Article and photos by Gregory Havel
Our perceptions and experience affect everything that we see and hear. If we are on vacation in the western United States and are awakened in the night by thundering hoof-beats, we would look out of the window expecting to see horses or a cattle stampede. We are less likely to expect to see buffalo because they are less common, and would not expect zebras or giraffes.
A few weeks ago, we looked at photos of a first-floor fireplace, chimney, and hearth that were supported on masonry foundations and concrete footings. Wood framing and structural members were separated from the masonry by the distances required in National Fire Protection Association 211, Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances NFPA 211), which is incorporated into the state building codes. Since the best support for a wood-burning masonry fireplace and chimney is a noncombustible foundation, that is what we expect to see-and we assume that the hearth (the noncombustible floor extending into the room in front of the fireplace) has the same kind of support.
Photo 1 shows a first-floor fireplace and chimney in a different house presently under construction. This one is supported on the masonry structure of the fireplace and chimney directly below in the walk-out basement, which is carried by a footing on undisturbed soil. The first floor is made of plywood on parallel-chord wood trusses. The recessed area in the floor in front of the fireplace is for the stone hearth, which will be set in cement mortar. The hearth’s surface will be flush with the finished floor in the rest of the room. This recessed area is built of several inches of concrete on corrugated steel decking.
Photo 2 shows the support for this hearth. The underside of the steel decking is visible, as are its supports (laminated veneer lumber [LVL] on each of the short sides, carried by a galvanized steel stirrup nailed into a doubled parallel-chord truss on each of the long sides). The edge of the plywood form board against the doubled truss is also visible. Although he strength of this support system meets or exceeds code requirements, and the separation of the masonry fireplace and chimney from combustible structural members meets the requirements in NFPA 211, the hearth is likely to become an unstable dead load if a structure fire affects the wood trusses inside the floor-ceiling assembly.
We need to assume that the supports for a fireplace hearth are similar to (or less sturdy than) the structure shown in photo 2; and that, during a fire, it is unstable until proven otherwise. If the hearth is supported as shown and remains stable during and after a structure fire, it should be celebrated as a great day in a good week. A fireplace and hearth system proved to be supported on masonry foundations, as show in the photos a few weeks ago, should be celebrated like a million-dollar winning lottery ticket.
Subjects: Masonry fireplaces, fireplace hearths, building construction for firefigher operations
