Gypsum-Board Firewalls, continued Once the wood-framed wall that adjoins the firewall on one side is in place, the bottoms of the 2-foot-wide gypsum panels are inserted into the base channel and tilted up to vertical (Figure 3).

Figure 3.After inserting the bottom edges of the gypsum-board panels into a metal track fastened to the foundation, a carpenter installs the steel H-stud that will connect the vertical edges of the panels where they butt together. The base of the H-stud fits into the base channel and will be fastened in place with pan-head screws (left). Two lengths of channel are screwed together back to back atop the panel edges that extend above the second floor, serving as a base for the next course of panels (right).

The edges of successive panels are joined where they butt together with 2-inch metal H-studs. Special L-shaped aluminum clips are screwed to the H-studs where they pass by the top plate or floor plate; the other leg of the clip is screwed or nailed to the plate itself (Figure 4).

Figure 4.The double layer of gypsum board at the center of the area separation wall assembly is fastened to the adjacent framing with aluminum "breakaway clips." In the heat of a fire, the clips soften and melt, allowing the burning unit to collapse without pulling down the firewall or the neighboring unit on the other side.

It's important to maintain a 3/4-inch to 1-inch air space between the gypsum panels and the wood-framed walls on either side — no direct contact between the gypsum board and surrounding combustibles is permitted.

Hot clips, cool clips. At normal temperatures, the aluminum clips hold the gypsum-board core of the area separation wall in an upright position. But in a fire, when one side of the system is exposed to temperatures in excess of 1,100°F, they soften and break away. (On the job site, in fact, the aluminum clips are generally referred to as "breakaway clips.") This allows the fire-side structure to collapse without pulling down the fire-resistive separation wall. The clips on the opposite side of the area separation wall remain intact, since temperatures on that side will be far below the point at which the clip will soften.

Moving up. The gypsum-board area separation wall is extended upward a floor at a time as the framers continue their work. As each floor is completed, the top edge of the previous course of gypsum panels is capped with an upside-down piece of steel track. A second piece of track is screwed back to back on top of it, and the next row of panels is erected and fastened to the framing with more clips. The area separation walls we build are usually limited to two stories and an attic, but walls up to four stories high are permissible if the first story is a basement.

Fireblocking and Roof Junctions

To slow the vertical spread of fire between floors on the same side of an area separation wall, the code requires fireblocking of the space between the gypsum panels and the adjacent floor joists. In most cases, the fireblocking consists of strips of the same 1-inch gypsum board used in the firewall itself. The gypsum-board fireblocking is fastened to the H-studs with screws.

The 2-inch gypsum core of the firewall is required to be continuous, with no openings or penetrations that could allow fire to pass through it. But the framing of the walls on either side can accommodate plumbing and wiring just like any other partition.

Sound control. The dense, continuous area separation walls also provide an effective barrier to sound transmission between units of a townhouse or apartment complex. To reduce transmitted sound even more and improve thermal performance, we insulate the adjacent walls with fiberglass batts. When the plumbing, electrical, and insulation subs are done, the walls and ceiling are finished with drywall in the usual way.

Where roof meets firewall. Building codes ordinarily require area separation walls to be constructed with a parapet — a vertical segment of the wall that extends through the roof to a specific height, usually 30 to 36 inches above the roofline. But for townhouse construction, the code in our area also accepts a method that doesn't require a parapet (Figure 5). We use this method in applications where we want to avoid the appearance of a roofline interrupted by one or more parapets.

Figure 5.Where the area separation wall meets the roof, sheets of 5/8-inch type X gypsum board protect the underside of the roof sheathing in the two adjoining rafter bays on each side.

This involves installing a code-approved roofing material and adding a layer of 5/8-inch type X gypsum board for 4 feet on either side of the area separation wall. The gypsum-board underlayment sits directly beneath the sheathing, where it's supported by ledgers nailed to the top chords of the adjoining roof trusses. A similar detail can be used where an area separation wall meets an exterior wall (Figure 6).

Figure 6.A carpenter installs a strip of fireblocking to close the gap where a short section of exterior garage wall will butt against the area separation wall.

It Works

We're confident that gypsum-board area separation walls perform every bit as well as masonry. About 12 to 15 years ago, we encountered a local township that had no history of working with gypsum-board area separation walls, and town officials were skeptical of our intention of using them. To show them how reliable the gypsum-board walls are, we built an 8x16-foot building and divided it in half with an 8-foot-high gypsum-board area separation wall. Under the watchful eye of the local fire department, we set the test facility on fire. The gypsum-board wall system performed well beyond expectation, and we were able to sell the project to town authorities as planned. The test wasn't completely scientific, but it confirmed our belief and convinced local fire officials that gypsum-board area separation wall systems work as designed. Robert Brownis vice president of construction for the Barness Organization in Warrington, Pa.