Outdoor Fireplace Dimensions That Draw Without Smoking

The Opening Sets Everything Else

Every other dimension follows from the size of the opening, width times height. EPA calls the venting system the engine that drives the burning process, and in a masonry fireplace that engine is sized to the opening. A big, showy opening on a short, narrow chimney is the classic outdoor fireplace that smokes.

The opening also sets the hearth. Under section R1001.10 of the 2023 Florida Building Code, Residential, the hearth extension runs at least 16 inches in front of the opening and 8 inches past each side; once the opening reaches 6 square feet, those become 20 and 12 inches.

Firebox Depth

Section R1001.6 sets a minimum firebox depth of 20 inches for a masonry fireplace. A shallow firebox puts the fire close to the opening, and the smoke spills out of the front instead of turning up into the throat.

The Rumford design is the exception the code allows: a tall, shallow firebox with angled sides, at least 12 inches deep and at least a third as deep as the opening is wide, with the throat at least 12 inches above the lintel and at least one-twentieth of the opening's area.

Throat and Smoke Chamber

Above the fire, the passage narrows and then funnels into the flue.

• The throat sits at least 8 inches above the top of the opening and is at least 4 inches deep.
• Every part of the passage above the firebox, throat and smoke chamber included, is at least as large in cross section as the flue.
• The smoke chamber is no taller than the inside width of the opening.
• Its walls lean no more than 45 degrees from vertical when rolled or sloped, and 30 degrees when corbeled.

Flue Size, With an Example

Section R1003.15 gives the simplest sizing rule. A round flue needs at least a twelfth of the opening's area, a square flue or a rectangle less than twice as long as it is wide needs a tenth, and a rectangle twice as long as wide or more needs an eighth.

Take an outdoor fireplace with an opening 36 inches wide and 30 inches high: 1,080 square inches. A square flue needs at least 108 square inches. The code's clay liner table gives a 12 by 12 inch nominal liner 102 square inches, too small, and a 13 by 13 inch liner 127 square inches, which passes. The code's second method sizes the flue by chimney height as well.

Chimney Height Outdoors

Height adds draft. Section R1003.9 wants the chimney at least 3 feet above a roof it passes through and at least 2 feet above any part of a building within 10 feet. A freestanding outdoor fireplace near the house, a pergola or a lanai roof is measured against those structures, which often means a taller chimney than the drawing first showed.

Factory-Built Units Come Proportioned

A factory-built outdoor fireplace arrives with its proportions fixed and tested. One outdoor wood-burning model comes in 36 and 42 inch sizes, and its manual gives the hearth extension for each, along with its own chimney height rule that matches the code's 3 feet and 2 feet within 10 feet. Follow the manual rather than the masonry tables for those units.

Designing or checking the proportions of a masonry outdoor fireplace is part of our outdoor fireplace work. A family-owned, local company with experienced masons does it, with a free inspection and a written estimate. Proudly American builders measure the opening and the flue before anything else, because a Proudly American fireplace should draw.