How to Measure for Chimney Brick: Stack Faces, Bands, and Net Area

Measuring chimney brick means measuring the outside faces of the stack, band by band, then turning the total face area into a brick count. A chimney stack is narrow and tall, so a single length times height figure hides the parts that change the count: a wider cap band, a shoulder at the roofline, and faces of different heights on a sloped roof.

What do you measure for chimney brick?

Record four things before any counting starts: the plan size of the stack, the height of each band, the datum the heights start from, and the brick size. The plan size is the two outside dimensions of the stack, for example 2 ft by 2.5 ft. The datum is the line the height is measured from, usually the roofline at the flashing base for an above-roof rebuild. Write the datum on the ledger, because a height measured from the ground and a height measured from the roofline describe different jobs.

Record the number of wythes as well. A wythe is one brick thickness of the stack wall. The coverage rate in this guide counts one face of one wythe, so each wythe gets its own line with its own perimeter. Doubling the outside face count for a two-wythe stack overstates the inner wythe, because its perimeter is smaller.

How do you measure a straight chimney stack?

Measure the four side lengths of the stack and add them. That sum is the perimeter. Multiply the perimeter by the stack height to get the face area. For a stack 2 ft by 2.5 ft in plan, the perimeter is 2 plus 2.5 plus 2 plus 2.5, which is 9 ft.

The fixture stack is 2 ft by 2.5 ft in plan and 7 ft high above the roofline, with a corbelled cap band 2.5 ft by 3 ft in plan and 1 ft high. The main stack gives 63 ft2 and the cap band gives 11 ft2, for a total net face area of 74 ft2.

How do you measure a stepped or corbelled section?

Give every change in plan size its own line. A shoulder where the stack widens at the roofline, a corbelled cap that steps outward near the top, and a tapered section each get a perimeter and a height, and their areas add to the total. Measuring the whole stack at its widest perimeter overstates the narrower bands, and measuring it at its narrowest perimeter understates the wider bands.

The cap band in the fixture shows the rule. Its plan size is 2.5 ft by 3 ft, so its perimeter is 11 ft, and at 1 ft high it adds 11 ft2. That band is counted even though it is only 1 ft high, because its wider perimeter holds more bricks per course than the main stack.

What changes on a sloped roof?

Measure each face separately, because the roof meets each face at a different height. The downhill face stands tallest above the roof, the uphill face stands shortest, and the two side faces fall between them. A single height applied to the whole perimeter fits only one face and mismeasures the other three. Measure each face from the roof surface where it meets that face to the underside of the cap.

The per-face total is 58.5 ft2. Using the downhill height of 8 ft for the whole 9 ft perimeter would give 72 ft2, an overcount of 13.5 ft2, or about 91 bricks at the modular rate. The error comes from one height, so the fix is one height per face.

How do you turn net area into a brick count?

Multiply the net face area by the coverage rate of the brick actually being used. The Brick Industry Association wall-area method works this way: net area times bricks per square foot. Its Table 4 gives 675 modular bricks per 100 ft2 at a 3/8 in joint, which is 6.75 bricks per ft2, and 655 standard bricks per 100 ft2, which is 6.55 bricks per ft2. Those table values exclude waste and assume running or stack bond.

Net bricks = net face area x bricks per ft2

For the fixture, 74 ft2 times 6.75 gives 499.5 bricks. A fractional brick cannot be laid, so the net count rounds up to 500 bricks.

The metric check matches. 74 ft2 converts to 6.8748 m2, and 6.75 bricks per ft2 converts to 72.66 bricks per m2, which returns the same 499.5 bricks before rounding.

The same 74 ft2 at the standard rate of 6.55 bricks per ft2 gives 484.7 bricks, so 485 bricks net, about 15 fewer than the modular count. Record the brick size before applying any rate, because the rate follows the size.

Waste and ordering belong to the ordering step, and this guide gives only the illustration the source supports: the Association states a general rule of at least 5 percent added to net brick quantities. Applied here, 499.5 times 1.05 gives 524.475, so 525 bricks. That figure is an illustration of the rule, not an order quantity for a real stack.

What do you not deduct?

Deduct nothing for the flue opening at the top of the stack. The opening sits in the horizontal top of the chimney, while the measured area is the vertical faces, so the opening takes no area from any face. The same holds for the cap slab that covers the top: it is horizontal, and it is not part of the face area.

Deduct nothing for mortar joints either. The coverage rate already includes the 3/8 in joint, so subtracting joint area a second time would undercount the bricks. Flashing at the base takes no deduction from the measured bands, because the bands are measured from the flashing line upward, not through it.

Common measuring mistakes

• Measuring the whole stack at one height on a sloped roof.
• Measuring from the ground when the job rebuilds only the stack above the roof.
• Counting the widest band perimeter for the full stack height.
• Doubling the outside face count for a two-wythe stack instead of measuring the inner wythe.
• Deducting the flue top opening from the face area.
• Applying the modular rate to a standard brick, or any rate to an unrecorded brick size.
• Subtracting mortar joint area that the coverage rate already includes.
• Recording one total without the band lines, so a later checker cannot find a missed band.

Safety and professional scope

Measuring a chimney stack plans quantities only. It does not design the chimney, select the flue liner, set the chimney height, or approve a rebuild. Those decisions rest with the project architect, engineer, and owner, as Technical Note 10 itself states for its estimating figures.

Measure from safe footing and with proper access equipment. Do not climb onto a roof or lean over an edge to reach a stack measurement without the access and fall protection the job requires.