Measuring Guide & Tips for Common Chimney Projects

Measuring Guide & Tips for Common Chimney Projects

Common measuring tips that come in handy when planning any type of chimney installation. From general measuring tips all the way to measuring clearance to your roof line or measuring roof pitch, we have the answers for you here. Still have questions? Contact us!

Measuring for Chimney Height

To function properly, a chimney should be at least 10'-12' in overall height.

The 10-Foot/3-Foot/2-Foot Rule: This rule is frequently misunderstood or ignored, and many people have drafting problems with their appliances because of that. This rule is fairly simple and must be followed in every chimney installation. This applies for all masonry or Class A pipe chimneys.

Where it projects above the roof, the chimney should be at least 3 feet tall, and at least 2 feet higher than anything within 10 feet of it including the roof, other buildings, & trees - this is called the 10'-3'-2' Rule.

Measuring Roof Pitch

The pitch of a roof is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. For example, a roof that slopes/rises 4 inches for every 12 inches of horizontal run has a 4-in-12 pitch. So measuring on the roof 12" horizontal, then measuring the distance to the roof, will determine the pitch of your roof.

You can also measure roof pitch safely from the inside of your attic. Using a 12" ruler or marking a 12" distance on a straight edge and holding it level. Then, on the underside of the roof in the attic, place the end of the 12" ruler against the bottom of the roof and hold it level. Now measure vertically from the 12-inch mark up to the underside of the roof. That measurement is the number of inches the roof rises in 12 inches.

Measuring Terminology & Definitions

The measuring definitions below are commonly used when measuring for your chimney installation.

  • Circumference - Measurement of the distance around a circle. Divide the circumference by 3.14 to get the diameter.
  • Diameter - Straight measurement across the center of a circle.
  • Radius - Measurement halfway across the center of a circle.
  • Outside diameter - (O.D.) The outer diameter of a pipe, including the wall thickness.
  • Inside diameter - (I.D.) The inner diameter of a pipe, measuring only the inner surface of the pipe/opening.
  • Vertical - Positioned up and down rather than from side to side.
  • Horizontal - Flat or level position.
  • Roof Pitch - Measurement of the rise of a roof, ie: 6/12 pitch. The higher the first number the steeper the pitch.


  • 1/8" = .125
  • 1/4" = .25
  • 3/8" = .375
  • 1/2" = .5
  • 5/8" = .625
  • 3/4" = .75
  • 7/8" = .875

DIY Center

(AKA ‘The Rockford Files’)

From video tutorials to product walkthroughs, we have a variety of DIY resources just for you! Click the button below to view our entire library.


Need Help?

We are here to answer all of your questions




Our 316Ti flexible chimney liner and chimney liner components have passed the rigorous testing at the Underwriters Laboratories with best in class status and are UL Listed. So if you are looking for UL listed chimney liner sold directly to homeowners, then look no further than Rockford Chimney

Customer Testimonials

Back in July you gave me excellent advice re. sizing/purchasing a flue liner for the chimney associated with my wood stove. I'd like to thank you once agai... More

—AE Northfield, MN

A few weeks ago I ordered a chimney liner system for my fireplace insert. Your salesman recommended the liner system plus the insulating blanket. I just wa... More

—AH Happy customer

Just got around to installing my new smooth flexible liner and deluxe terra cotta cap (wow). The quality of your product is second to none. Very pleased an... More

—B.B. from Massachusetts

Thanks so much for the great tech service from Dennis, very knowledgable, friendly and proffessional.Your website is the best, beats your competition by fa... More

—Bill in Knoxville

Sue and Dennis, I received the final part of my order yesterday (the chimney cap that actually fits over the chimney chase). It looks wonderfu... More


Hello, I was shopping for an fireplace insert and called RCS to ask why they were so much cheaper than my local guy, the reply was " we sell thousand of th... More

—Chris, PA.