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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

Roof and Chimney DiagramTo 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

Level Measuring Roof PitchThe 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.

Roof Pitch Examples

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

Different Measuring Terms