Local chimney chase cover replacement in Callahan

Why thin sheet dents and holds puddles

Sheet metal is sold by gauge, and a higher gauge number means a thinner sheet. A thin sheet is easy to cut and bend, but it flexes under a ladder foot, dents when a branch falls on it, and sags between its edges on a wide chase. Every dent becomes a small basin. Manuals give a minimum gauge, which is where a design starts rather than where it has to stop. For a larger chase, or one that sees foot traffic during chimney service, we propose a heavier sheet so the slope and cross-breaks keep their shape, and the gauge is named beside the metal.

Folded, welded and sealed seams

One-piece chase tops have seams only at their corners, where the skirt folds down. A very large chase, or one with an unusual shape, may need two sheets joined across the top.

Every seam is a possible leak, and fireplace manuals call for caulking all seams on the chase top. Corner seams are folded so water runs over them rather than into them. Joints across the top are folded and locked or welded, then sealed, and placed where water runs across them rather than along them. Fewer seams in the flat means fewer places to fail, which is why we cut one sheet where the chase size allows.

A water heater or furnace vent in the same chase

Some chases carry a vent from a heating appliance or water heater next to the fireplace chimney. That vent is its own listed system, with its own maker's cap and clearances, and the chase top treats it as a separate penetration. It gets its own collar sized to its pipe and its own storm collar, spaced from the fireplace pipe as its instructions require.

We do not change that appliance's vent or cap. If its cap or storm collar is damaged, we name the problem and who should replace it, so the fix stays within that system's own instructions.