Local chimney cap replacement in Bellair-Meadowbrook Terrace

The half-inch and three-eighths rule for screen openings

Florida's residential code sets the screen opening size on a masonry chimney spark arrestor with two spheres. An opening may not let a sphere larger than 1/2 inch pass, which holds back embers large enough to start a roof fire. It also may not block a sphere smaller than 3/8 inch, which keeps the mesh open enough for the chimney to breathe. Screens outside that window fail in opposite ways: too coarse lets embers out, too fine clogs with soot and slows the draft. Any cap we propose has its mesh checked against both limits, and caps sold for other uses, such as gas vents or roof vents, may not meet them.

How a galvanized cap runs out of zinc

A galvanized cap is steel protected by a zinc layer that corrodes in the steel's place. The zinc wears away fastest where water lingers: along the screen wires, at folded seams, and around fastener holes.

Once it is gone in a spot, the steel beneath rusts, and rust takes up more room than the steel it came from, so the wires swell and break. Streaks of orange on the crown or the brick below are the visible sign of that stage. By then recoating is not practical, since the zinc cannot be put back on the roof, and the cap is replaced in a longer-lived metal with the old fasteners removed as well.

Water the cap cannot stop

The cap guards only the flue opening. The crown around it takes the same rain, and when it cracks or pulls away from the flue tile, water enters the top of the masonry and runs down inside the chimney. The result looks just like an uncapped flue from below: water in the firebox and rust on the damper. That is why a cap visit includes a close look at the crown surface and the joint where the crown meets the tile. When crown repair is needed, it appears as its own line, priced apart from the cap, making it clear that the new cap was not sold as a cure for a crown leak.