A rooftop finished with brown roof tiles that has three chimney stacks with brown clay chimney pots.

How to Measure & Install a Chimney Cap

Discover what kind of cap you need and how to mount it correctly! 

Last Updated: October 4, 2021

Chimney caps are more than just decorative accents for your roof; they’re also important safety features that prolong the life of your fireplace, prevent damage to your chimney, and keep hazards out of your home.   

Chimney caps are protective coverings that sit on top of your chimney crown. They're available in various styles to match your house and are typically made of metal, steel, or copper.

Wire mesh surrounds the sides to keep curious critters from seeking refuge in your chimney while keeping rain, snow, and debris, like leaves and sticks, out. 

Besides preventing critters and debris from accumulating in your chimney and damaging the flue, liner, and dampers, chimney caps also protect against potential fires by blocking sparks from floating up and out of your chimney and onto the roof.  

Covering your chimney is a must and measuring for the correct cap size is crucial for getting the best fit and protection for your fireplace.

To make this process easier, we’ve put together a guide that outlines all the measurements you'll need to choose the right cap for your chimney and get it installed properly!


Make sure you have a sturdy ladder that extends a few feet above your roof, so you can climb to the top of it safely.  

Grab a pair of work gloves, a power drill or screwdriver, a tape measure, a pencil, and a piece of paper.

We recommend measuring everything at least twice to guarantee all of your numbers are accurate. 


Once you’re on the roof, you’ll need to determine what kind of flue you have. The flue will be a clay or metal tube that extends out from the top of the chimney. 

 If you have a traditional, masonry-style fireplace, you’re likely working with a clay flue. Newer, factory-built fireplaces typically have metal flues.   

Single-flue chimneys are the most common style and require one cap. However, if your chimney has more than one flue, you’ll need to decide if you’re going to purchase multiple single-flue caps, or one, large cap that covers the entire area surrounding the flues.

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Before you start measuring, take note of your flue’s shape. If you have a round flue, you'll measure the diameter instead of length and width.  

How to Measure a Single-Flue Chimney

Standard, single-flue chimneys typically require an outside-mounted cap, which fits around the top of the flue —similar to a shoebox lid.   

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Proud, or protruding, liner

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Flush liner

  1. First, determine if your liner is proud, or protruding, from the cement crown, or if it’s flush.   
  2. If your liner is proud, you’ll measure the outside length, width, and height of the liner.   
  3. If your liner is flush with the cement crown, measure the inside length, width, and height of the liner.   

How to Install a Single-Flue Chimney Cap

Single-flue caps are relatively easy to mount because they’re designed to be fastened directly onto the chimney liner.  

  1. To securely install your single-flue cap, position it on top of the flue. 
  2. Screw the cap into place by hand turning the included bolts. Once the cap is secure, make an extra quarter-inch turn on each bolt to lock it down.

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If you don’t have a chimney liner, or if your liner is flush with the cement crown, the chimney cap will be a compression fit.

How to Measure a Multi-Flue Chimney

Chimneys with multiple flues require a top-mounted cap. You’ll need to take five basic measurements to find the right fit.  

  1. First, measure the length and width of all the flues together, from the outermost sides.   
  2. Next, measure the width and length of the entire cement crown. To ensure the cap fits properly, make sure it's big enough to cover the area surrounding the flues, but doesn't hang over the sides of your cement crown.   
  3. Finally, measure the height of your tallest flue.  

It’s important to note that top-mounted caps have 1½-inch-long flanges along the outside that are fastened to the cement crown.

You’ll want to factor the extra length into your total measurements to make sure the chimney cap sits at least 5 inches above your tallest flue.  

If you can’t find a pre-manufactured chimney cap in the exact dimensions you need, you’ll need to custom-order one.

Call our team of NFI certified experts at 800.919.1904 for help building your custom cap!

How to Install a Multi-Flue Chimney Cap

  1. To install your multi-flue, top-mounted cap, position it on top of the chimney, aligning the edges with the sides of the cement crown until it's centered.  
  2. Find the holes along the bottom flange of the cap, so you know where to drill pilot holes into the crown. It's crucial to drill pilot holes before screwing your cap into place. If you don't, you risk cracking the cement crown.  
  3. Secure the cap in place with the included screws.  

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If you’re looking for maximum protection against weather-related damage, you can choose a large cap that covers the entire top of the chimney. For a more economical solution, you can select a chimney cap that only covers the top of the flues.



If you have any questions about how to measure for or install a new chimney cap, we’d love to help! Call our team of NFI certified experts today at 800.919.1904.   


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