4.5 Fireblocking, Draftstopping & Concealed Space Protection

1. Why Concealed Spaces Fail Plan Review

Concealed combustible spaces are the hidden highways that let fire and hot gases bypass every rated assembly on the drawings. IBC Section 718 (Concealed Spaces) controls two different tools that plans examiners constantly confuse:

• Fireblocking cuts off concealed draft openings — vertical and horizontal — so a fire in a stud cavity cannot run from floor to floor or into a soffit. Fireblocking has no hourly rating requirement; it is a prescriptive material list.
• Draftstopping subdivides large combustible concealed areas inside floor/ceiling assemblies and attics so smoke and fire cannot spread across an entire floor plate above the ceiling.

The sprinkler distinction in that table is the single most-missed item on plan review: an NFPA 13 system throughout eliminates draftstops but never eliminates fireblocking.

2. Fireblocking Materials (IBC 718.2.1)

The code lists acceptable fireblocking materials explicitly, and a plans examiner should reject anything not on the list unless it has been tested in the form and manner intended for use:

1. Two-inch (nominal) lumber.
2. Two thicknesses of 1-inch nominal lumber with broken lap joints.
3. One thickness of 0.719-inch (23/32) wood structural panel with joints backed by 0.719-inch wood structural panel.
4. 0.75-inch particleboard with joints backed by 0.75-inch particleboard.
5. One-half-inch gypsum board.
6. One-fourth-inch cement-based millboard.
7. Batts or blankets of mineral wool or mineral fiber securely retained in place.
8. Cellulose insulation tested in the form and manner intended for use.
9. Mass timber complying with IBC Section 2304.11.
10. 19/32-inch fire-retardant-treated wood structural panel complying with Section 2303.2.

Two material sub-rules generate correction notices:

• Unfaced fiberglass batt (IBC 718.2.1.2) must fill the entire cross section of the wall cavity to a minimum height of 16 inches, packed tightly around pipe or conduit obstructions. A thin friction-fit scrap is not fireblocking.
• Loose-fill insulation, insulating foam sealants and caulk (IBC 718.2.1.3) are not permitted as fireblocking unless specifically tested in the form and manner intended for use. Details that call out "spray foam at top plate — fireblocking" without a listing are noncompliant.

3. Required Fireblocking Locations (IBC 718.2.2 through 718.2.7)

Mark up the wall sections and details against this list:

• Concealed wall spaces (718.2.2): vertically at ceiling and floor levels, and horizontally at intervals not exceeding 10 feet. Tall single-story spaces such as gymnasiums, warehouses and atrium walls almost always need an intermediate horizontal fireblock that the drawings omit.
• Interconnections between horizontal and vertical spaces (718.2.3): at soffits, drop ceilings, cove ceilings and similar transitions where a stud cavity opens into a joist or truss space.
• Stairways (718.2.4): in concealed spaces between stair stringers at the top and bottom of the run.
• Ceiling and floor openings (718.2.5): the annular space around vents, pipes, ducts, chimneys and fireplaces at ceiling and floor levels, using material specifically tested to remain in place and resist the passage of flame and combustion products.
• Exterior wall coverings (718.2.6): within concealed spaces of combustible exterior wall coverings and architectural elements, at maximum 20-foot intervals in either dimension so no concealed space exceeds 100 square feet. Assemblies tested to and complying with NFPA 285 are excepted, as are coverings on noncombustible framing faced with 0.019-inch aluminum or 0.016-inch corrosion-resistant steel.
• Concealed sleeper spaces (718.2.7): where wood sleepers carry wood flooring over concrete or masonry fire-resistance-rated floors, the space must be filled or fireblocked into areas not exceeding 100 square feet, filled solid under permanent partitions. Gymnasium slab-on-grade floors are excepted, and bowling lanes only need fireblocking at alternate lanes and lane ends.

4. Draftstopping in Floors and Attics (IBC 718.3, 718.4)

Floors (718.3): In other than Group R occupancies, draftstops subdivide combustible floor/ceiling assemblies so that no horizontal concealed floor area exceeds 1,000 square feet. Draftstops are also required wherever Section 708.4.3 calls for them — that is, where required fire partitions stop at a ceiling instead of continuing to the deck above.

Attics (718.4): In other than Group R occupancies, draftstops subdivide combustible attic and concealed roof spaces so that no horizontal area exceeds 3,000 square feet, and required attic ventilation under Section 1202.2.1 must still be maintained across the draftstop. Openings through draftstop partitions must be protected by self-closing doors with automatic latches constructed as required for the partition (718.4.1.1).

Both are waived where the building is equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1 — note that this exception names the NFPA 13 section only.

Draftstop materials (718.3.1): not less than 1/2-inch gypsum board, 3/8-inch wood structural panel, 3/8-inch particleboard, 1-inch nominal lumber, cement fiberboard, or batts/blankets of mineral wool or glass fiber, adequately supported, with the integrity of the draftstop maintained. Attic draftstops use the same material list.

5. Combustible Materials in Type I and II Concealed Spaces (IBC 718.5)

Combustible materials are prohibited in concealed spaces of Type I or II construction, with six narrow exceptions: materials permitted by Section 603; combustible materials exposed within plenums complying with IMC Section 602; Class A interior finish classified under Section 803; combustible piping within partitions or shaft enclosures; combustible piping in concealed ceiling spaces installed per the IMC and IPC; and combustible pipe insulation and covering complying with Section 720.7. A plan set showing PVC storm piping above a Type IB corridor ceiling is acceptable only under the IMC/IPC pathway; the same detail showing combustible blocking or furring is not.

6. Plan Review Scenario: 3-Story Type VB Business Building

Submittal: 3-story Group B office building, Type VB, not sprinklered, 9,000-square-foot floor plate, wood floor trusses with a suspended gypsum ceiling, 14-foot-tall stud walls in the ground-floor lobby, boxed soffits over the corridor, and a vented attic of 9,000 square feet.

Examiner findings:

1. Floor draftstops missing. The 9,000-square-foot combustible floor/ceiling cavity must be subdivided into concealed areas of 1,000 square feet or less (718.3). At least nine compartments are required, or the applicant can add an NFPA 13 sprinkler system throughout and delete them.
2. Attic draftstops missing. The 9,000-square-foot attic requires subdivision into areas of 3,000 square feet or less (718.4) — a minimum of three compartments — with ventilation maintained across each draftstop.
3. Lobby wall fireblocking. The 14-foot stud cavities exceed the 10-foot horizontal interval in 718.2.2; add a horizontal fireblock.
4. Soffit interconnection. The corridor soffit detail shows the stud cavity open into the truss space; add fireblocking at the interconnection (718.2.3).
5. Foam at penetrations. The plumbing detail specifies unlisted expanding foam at pipe penetrations of the floor; replace with a tested fireblocking material (718.2.5) or a listed firestop system under Section 714 where the assembly is rated.

7. Common Plan Review and Exam Traps

• Trap 1: Believing sprinklers delete everything. Sprinklers waive draftstops only; fireblocking survives.
• Trap 2: Mixing up 1,000 and 3,000. Floors are 1,000 square feet; attics are 3,000 square feet.
• Trap 3: Treating fireblocking as a rated firestop. Penetrations of rated assemblies need a tested penetration firestop system under Section 714 (F/T ratings); fireblocking under 718 applies to concealed combustible spaces that are not rated.
• Trap 4: Forgetting the 20-foot/100-square-foot exterior rule. Combustible cornices, cladding cavities and column wraps need fireblocking under 718.2.6.
• Trap 5: Accepting caulk or loose fill. Only tested products qualify (718.2.1.3).