Chimney Brick Mistakes to Avoid

A chimney brick mistake usually shows up after the order is placed or after the first courses are laid. By then the fix means a second delivery, a second lot with a different colour, or bricks pulled back out of the stack. Every mistake below can be caught on paper first, with the sibling pages already set out.

Which chimney brick mistakes cost the most?

The costly mistakes fall into three groups. A wrong grade or mortar damages the stack itself. A wrong measurement or rate orders the wrong number of bricks. A missing waste basis or a combined order line leaves the job short or buys a whole extra pack. Check the product choice first, then the measurement, then the order record, in that order.

Did you choose the wrong brick grade?

An exposed stack stands above the roofline on all four faces, so its brick must suit severe weather exposure where freezing and thawing cycles occur. ASTM C216 gives two grades for facing brick. Grade SW covers severe weathering, where high resistance to damage from cyclic freezing and thawing is desired. Grade MW covers moderate weathering, where moderate resistance is permissible. A MW brick on a stack that needs SW brick is a durability mistake, and no order quantity fixes it.

Record the grade and the type on the order, next to the size and the sample reference. The types are FBS for general use, FBX for higher precision, and FBA for architectural variation. C216 has no NW grade, so a grade written as NW on an order is a sign that the specification was guessed. Check the grade against the project specification before ordering, because the specification controls where it names a grade.

Did you pick the strongest mortar by default?

Stronger mortar is not automatically better mortar for a chimney stack. ASTM C270 sets minimum average compressive strengths at 28 days of 2500 psi for Type M, 1800 psi for Type S, 750 psi for Type N, and 350 psi for Type O. Type S is 2.4 times Type N on those minimums, and Type M is about 3.3 times Type N. A mortar harder than the brick leaves the brick to take the movement and moisture stress, and the brick face can spall while the joints stay intact.

The Brick Industry Association guidance in Technical Note 8 uses Type N mortar for normal use, including most veneer applications. Type S belongs where the project specification calls for higher strength, such as severe weather exposure, and Type M belongs where the specification calls for it. The choice is recorded as a mortar type, not as a strength target. Where the stack uses soft or historic brick, the mortar must suit that brick, and the project specification settles the choice.

Did you put stack materials in the firebox?

The outer stack and the firebox are different jobs with different materials. Stack brick and stack mortar do not become firebox materials because they sit in the same chimney. ASTM C1261 covers brick intended for use as the lining in the fireboxes of residential fireplaces. Firebox brick are typically installed using ground fire clay mortar or refractory mortar, with joints typically 1/8 in. thick.

Keep the two material lists apart on paper before anything is ordered. A single combined list invites the wrong brick into the firebox and the wrong mortar into a heat-contact joint. The firebox choice, the flue liner, and the crown each follow their own specification, and none of them is settled by the stack brick order.

Did you measure from the wrong datum or use one height for all faces?

Two measurement mistakes change the count on almost every stack. The first is the datum. A height measured from the ground and a height measured from the roofline describe different jobs, and only the above-roofline height belongs in the stack count. Record the datum with the heights, so a second person reading the ledger measures the same stack.

The second mistake is one height for all faces on a sloped roof. Each face keeps its own height, measured face by face. The sloped-roof fixture from the measuring guide has faces of 20 ft2, 12.5 ft2, 13 ft2, and 13 ft2, for 58.5 ft2 in total. Perimeter times the downhill height alone gives 9 ft times 8 ft, or 72 ft2. That overstates the area by 13.5 ft2, which is 91 bricks at the modular rate of 6.75 bricks per ft2. The fix is a ledger with one line per face, each with its own height.

Did you merge the cap band into the main stack?

A stepped or corbelled band has a wider plan than the main stack, so it needs its own ledger line. The fixture stack is 2 ft by 2.5 ft in plan and 7 ft high, which gives a 9 ft perimeter and 63 ft2. The cap band is 2.5 ft by 3 ft in plan and 1 ft high, which gives an 11 ft perimeter and 11 ft2. Together they give 74 ft2 and 500 bricks net at 6.75 bricks per ft2.

A single perimeter times a single height for the whole stack hides the band entirely. A shoulder at the roofline hides the same way. Give every band its own perimeter and height, and add the bands only at the end of the ledger.

Did you deduct the flue opening or count the crown in bricks?

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. Deducting it understates the count for no reason.

Count no crown, cap slab, mortar, or flue liner in the brick pack count. The crown or cap slab follows its own specification, mortar follows its own specification and supplier quantity, and the flue liner follows its own specification. A brick count that absorbs those items cannot be checked against a delivery of bricks, and a delivery check needs bricks only.

Did you apply the wrong size rate?

The coverage rate belongs to one brick size at one joint width. The Brick Industry Association wall-area method 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 at the same joint, which is 6.55 bricks per ft2. On the 74 ft2 fixture stack, the standard rate gives 484.7 bricks, or 485 bricks net, which is about 15 bricks fewer than the modular count of 500 bricks.

Table 4 values exclude waste and assume running or stack bond. A rate taken from a different size, a different joint width, or a different bond does not hold for this stack. Record the size, the joint width, and the bond with the rate, and change the rate whenever any of the three changes. The Association figures must be used with good technical judgment, and final decisions rest with the project team.

Did you order the net count with no waste basis?

Ordering the net count exactly is the most common ordering mistake on this page. Cuts and breakage take bricks that the net count does not include. The Association states a general rule of at least 5 percent added to net brick quantities for breakage and waste. On the fixture stack, 499.5 bricks at 5 percent gives 524.475 bricks, or 525 bricks. Ordering 500 bricks leaves the order 25 bricks short of the waste-covered count before any cut is made.

Write the waste basis on the order record, with what it covers: cuts, breakage, or both. An unstated percentage cannot be audited later, and a percentage carried from another job cannot be checked against this stack. The ordering page owns the waste step and the pack rounding that follows it.

Did you combine wythes or round the combined total?

A wythe is one brick thickness of the stack wall. The inner wythe has a smaller perimeter than the outer wythe, because it sits inside the outer wall thickness. Doubling the outer wythe count overstates the inner wythe, and ordering only the outer count leaves the inner wythe short. Give each wythe its own line. The line carries its own perimeter and area, plus a net count. Waste and pack figures go on the same line.

Round each line up separately, and round up to whole packs on each line. On the fixture stack, the 525-brick order need at an example 500 bricks per pack rounds up to 2 packs, or 1000 bricks, which is a surplus of 475 bricks over the order need. That surplus is the cost of whole-pack rounding on a small job, and it belongs on the record before the order is placed. Ask the supplier about a split-pack policy before rounding, because it changes the delivered quantity. The 500 bricks per pack figure is an example supplier figure for this check only, not a standard.

Did you order by colour alone or mix lots?

Colour alone does not identify a brick. An order that names only a colour leaves the supplier to guess which grade and size to supply, and either can come back wrong. Record the product as clay facing brick with its ASTM C216 grade and type. Add its size and joint width, with the sample reference on the same line, as the ordering page sets out.

Order a single lot where the supplier can supply it. A second lot can shift colour visibly, and a shortfall order placed later is the usual way a mixed-lot stack happens. A colour check against the sample belongs before the order, not at delivery, when the only fix is another delivery.

Pre-order checklist

Run this checklist against the order record before buying. Every line that fails goes back to the sibling page that owns the step.

Safety and professional scope

Checking a quantity plans materials only. It does not design the chimney, size the flue, set the chimney height, or approve a rebuild or repair. Those decisions rest with the project's design team and owner, under the governing code and the project specification.

Work on a chimney stack needs safe access and fall protection. Do not climb onto a roof or lean over an edge to check a stack without the access and protection the job requires.