Choosing a chimney cap can be an overwhelming task but fortunately determining what size chimney cap you need isn’t as difficult as it seems. Grab a tape measure, pencil, paper, and a ladder. You will need these items to get up on your roof to physically inspect and measure your masonry chimney flue and crown.
Today I will be covering how to determine which flue type you have and what measurements you will need to take before you can choose a chimney cap.
What to Measure for a Chimney Cap
A single flue chimney with an extended flue is going to be the most common chimney style. It will also be the easiest cap to measure for; you will simply measure the outer width and length of the flue extending from the chimney. Single flue caps will most often mount with pressure screws and a flue size range that it will mount to. For example an 8″ x 8″ cap may mate to flues from 6″ to 8.5″ in both width and length.
If your flue is oval or round you will need to use a top mount style cap. Measure the total crown width and length, length and width of the area surrounding the flue, and the height of the flue if extended.
Now if you have a single flue chimney with a flue that is either flush or doesn’t extend high enough above the chimney to allow for a flue mount cap, gather the outer length and width of the flue. You can still use a flue mounted cap you will just need to use a chimney cap leg kit that will allow the cap to be held into place from the inside of the flue. If you live in a very windy are I would suggest converting flue mount cap to a crown mount cap instead with mounting brackets.
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 the total crown width and length, length and width of the area surrounding the flue, and the height of the tallest flue.
Measuring the height of the tallest flue is to make sure that the cap is at least five inches taller than your tallest flue as to not interrupt the draft of your chimney.
The Multi-Flue cap can be sized to fit just around the flues or to cover the entire chimney crown. You can save on the caps cost by sizing it to the minimum needed to cover the flues (especially if you are using an expensive material like copper). But for the best looking cap and the added protection of covering the chimney crown most people choose a full size cap.
I hope you have found this information helpful.