A chimney cap sits on top of the flue, a few inches above the masonry crown, and it is one of the few metal parts on a roof that often has no metal named next to it. When a roof is replaced, the cap is either left as it is, swapped for whatever metal the crew has on the truck, or replaced without the bid saying which metal went on. Each choice leaves a different piece of metal exposed to the same rain, wet snow, road salt drifting off the street, and the freeze and thaw cycles that run through St. Louis from December into March. This page covers the three metals offered for residential caps, how each one behaves in this climate, and what to write into the proposal so the cap you receive matches the cap you priced.
Why the cap belongs in the roof spec
The cap does three jobs. It keeps rain from running straight down the open flue, it sheds water off the top of the masonry so the crown does not hold a pool, and it holds a screen that keeps birds, squirrels and leaves out of the flue. The top surface takes direct rain and hail, and the sides take wind-driven rain from the storms that cross the metro in spring and summer. Wet snow can sit on a cap for days, and when the temperature drops again that water freezes in every seam with an open edge. A cap that rusts, loosens or separates at its seams lets water into the flue and down into the chimney body, where the crown mortar and the brick below it take the damage. Replacing a cap usually costs much less than repointing a crown, so the cap is a practical place to stop that damage, and it only works as a spec item if the metal is named.
Stainless steel
Stainless steel is the metal most homeowners picture when they hear the word cap, and in a residential spec the usual grade is Type 304. It resists rust in ordinary rain without a coating that can wear through, and it keeps its finish through repeated freeze and thaw cycles without flaking. Type 316 adds molybdenum, which gives it better resistance to chlorides, the salt in road de-icer and the spray a street can throw onto a low chimney. If the house sits close to a road that gets salted every winter, the grade is worth naming. Stainless costs more than galvanized, and that difference is the line a bid most often leaves out or buries inside a larger chimney figure.
Ask for the screen to be stated separately. Stainless screen is usually welded or woven wire, and the spec should give the material, the wire gauge and the mesh opening in inches. A screen that is too fine clogs with soot and leaves, and a screen that is too coarse lets in the animals it was meant to keep out. The screen also needs a fastening method that keeps it from rattling or lifting in wind, and the spec should say what that method is.
Galvanized steel
Galvanized caps cost less than the other two and show up often on older houses and rentals. The zinc coating protects the steel by giving up first wherever the steel is exposed, so the protection depends on the coating being intact. Cut edges, bent folds and the seams where the cap meets its base lose coating first, and rust usually starts there rather than on the top surface. The spec should name the coating weight, which is written as a G number under the steel standard. A G90 coating is a common designation for this kind of part, and a lighter coating will show rust sooner. Expect a galvanized cap to need replacing before a stainless one does, and plan the next chimney visit with that in mind.
Copper
Copper lasts longer than the other two in ordinary exposure, and it bends into a tight seam with less effort than steel. It develops a patina over several years, which is a color change and not a failure. Copper is softer than steel, so a hailstone or a ladder can dent it, and those dents show clearly in photos taken before and after a storm. Copper runoff is the one thing to plan around: rain running off copper carries copper ions that corrode zinc, so a galvanized gutter or galvanized flashing sitting below a copper cap can fail sooner than it should. The spec should say whether the cap is copper alone or copper with a different metal beside it, and where its runoff lands.
Aluminum and other metals
Aluminum caps appear in the same stores as the other three and are priced low. Fresh masonry and mortar are alkaline, and alkaline material attacks aluminum in contact with it, so an aluminum cap set on a new crown can corrode at the contact line. If an aluminum cap appears in a bid, the spec should name it and the contractor should explain how it is isolated from the masonry. If the spec does not name it, the bid should be read as excluding it.
What the spec should name
- The metal and grade, such as 304 stainless, 316 stainless, galvanized steel with a stated coating weight, or copper with its weight in ounces.
- The screen material, wire gauge and mesh opening in inches.
- The outside dimensions of the cap in inches, and the clearance it leaves around the flue liner.
- How the cap is fastened to the crown or the flue, including the fastener type and count.
- The sealant at each seam, named by product and by its rated temperature range.
- Whether the existing cap is reused after inspection or replaced, and what the crew does if it is cracked or loose.
- Where the old cap goes when it comes off the roof.
What it costs
A bid should show the cap as its own line, with its metal and size stated, and not as a few words inside a flashing or chimney line. Galvanized usually comes in below the other two, stainless sits higher, and copper higher again. A custom size, a heavier gauge or a larger screen adds to the figure. Ask whether the cap is inside the roof total or priced as an add, and ask what the add covers. If a bid says chimney cap included with no metal named, the number cannot be compared against another bid. Two bids can name different caps and still look the same on the page, which is why the metal belongs in the figure's description.
What to ask
- Is the existing cap being reused, and who decides after the inspection?
- Which metal and grade will go on, and does the bid name it?
- What is the screen material and mesh opening, and is the screen included in the price?
- Is the cap sized to the flue, and is the clearance stated in inches?
- How is the cap fastened, and what sealant goes at the seams?
- If the cap is copper, where does its runoff land on the roof, the gutter and the masonry?
- Will the cap be photographed from the top and both sides before the tear-off starts?
- Who takes the old cap away when it comes off?