Local chimney cap replacement for Atlantic Beach homes

Why a screen needs four times the flue's open area

Beyond the opening size, the code sets how much total open space the screen must have. After the wire and frame are subtracted, the screen's clear area has to reach at least four times the clear area of the flue outlet under it. Wire takes up part of every square inch of mesh, so the screen's overall size must be well beyond the flue opening to leave that much air path. The rule explains why a cap for a large flue is noticeably taller or wider than one for a small flue. A cap that looks generous from the yard can still fall short once the wire is counted, which is why we size from the maker's data rather than from appearance.

Copper caps and the fasteners they need

Copper and zinc-coated steel make a small battery when rain connects them, and the zinc and the steel lose. That is the reason a copper cap is fastened with copper or stainless hardware and kept clear of galvanized flashing or an old galvanized screen. Roofing metal suppliers publish the same rule for copper used anywhere on a roof.

When a copper cap replaces a galvanized one, every old screw and clip comes out of the crown. Any nearby galvanized part that has to stay, such as a flashing piece a roofer owns, is kept out of contact with the copper, and its position is noted.

A damper that doubles as the rain cap

Fitted to the flue tile in place of a cap, a top-sealing damper has a hinged lid with a gasket and is opened and closed by a stainless cable that runs down to a bracket inside the firebox. Closed, it seals the flue at the top, which keeps conditioned air in the house and rain out of the flue. Models with a screen in the frame also keep animals out, as a cap would. It is chosen when the throat damper is rusted or warped, and it follows its maker's sizing just as a cap does. The one thing to remember is to open it fully before any fire is lit.