Q. I have to build a framed chase for a gas fireplace's 6-inch vent pipe. What's the best way to finish the top of the chase? I had planned to use a metal cap, but I've noticed that a lot of these caps tend to be rusted.
A. Patty McDaniel, owner of Boardwalk Builders in Rehoboth Beach, Del., responds: We finish chimney chases with sheet-metal caps made for us at a local metal shop. We've used both galvanized and stainless steel, though we prefer stainless because galvanized will rust after several years in our coastal environment.
For stability against high winds, we typically frame the chases with full-length 2x4 studs that run back to the attic floor level. We sheathe the chimney on all four sides and the top with exterior-grade plywood. The chase gets insulated to the ceiling or roofline, with intermediate fire blocking and air-sealing at each floor level. The interior of the chase is drywalled and taped.
We cover the plywood top with a peel-and-stick modified-bitumen membrane, lapping it over the weather barrier on the chase walls.
The chase cap looks somewhat like a shoebox lid, with a collar around the hole for the pipe, and a hem bent down around the chase on all four edges. On a small chase, we use a flat cap, with a 3- to 4-inch-tall collar sized to fit snug to the chimney pipe. On a larger chase, the metal cap is creased to make it more rigid - like a miniature hip roof, in which case the plywood cap is also detailed as a mini-hip.
