How to Measure for a Chimney Cap

A proper fitting chimney cap is essential to the structural integrity and overall performance of a fireplace chimney. An ill-fitting cap or a chimney that doesn't have a cap at all leaves room for birds, leaves and other foreign materials to enter the chimney system and cause structural damage and hinder the chimney's performance and draft.

Experts say that every chimney should be capped. With that in mind, there are many different sizes and types of chimney flues and just as many different types and sizes of chimney caps to fit them. Follow these guidelines to choose the correct chimney cap for you.

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Masonry Chimneys

Single Flue Chimneys With Extended Flues (most common)

• These chimneys have flues that extend out of the chimney allowing you to attach your chimney cap with screws that tighten to the ceramic or clay flue. You will need to measure the width and length of the outside dimensions of the flue to get a proper fit. Most single flue caps will require you to have your measurements and will fit within a range of sizes. Depending on the length of the screws the cap will fit a range of different flue sizes.

Single Flue Chimneys Without Extended Flues

• These chimneys do not extend (flush with crown or chimney top) and will require you to use a bracket that will extend down into the flue and grip or bolt to the sides. Leg Kits are good for easy installation and low wind areas but will not hold up in high wind areas, for these areas you will want to use the more secure brackets. You will need to measure the width and length of the opening itself. Most single flue caps will require you to have your measurements and fit within a range of sizes. Caps with screws on the outside will still work well even though you are fitting them to the inside opening of the flue itself. You will need to measure the inside dimensions of the flue opening (length and width). Next you will match these measurements to the range of sizes given by a specific chimney cap and order the corresponding leg kit or brackets.

Multi Flue Chimneys With or Without Extended Flues

• In this application the extended flue does not matter. With or without the flue extending you will need to measure the total outside width and length of the combined flues as well as how high the highest one extends out of the chimney where applicable. You will need to then choose a cap that will not only cover the flue's but also fit your chimney and clear the top of the flue's with the lid. The screen height is very important since you are mounting this to the chimney crown. You want at least 5" of clearance from the top of the flue to the top of the cap to provide a proper draft.
  
  An example of measuring proper screen height: If your highest flue sticks up 3" from the chimney crown, then your screen height needs to be at least 8". This will give you 5" of clearance between the top of the flue and the chimney cap lid to provide proper drafting.

Round Metal Chimneys

There are three common types of metal chimneys, double-wall solid-pack insulated chimney pipe and double or triple-wall air-insulated chimney pipe. To determine which style you have is simple, take a look down the pipe itself. If you are looking at double-wall pipe there will be an inside pipe and an outside pipe. While looking in between the two pipes you will see either a metal cap blocking your view, an insulation type material, or nothing but air. If it is capped or has insulation in it, it is solid-pack insulated chimney pipe. If it has nothing but air it is air-insulated chimney pipe. Triple wall pipe is almost always air-insulated.

Double-Wall Solid-Pack Insulated Chimney Pipe

• First measure the diameter of the inner pipe. Next measure the diameter of the outer pipe. The inner diameter is the measurement used to order the cap. The outer pipe needs to be covered to keep out rain however it does not hurt if the outer pipe is overlapped by the cap. As long as the outer diameter pipe is covered rain can not get in.
  
  Example: Inner pipe measures 6" and outer pipe measures 8". You need a 6" cap with and bottom ring that is larger than 8".

Double-Wall or Triple-Wall Air-Insulated Chimney Pipe

• Next measure the diameter of the outer pipe. If you are working with triple wall you can ignore the middle pipe. The inner diameter is the measurement used to order the cap. The outer pipe needs to be covered to keep out rain however it does not hurt if the outer pipe is overlapped by the cap. As long as the outer diameter pipe is covered rain can not get in.
  
  Example: Inner pipe measures 6" and outer pipe measures 12 3/8". You need a 6" cap with and bottom ring that is larger than 12 3/8".