Contrary to concerns over its combustibility, wood can be an excellent performer under fire conditions. This is because of wood’s unique charring properties, which actually protect it from fire. Properly designing a building for fire safety means faithfully executing building code regulations.
Framing Around Chimneys and Fireplaces
Wood framing must be adequately separated from fireplace and chimney masonry. See Figure 5 for fireplace framing details. All headers, beams, joists, and studs must be kept at least two inches from the outside face of chimney and fireplace masonry. Prefabricated metal fireplace and chimney assemblies must be installed in accordance with the manufacturer’s recommendations and approved by the code authority.
Fireblocking is an important fire safety feature in wood-framed buildings with concealed spaces that can serve as ducts or chimneys for the spread of flames, gases, and smoke. Fireblocking is an integral part of a building’s passive fire protection. Fireblocking can protect against the passage of flames, deadly gases and toxic smoke through hollow vertical spaces created by joints and gaps in walls, floors, and floor/ceiling assemblies. In short, hollow vertical spaces need to be firestopped at every floor level.
In light wood-frame construction, 2″ dimension lumber has proven to be very successful in fireblocking if tightly fitted in place. Codes recognize 2″ lumber for use as fireblocks, as well as wood panels if the joints are backed. For sealing around vents, pipes, and ducts, the codes permit the code official to approve other fireblocking materials available in cans, tubes, and strips for ease of installation. Typically, materials meeting ASTM E814 Standard Method of Fire Tests for Through-Penetration Firestops for use with tested commercial firestop systems are acceptable for fireblocking around ducts and pipes.
Although fireblocking between floors is automatically built into the framing in most instances, care still needs to be exercised where certain designs require studs to pass floor levels. The bottom sill plate of wall framing usually acts as a fireblock. The use of furring strips, which create horizontal or vertical concealed spaces, must be fireblocked. For example, in ordinary construction, the space created when a masonry wall is furred needs to be fireblocked.
Fireblocking should be installed in these locations:
- Concealed spaces of stud walls and partitions, including furred spaces at ceilings and floor levels.
- Concealed spaces between stair stringers at the top and bottom of the run.
- Openings around vents, pipes, ducts, chimneys, and fireplaces at ceiling and floor levels.
- Interconnections between concealed vertical stud wall or partition spaces and concealed spaces created by an assembly of floor joists. With respect to concealed spaces created by an assembly of floor joists, fireblocking should be provided for the full depth of the joists at the ends and over supports.