Chimney Length Ratios

Roof Clearance

• The overall height of you chimney is dependent on the height of your roof, the slope of your roof and the location of your chimney in relation to the roof's ridge line. Your chimney should always measure at least 36 inches in height above above the spot where it penetrates the roof. The height of the chimney must also reach 24 inches above the height of the roof at a distance of 10 feet away measured toward the nearest spot on the roof's ridge line. If your chimney is less than 10 feet away from the ridge of your roof, it should measure at least 24 inches above your roof's ridge line.

Chimney Footers

• Masonry chimneys are heavy, freestanding structures that require a concrete footer for support. The footer of the chimney is a concrete slab that distributes the weight of the chimney evenly across the ground beneath it. The footer should extend a minimum of 6 inches beyond the sides of the chimney on each side. In order to prevent freeze and thaw action from shifting the chimney, the footer should extend below the ground so that the underside of the footer rests below the average depth of the frost line in your area. In warmer climates, the footer should have a depth of at least 8 inches for a single-story home or 12 inches in two-story designs.

Flue Size

• The size of your fireplace dictates the size and height of the space inside your chimney, known as the chimney flue. According to the San Jose State University Engineering Department, the area of your chimney flue should measure at least one-10th the area of the opening for your fireplace, if you plan on having a chimney greater than 15 feet in height. Shorter chimneys only require a flue with an area equal to one-eighth the size of the opening for the fireplace.

Considerations

• The minimum height requirements for chimneys are designed to ensure that the roof around the top of the chimney does not force air down the chimney. Building your chimney closer to the ridge line of your roof reduces the amount of heat lost through the walls of the chimney. Building a well-insulated chimney with good clearance above the roof will improve the draft of your chimney, allowing it to vent smoke effectively. This helps reduce the buildup of creosote on your chimney walls over time and reduces the chances of a chimney fire.