How to Measure for a Chimney Cap

Why there is no standard chimney cap size

Clay flue tiles come in a range of nominal sizes, and older chimneys add their own variety: tiles that are slightly out of square, oval or round flues, flues that were cut down or patched, and stainless liners of different diameters. On top of that, crowns vary in size and shape, and many older chimneys have two or three flues of different sizes and heights.

That is why caps are sold in many sizes and why custom caps are common on older homes. Ordering by guesswork is the most frequent reason a cap does not fit or blows off in the first strong wind.

What gets measured, by cap type

The measurements depend on the kind of cap. These are the usual ones.

• Single-flue, square or rectangular: outside length and width of the flue tile, plus how far the tile stands above the crown
• Single-flue, round: outside diameter of the round flue, with an eye on cracked or chipped tile tops
• Stainless liner: the liner diameter and its top plate, since caps are made to fit the liner
• Multi-flue, outside mount: length and width of the crown or chimney top, and the height of the tallest flue
• Chimney pot: the pot's opening and outside diameter, for caps or guards made to fit pots

Single-flue caps: outside of the tile

A single-flue cap slides over the flue tile and is held by legs or a band that clamp to the outside of the tile. So the key numbers are the outside length and width of the tile at its top, measured across the tile edges, not the opening inside. Rounded corners and slight warping on old clay tile mean each side is measured, not just one.

The tile also has to stand far enough above the crown for the cap's mounting to grip it. A tile cut almost flush with the crown may not leave room for a clamp-on cap, and an outside-mount cap or a crown repair may be the better answer.

Multi-flue caps: the crown

A multi-flue cap covers the whole top of the chimney and is fastened to the crown. It is sized from the length and width of the crown or the area it will sit on, and its height has to clear the tallest flue with room for the exhaust to leave. Flues of different heights often get trimmed or extended so the cap can do its job.

Because it bolts to the crown, the crown has to be sound. Fastening a new cap to a cracked crown just moves the leak.

Height and open area

A cap must not choke the flue. The space between the top of the flue and the cap's hood, and the open area of the screen around it, together need to let exhaust leave at least as freely as the flue itself. Caps that sit too low, or whose screens clog, are a common cause of smoky fireplaces and spillage from boilers and water heaters.

Gas and oil appliances have their own considerations. A flue serving a boiler or water heater may call for a cap listed for that use, and some appliance manufacturers specify the termination. Wood flues need a screen that stops sparks and animals without clogging quickly with creosote.

Screen mesh size

The mesh on a cap's sides keeps out birds, squirrels and raccoons and stops sparks from leaving a wood flue. Cap makers use mesh sizes chosen for that balance. Mesh that is too fine catches soot and creosote and clogs, especially on stoves run cool. Mesh that is too open lets animals in.

Some towns and insurers have their own spark arrestor requirements, and a few appliance manuals specify caps without screens. Check the appliance's instructions and your local rules before choosing a mesh.

Materials

Galvanized steel caps are the least expensive and rust in a few seasons on a busy flue. Stainless steel resists rust and is the usual choice for long service. Copper is used where appearance matters, for example on a landmarked facade, and weathers to a dark finish. Whatever the material, the fasteners should match it, so a stainless cap is not held on with plain steel screws that rust out.

Appearance matters on brownstones and rowhouses where the chimney top is visible from the street. A low-profile cap in a dark finish reads as part of the chimney. In a historic district, a quick check on what the preservation body allows can save a second installation.

Signs the cap you have does not fit

Many chimneys already have a cap that was the wrong size from the start. The signs are easy to spot from below or from inside. A cap sitting crooked, or one that rattles and hums in wind, usually has legs that never gripped the tile properly. Caps that blew off in a storm often had the same problem. A cap that sits very low over the flue, or whose screen is visibly black and clogged, can cause smoke in the room or slow draft at the boiler.

Rust streaks running down the brick from the top point to a galvanized cap reaching the end of its life. Any of these is a reason to measure and replace rather than reattach.

Why the installer should measure

Every measurement above is taken on top of the chimney, often on a steep roof, two or three stories up. We do not recommend owners climb up to do it. Photos from the ground or a neighbor's window can show how many flues there are and roughly what shape the top is, but they cannot give dimensions.

An installer measuring in person also sees what you cannot: a cracked tile top that will not hold a clamp, a crown that needs repair before a cap goes on, or a flue that serves a gas appliance and needs a specific cap. Caps installed by our own crews are measured and fitted on the same visit wherever possible, with a free written estimate first.

A few details help the installer arrive with the right cap or the right questions.

• How many flues the chimney has, and what each one serves
• Whether any flue has a stainless liner (often listed on an inspection report)
• Photos of the chimney top from the ground, taken with zoom
• Any recent inspection report with flue sizes
• Whether the building is landmarked or in a historic district