Height of Chimney Flue Vs. Cap

Building and Fire Safety Codes

• Local and state jurisdictions mandate requirements for chimney flues and caps. The requirements vary by jurisdiction, but many adhere to the Underwriters Laboratories international codes related to chimney flues and caps, and the National Fire Protection Association’s codes. Check with your local jurisdiction’s codes for chimney building and covering to ensure the chimney and cap is the correct height.

Manufacturers

• Prefabricated chimney flues and caps that adhere to codes related to safety and construction are marked with a UL and NFPA number on them. The markings verify the cap is compliant. Although each jurisdiction’s building and fire safety codes vary, manufacturers are required to comply with Underwriters Laboratories international mechanical and residential codes.

Chimney Height

• To adhere to federal regulations for chimney building and installation, a chimney must be at least 3 feet higher than its entrance point on the roof. It must also be 2 feet higher than any structure within 10 feet of the chimney. This allows the chimney to ventilate air properly without interference from surrounding objects or the roof itself. The height of the chimney reduces the chance of a potential roof fire from sparks going up the chimney. The overall height of a chimney with an average-sized firebox should be at least 15 feet.

Cap Height

• Chimney caps come in a variety of sizes and shapes. Some are designed to improve the look of the chimney, and others are designed to cover several chimney flues. As a general rule, the cap should measure 5 inches higher than the tallest flue. Many home improvement stores carry standard sizes of flue caps that fit a variety of chimney flues.

Other Considerations

• Along with selecting the appropriate size of a chimney cap, consider outside environmental conditions. Caps are manufactured for different environments such as strong winds, heavy rains and to keep birds and other animals from getting trapped in the chimney. Strong winds can create downdrafts, and heavy rain can affect a furnace's or boiler's functionality. Select a cap best suited for the outside elements.