstopping details against the adopted code with your plan reviewer. The full code text is available from the International Code Council.