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Chimney Cap Measuring Guide

Chimney Cap Measuring Guide

An improperly sized chimney cap is a lot like an ill-fitting hat; it can't do its job properly and tends to look strange rather than stylish. Fortunately, determining what size chimney cap you need isn't as difficult as it seems. This guide will take you through every step of what is actually very simple process.

Identifying Your Flue Type

The kind of flue you have dictates what chimney caps you can use and, consequently, the measurements you'll need to select the right one. Below is some information about the most common flue constructions. If you're not sure what type of flue you have, just take a look inside your chimney and pick the description that best matches what you saw.

Chimney Cap Measuring Guide
Masonry Chimneys

Single-Flue Chimneys with Extended Flues — If your chimney has a single flue that extends above its crown, this is the type of system you have. Extended flues are amongst the most common and easiest to cap - chimney caps for this kind chimney can be directly attached to the flue with clamps or screws.

Single-Flue Chimneys without Extended Flues — This kind of chimney features a flue that is completely flush with its crown. Capping one of these chimneys requires either a standard single-flue chimney cap with legs or brackets or a top-mounted model.

Single-Flue Chimneys with Flexible Metal Liners — In these systems, the smoke from your fireplace, insert, or stove is vented through a flexible liner housed inside your chimney rather than the through the chimney itself. It's unlikely that a system like this would be installed without a chimney cap, but if you look into your chimney and see a soft round metal tube, then that's exactly what you've got. When capping this kind of chimney, you have to use a slip-in model.

Multi-Flue Chimneys — A multi-flue chimney, is as you've probably guessed, a chimney that houses more than one flue. This kind of venting system requires a top-mounted chimney cap regardless of whether the flues are extended or not.

Chimney Cap Measuring Guide

Factory-Built Round Metal Chimneys

Single-Wall Metal Chimneys — If you have an older wood burning stove, there is a chance it is venting using single wall stovepipe. Single-wall chimneys are literally just metal tubes; they don't contain any insulation or other internal features. This type of chimney is no longer acceptable by code and is becoming less common. When it comes to capping, they require products specifically designed to work with single-wall pipe.

Double-Wall Air-Insulated Chimneys — This kind of chimney features an inner pipe and an outer pipe separated by open space and uses air to cool the flue. Because they are so specialized, it's best to use a chimney cap produced specifically for your brand of double-wall air-insulated vent pipe when possible.

Double-Wall Solid-Pack Insulated Chimneys — Double-wall solid-pack insulated chimneys also feature an inner and outer wall, but use insulation to separate the two. In some instances the insulation is left exposed, while in others it is covered with a metal cap. Both varieties require a chimney cap specifically designed for that type of double-wall solid-pack insulated pipe. As with air-insulated models, it's best to use a cap produced by the same manufacturer as your pipe.

Triple-Wall Air-Insulated — These work just like double-wall air-insulated pipes. They feature three concentric pipes separated by air and should be covered with a triple-wall chimney cap made by the same manufacturer whenever possible.

Triple-Wall Solid-Pack Insulated Chimneys — Triple-wall solid-pack insulated pipes feature three walls separated by some kind of insulation. You'll find models that leave the insulation exposed as well as models that cover it with a metal cap. This kind of chimney requires a cap specifically designed for triple-wall insulated pipe and, as with other types of metal chimney pipe, it's best to use one produced by the same manufacturer.

Flue Shape

It's important to know how your flue is constructed when selecting a chimney cap, but you also need to be aware of your flue's shape. Flues can be round, oval, square, or rectangular and a cap that's appropriate for one shape isn't going to be suitable for another. The shape of your flue can limit your chimney cap options and will, of course, affect the measurements you need to determine a proper size.

Understanding What to Measure

Once you know what kind of flue you have, you're ready to take some measurements. To learn more about the information you'll need, locate your flue type in the list below.

Measuring for Masonry Chimneys

1. Single Flue Chimneys with Extended Square or Rectangular Flues: If you have this type of chimney a chimney all you need to do is measure the length and width of the outside of your flue. Then you can simply search for a chimney cap designed to fit those dimensions.

2. Single Flue Chimneys with Square or Rectangular Flues that are not Extended: The measurements you'll need if you have one if these depends on what kind of chimney cap you've decided to get.

  • Chimney Caps with Legs or Brackets — If you want to install a chimney cap with legs or brackets you'll need to measure the outside width and depth of your flue. Then, as when looking for a chimney cap for an extended flue, you can search for a model designed to fit your specifications.
  • Top-Mounted Chimney Caps — Measuring for a top-mounted cap that attaches directly to your crown is slightly more complicated. Start by measuring the entire length and width of your chimney. Then measure the length and width of the area that encompasses your flue. And, finally, measure the height of your flue. Now, depending on your goals, you can either choose to get a chimney cap big enough protect your entire chimney from weather damage or, if you're looking to save money, you can get a one that will cover just the area surrounding your flue. Regardless of what kind of coverage you want, though, your cap must be at least five inches taller than your flue.

3. Single Flue Chimneys with Round Flues: There are two different types of chimney caps suitable for round flues. Each one requires different measurements.

  • Chimney Caps that Fasten to the Flue Exterior — If you want this kind of cap all you need to do is measure the outside diameter of your flue. Then you can simply look for a chimney cap with the same diameter.
  • Chimney Caps that Slip into the Flue — Finding a chimney cap for this type of flue is also very simple. Just measure the inside diameter of your flue and search for a cap with that diameter.

4. Single Flue Chimneys with Flexible Metal Liners: The inner diameter is the only measurement you need when capping a metal chimney liner. Once you have that number you can search for a slip-in chimney cap of the same size.

5. Single Flue Chimneys with Oval Flues: You need to measure the outside length and width of your flue in order to cap an oval chimney termination. Then you can search for a chimney cap designed to fit an oval opening of those dimensions.

6. Multi-Flue Chimneys: Chimneys with multiple flues require top-mounted chimney caps regardless of whether any of their flues are extended or not. The measurements you need for this kind of chimney are essentially identical to those required if you're buying a top-mounted cap for a single flue. You need the length and width of your chimney, the length and width of the smallest area that will encompass all of your flues, and the height of your tallest flue. Then, depending on your priorities, you can get a cap large enough to protect your entire chimney or a more cost-effective model that only accommodates your flues. Just remember to make sure your cap is at least five inches taller than your tallest flue.

Measuring for Factory-Build Round Metal Chimneys

1. Single-Wall Metal Chimneys: For this type of chimney you need a chimney cap that fastens to the flue exterior. The only measurement required for this kind of cap is your pipe's outside diameter. Once you have that you can just pick a model with the same dimensions. 

Chimney Cap Measuring Guide

2. Double-Wall Air-Insulated and Solid-Pack Chimneys: You need to know both the inner and outer diameter of your pipe when selecting a chimney cap for any type of double-walled venting system. The diameter of the chimney cap you choose needs to be the same as your pipe's inner diameter; a pipe with a seven-inch inner diameter, for example, requires a seven-inch chimney cap. The outer wall of your pipe also needs to be covered by your cap, but it doesn't matter if your cap extends beyond the edges of your pipe.

Chimney Cap Measuring Guide

3. Triple-Wall Air-Insulated and Solid-Pack Chimneys: Even though these chimneys have three walls, you only need to know the diameter of the inner and outermost ones. Your pipe's inner diameter is the number you need to match when selecting a chimney cap; a pipe with a six-inch inner diameter, for example, requires a six-inch chimney cap. The outer wall of your pipe also needs to be covered by your cap, but it doesn't matter if your cap extends beyond the edges of your pipe.

Chimney Cap Measuring Guide

Measuring for Specialty Chimney Caps

Some specialty chimney caps require measurements other than the ones listed above. If you're planning to purchase a non-standard model, the following instructions will tell you how to get the information you need.

1. Caps Designed to Increase Draft: Because this type of chimney cap differs from standard models in the way the top is constructed rather than the way it connects to the flue, you can usually measure for them the same way you would a regular cap. This holds true even if your installation requires an adaptor.

2. Outside-Mount Chimney Caps (for Masonry Chimneys): Because masonry chimneys are hand-built, many outside-mount chimney caps are made to order. In those instances it's best to talk to your retailer before taking any measurements. If, however, you want to buy a stock model, you'll need the length of each of your chimney's exterior walls and the height of your tallest flue. As when selecting any kind of cap, it's crucial that your cap's height be at least five inches taller than your tallest flue.

3. Chimney Cap and Damper Combinations (for Masonry Chimneys with Square or Rectangular Flues): For this type of cap, you'll need to measure the inside length and width of your flue. Then you can simply search for a model that will fit those dimensions.

But What If I Don't Have a Flue?

So you've hauled out your ladder and climbed up to the roof, ready to get down to some serious flue measuring, only to discover that your masonry chimney does not, in fact, have a flue. Don't worry. It's still possible for you to have a chimney cap. You can get a standard single flue cap and attach it with mounting brackets or buy a multi-flue model. Regardless of which type you want, you need to start by measuring the length and width of your chimney's opening. Then measure the length and width of your entire chimney.

Now you've got a choice. If you want protect as much of your chimney as possible, you should opt for a largest chimney cap available that will be able to sit on your chimney. If, however, you're more interested in saving money, you can get a cap that is only large enough to cover your chimney's opening. Just remember that, no matter what approach you choose, you still need to make sure that your chimney cap is at least five inches taller than your chimney's crown.

A Few Measuring Tips

Measuring for a chimney cap might seem like a difficult task, but the hardest part is actually determining what you need to know. Now that you've done that, you'll find that all of the steps involved are really quite easy - except, perhaps, for dragging the ladder out of the garage. Here a few more tips for making the measuring process as quick and simple as possible:

1. Take a pencil and paper with you when you go up to your roof to take measurements. Nothing is more frustrating than winding up with the wrong size chimney cap because of a misremembered measurement.

2. Measure at least twice. It might seem tedious, but making sure your measurements are accurate before you place an order will save you a lot of work later.

3. If your chimney system doesn't match any of the descriptions provided in this guide or you're still unsure of what to measure, ask an expert. They'll be able to help you determine what you need to do.

About the Author

Collin Champagne

With over 13 years in the industry, Collin is a National Fireplace Institute (NFI) certified technician and managed content for the eFireplacestore and eCanopy brands. He has achieved the highest NFI certification possible as a Master Hearth Professional and is certified in all three hearth appliance fields: wood, gas, and pellet. With experience with sales and in-field installations, his expertise shines through his technical knowledge and way with words.

Customer Q&A with Product Specialists

Andy G. from nashville asked:
What is the size of the shut-off on fireplace piping?
What is the size of the shut-off on fireplace piping?
Sizes of valves and shut-offs vary depending on the type of pipe and the particular application.
Answered by: Tyler M. - NFI Master Hearth Professional


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