If you decide to attempt the construction of a masonry fireplace from scratch, Tables 1 and 2 will aid you in the proper design. Masons and engineers have evolved these recommendations after years of practical experience. Table 1 provides you with the width : depth : height ratios, while Table 2 helps you to determine adequate flue sizes.
Note: There may be slight variations from these tables depending on the geographical location of your property. Wind patterns and other characteristics may affect the draft. Check your local building codes. If any provision is required, it will indicate the procedure to follow.
Width (W) |
24 to 48 |
Height (H) |
2/3 to 3/4 Width |
Depth (D) |
1/2 to 2/3 Height |
Flue (effective area) |
1/8 W X H for unlined flue |
Throat (area) |
1.25 to 1.5 in addition to flue area |
Throat (width) |
3" minimum to 4.5" maximum |
The proportional requirements of a fireplace originate with the width of the firebox opening, as shown in Figure 1. All other dimensions for height, depth, etc., are relative to this width. While studying these tables, keep in mind a couple of general points.
Figure 1 - Fireplace measurements
- The higher you make the opening, the greater the risk of a smoky fireplace.
- The wider the fireplace opening, the deeper the firebox should be.
- Shallow fireplace openings radiate more heat than deep ones, but hold less wood or coal.
- A minimum firebox depth of 18” is recommended to prevent the danger of burning wood falling out of the firebox onto the surrounding floor.
- The damper should be placed as far forward as possible to permit greater depth for the smoke shelf.
- To allow for good exhaust of smoke and gases, make sure to provide an adequate-sized flue. If the size you want is not available, get a larger size – never get a smaller size!
Here are a sample set of fireplace design calculations. Let’s assume a 36” wide fireplace. According to Table 1, the height will be H = 3/4 W, or 3/4 X 36” = 27”. Using the same table, we find that the depth is D = 2/3 H, or 2/3 X 27” = 18”. Since we are using a rectangular flue, we find that the minimum flue area should be equivalent to 1/10 W X H, or 1/10 X 36 X 27 = 97.2 square inches. To double-check, we refer to Table 2 and find that for a 36” to 44” wide fireplace a 13” X 13” flue with an effective area of 99 square inches is recommended. Going back to Table 1, we see that the throat should be 1/4 to 1/2 larger than the flue area, making it anywhere between 125 and 150 square inches. The throat width can range between 3” and 4 1/2”.
Fireplace Width |
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Nominal or |
Inside Dimension |
Effective Area |
Inside Diameter |
Effective Area |
|
24 |
8.5 X 8.5 |
7.25 X 7.25 |
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30 to 34 |
8.5 X 13 |
7 X 11.5 |
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36 to 44 |
13 X 13 |
11.25 X 11.25 |
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46 to 56 |
13 X 18 |
11.25 X 6.25 |
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58 to 68 |
18 X 18 |
15.75 X 15.75 |
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70 to 84 |
20 X 24 |
17 X 21 |
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Fireplace Foundation:
Visualize for a moment the construction of a masonry fireplace. Brick is laid over brick, or stone over stone all the way up to the tip of the chimney. It should be clear to you that the fireplace and the chimney combined are probably too heavy for a simple floor to carry without failing structurally. For this reason, masonry fireplaces are provided with their own foundations independent from those of the rest of the house. This foundation will place the masonry load evenly to the ground without disturbing the other house components.
The criteria for the design of fireplace foundations are the same as that for the rest of the house. In cold climates they are set a minimum of 1 foot below the frost line, or as indicated by your local building code. They should not, however, be less than 18 inches deep. In addition, the foundations should over-lap the fireplace by a minimum of 6 inches in all directions.
Additional information on fireplaces
Additional information on fireplace chimney design
Additional information on smoke chambers, smoke shelves and dampers
Additional information on drying logs for fireplaces
Additional information on marble mantels for fireplaces
Additional information on how to build a masonry fireplace