What does fire blocking do?

Fire blocking is an important fire safety technique used in building construction to slow the spread of fire and smoke through concealed spaces in walls, floors, and ceilings. Fire blocking works by subdividing concealed spaces into smaller compartments to limit the passage of flames, heat, and smoke from spreading rapidly throughout a building. In the event of a fire, fire blocking provides more time for building occupants to escape safely and allows firefighters to better contain and extinguish the fire.

Why is fire blocking required?

Fire blocking is required by building codes for new construction in many jurisdictions because concealed spaces in walls, floors, and ceilings create channels that allow fire and smoke to spread rapidly throughout a building. This can lead to flashover, which is when the contents of a room ignite nearly simultaneously due to intense radiant heat. Flashover makes fire suppression extremely difficult and significantly increases the risk of life loss and building damage. Fire blocking subdivides concealed spaces and limits the amount of oxygen available to feed the fire, slowing flashover and containing the fire in a smaller area.

How fire spreads in concealed spaces

Fires start and spread very quickly in concealed spaces inside of wall and floor cavities. This is because:

• Concealed spaces allow fire and smoke to travel unseen throughout a building undetected
• The framing materials used to build walls and floors are highly combustible
• The confined space concentrates heat and allows rapid fire spread
• Concealed spaces contain a ready fire fuel source from utilities and building materials

Without fire blocking, a fire that starts in a concealed floor or wall space can quickly spread up walls and horizontally throughout a building until the fire breaks out in multiple remote locations. This makes fire suppression very difficult as firefighters chase flames popping out in disparate, disconnected areas.

Where is fire blocking required?

Fire blocking is required in concealed spaces of walls, floors, ceilings, stairways, and other concealed channels. Some specific locations where fire blocking is typically required include:

• Between floors at stairwells
• At soffits, drop ceilings, and other concealed ceiling spaces
• In concealed floor and ceiling spaces at room partitions
• In cornices and wall cavities at each floor level
• At chimneys to cut off vertical fire spread
• In double stud walls to block off hidden channels
• At openings around pipes, ducts, cables, and other penetrations

Fire blocking is also required at maximum intervals along wall and ceiling channels to subdivide extensive concealed spaces. Common code requirements specify maximum distances between 16 feet to 20 feet o.c. for fire blocking partitions.

Fire blocking materials and installation

Fire blocking materials are noncombustible substances that fit tightly inside concealed spaces to obstruct the path of fire. Common fire blocking materials include:

• Mineral wool insulation
• Sealants like caulk, foam, and mortar
• Sheet metal, cement board, or gypsum board
• Spray applied cellulose insulation
• Plywood or OSB sheathing
• Solid wood blocking

Fire blocking must completely fill concealed spaces to be effective. Gaps allow flames and smoke to circumvent. Fire blocking should be securely installed to remain in place if impacted by fire and building collapse.

Installation guidelines

Proper fire blocking installation involves:

• Cutting materials to fit snugly within spaces
• Friction fitting in cavities or attaching with adhesive or fasteners
• Sealing edges with caulk, foam, or mortar to prevent gaps
• Packing enough material to completely fill void spaces
• Checking for stability and absence of gaps

Poor installation reduces effectiveness. Common problems include loose-fitting materials that fall out of position and gaps at edges that allow smoke and fire to pass through. Taking care to properly install fire blocking is important.

Fire Blocking Material	Installation Method
Mineral Wool Insulation	Friction fit batts or loose-fill insulation
Spray Foam Insulation	Apply foam to completely fill voids
Intumescent Caulk/Mortar	Apply continuous bead around penetrations and connections
Sheet Metal/Cement Board	Screw or nail panels to framing as partition
Wood Blocking	Nail between framing at intervals to act as barrier

When should fire blocking be installed?

Fire blocking must be installed at the time of construction when wall, floor, and ceiling framing is open and concealed spaces are accessible. It is very difficult to properly install fire blocking in finished assemblies, so it is critical that this fire protection measure is not overlooked during initial building construction.

Some typical stages when fire blocking installation occurs include:

• Platform framing – In each framed wall section as floors are added
• Balloon framing – At intervals as the full wall height is assembled
• Open ceiling framing – Before installing ceiling finish materials
• Stair construction – When framing stringers and before enclosing
• Top story – Before roof or attic space is constructed

Verifying complete fire blocking installation should be a required inspection point by the building code official at logical construction points when concealed spaces become inaccessible.

Fire blocking building code requirements

Model building codes contain fire blocking installation requirements, which are then typically adopted fully or in part as law by states and local jurisdictions. Builders must comply with the fire blocking requirements mandated by the governing codes and standards in the location of the building site.

The International Building Code (IBC) and International Residential Code for one- and two-family dwellings contain fire blocking requirements builders must follow. Specifics vary slightly between the commercial and residential codes.

International Building Code requirements

The IBC fire blocking provisions mandate where fire blocking must be installed in larger commercial buildings. Key requirements include:

• Fire blocking at maximum 20 foot intervals along concealed wall and ceiling spaces
• Fire blocking at interconnections between concealed vertical and horizontal spaces
• Fire blocking at concealed draft openings into roof spaces and attics
• Fire blocking materials tested in accordance with ASTM E119 to provide 1-hour fire resistance

International Residential Code requirements

The IRC stipulates where fire blocking is required in residential homes and townhouses. Notable provisions require fire blocking:

• In concealed spaces of stud walls and partitions, including furred spaces, at 10 foot intervals along the length
• At interconnections between concealed vertical and horizontal spaces such as soffits and dropped ceilings
• In concealed spaces between stair stringers at the top and bottom
• At openings around vents, pipes, ducts, chimneys, and other penetrations between floors

Local jurisdictions may amend model code requirements with stricter or additional fire blocking specifications so builders should verify local codes.

Fire blocking inspection

Fire blocking should be inspected at key points during construction when concealed spaces become inaccessible. Typical inspection points include:

• Stud walls before installing drywall
• Floor cavities before installing ceiling finish below
• Top floor before roofing
• Inside of chases around chimneys and utility risers
• Stair stringers before enclosing

Inspectors check that fire blocking materials completely fill concealed spaces without gaps and are securely installed to avoid displacement. Thermal imaging can help identify heat loss through gaps. Failed inspections require installing or repairing fire blocking before construction continues.

Special fire blocking details

Fire blocking requires careful detailing at intersections, openings, and changes in construction. Common areas that require special attention include:

• Floor-to-wall connections – Seal top of wall gap before installing ceiling below
• Double walls – Install blocking to close off hidden cavity between wall sections
• Openings – Caulk or pack openings at pipes, wiring, ducts, etc. to seal penetration
• Chases – Ensure fire blocking at openings into vertical chases around flues, ducts, etc.
• Knee walls – Block off space between ceiling and bottom of knee wall
• Soffits – Install fire blocking at soffit connections with walls or ceilings

Architectural drawings should indicate all required fire blocking locations. The installer may also need to identify areas requiring fire blocking that are not explicitly shown.

Fire blocking mistakes to avoid

Some common fire blocking installation errors that compromise effectiveness include:

• Improper materials – Using paper, plastic, or other combustibles that won’t slow fire spread
• Loose installation – Not packing materials tight enough so they fall down or shift
• Gaps and voids – Leaving even small gaps at edges and openings
• Forgetting locations – Missing key areas like soffits, chases, and ceiling openings
• Penetration seals – Not sealing pipes, wiring, ducts, etc. passing through subdivisions
• No inspection – Failing to have fire blocking verified before concealment

Taking diligent care to install compliant fire blocking correctly avoids these mistakes that allow concealed fire spread.

Fire blocking costs

Fire blocking adds marginal cost to new building construction. Materials are relatively inexpensive, with basic items like sheets of mineral wool insulation and wood blocking ranging from $0.15 to $1.50 per square foot installed. Labor is also minimal, with most fire blocking able to be installed concurrently with other framing tasks.

A typical 2000 square foot home would incur less than $1000 in direct fire blocking expenses. This represents only a fraction of the total building cost, yet provides critical fire safety. Attempting to retrofit missing fire blocking after construction is substantially more expensive.

Alternatives to fire blocking

Fire sprinklers can sometimes substitute for fire blocking by suppressing fire spread in concealed spaces. However, sprinklers rely on detection and water to activate. Fire blocking is passive protection. Other limitations of sprinklers include:

• Water supply must be available and adequate
• Sprinklers only cover areas they are installed within
• Concealed fires can spread undetected until too late
• Delays allow smoke and heat to spread through channels

Fire blocking and sprinklers provide complementary protection working together. But fire blocking should not be considered redundant or safely eliminated.

Fire blocking exemptions

Fire blocking exceptions are allowed in certain cases where concealed fire spread risks are reduced. Common exemptions where fire blocking is not required include:

• Concealed roof spaces with ventilation between eave soffits and roof vents
• Concealed attic spaces under 30 inches of headroom
• Concealed spaces within noncombustible exterior wall assemblies
• Concealed spaces in walls which are fully lined with noncombustible materials

Light frame wood construction typically necessitates full fire blocking. See specific codes for permitted exceptions and alternatives.

Conclusion

Fire blocking is a vital fire protection measure required by building codes to compartmentalize concealed spaces and slow fire and smoke spread through wall, floor, and ceiling cavities. Proper materials and installation of fire blocking during construction provides passive protection that saves lives and property in building fires. Understanding where fire blocking is required by code, inspecting for correct installation, and carefully fire blocking at construction are simple, low-cost actions that make buildings significantly safer.