Local chimney chase cover replacement in Nassau Village-Ratliff

A quarter inch per foot away from the pipe

One fireplace maker's manual gives the minimum fall for a chase top: at least 1/4 inch per foot, sloping down away from the pipe opening. That may look slight, but it is enough to keep water moving on sheet metal as long as the sheet is stiff enough to hold its shape. A slope that reverses anywhere, even over a few inches, creates a low spot where water sits and the metal stays wet. A level goes on the finished top in several directions, and on a larger chase the sheet gauge is chosen so the slope holds across the span between the supports.

Checking the top pipe section while it is exposed

With the old cover and the cap off, the top section of the chimney pipe is in plain view. Rust on its outer wall, a dented section, or a loose joint between sections all show clearly at that moment, and they are hard to see at any other time. Damaged chimney pipe is not patched: sections are part of the listed system and are replaced with the maker's parts for that model.

If we see damage, you get the part name and a description of what we saw. If the pipe is sound, the new cover and the cap go back on and the section is covered again.

Why sealant cannot fix a flat top

Some old chase tops are covered in layers of roof cement or caulk, spread over rust spots and seams in the hope of stopping a leak. Sealant can bridge a pinhole for a while, but it cannot change the shape of a flat sheet, so the water still sits where it did before. Each layer also hides the metal, making it hard to see how far the rust has spread.

When a sealed-over top comes off, it can show more damage than its surface suggested. A shaped new top removes the need for the sealant, and the old layers go into the dumpster with the rusted sheet.