Choosing Brick and Mortar for a Chimney Stack

Choosing brick and mortar for a chimney stack means naming three things before anything is ordered: the brick grade, the brick type, and the mortar type. The stack above the roof stands fully exposed to weather above the roofline, so the choice has to suit that exposure. The same materials do not belong in the firebox, where heat, not weather, sets the requirement.

What brick and mortar does a chimney stack need?

For a new exposed stack, the default choice is a clay facing brick in Grade SW, in Type FBS, laid in Type N mortar. Grade SW gives high resistance to damage from cyclic freezing and thawing. Type FBS is the general-use facing brick. Type N is the normal-use mortar for above-grade brickwork. That combination suits a typical new stack, and the project specification confirms it or changes it where the exposure, the existing brick, or the structure calls for something else.

Which brick grade suits an exposed stack?

ASTM C216 gives facing brick two grades. Grade SW, Severe Weathering, is brick intended for use where high resistance to damage caused by cyclic freezing and thawing is desired. Grade MW, Moderate Weathering, is brick intended for use where moderate resistance to cyclic freezing and thawing damage is permissible. The stack above the roof is the most exposed part of the chimney: every face sees weather, and the top courses sit where water collects and freezes. That exposure is why Grade SW is the default for a new stack in a climate with freezing and thawing.

ASTM C216 has no negligible-weathering grade. A brick graded only for negligible weathering does not belong on an exposed stack. Where the stack must match an existing chimney, the grade still has to be checked: matching colour with a lower grade trades a short-term visual match for a shorter service life.

Which brick type should you specify?

ASTM C216 also gives facing brick three types. Type FBS is brick for general use. Type FBX is brick for general use where a higher degree of precision and more precise size and dimensioning are required. Type FBA is brick with more variation in size and texture, made to produce greater architectural effects.

Type FBS suits most chimney stacks. Type FBX suits a stack where tight joints and precise coursing matter, for example where the new stack must align course for course with existing brickwork. Type FBA suits a stack where varied texture is part of the intended look. The type changes appearance and dimensional control. It does not change the weathering grade, so type and grade both appear on the order.

Which mortar type suits the stack?

Mortar for brickwork is specified to ASTM C270. The Brick Industry Association recommends Type N mortar for normal use, including most veneer applications, and directs the reader to Technical Note 8B for selection. The four main mortar types differ in minimum average compressive strength under the C270 property specification, measured at 28 days on laboratory-prepared mortar.

The proportions are specification reference values. Bagged mortar follows the manufacturer's formulation for the named type, so the type name on the bag is the check, not a site-mixed ratio. Type S at 1800 psi is 2.4 times the Type N minimum of 750 psi, and Type M at 2500 psi is about 3.3 times it. Those ratios compare specification minimums only. They do not make the strongest mortar the right mortar: the mortar has to suit the brick it joins and the exposure it faces, and the project specification makes that call.

What changes the default choice?

Three conditions can move the choice away from Grade SW, Type FBS, and Type N. Each change belongs in the project specification, not in a verbal instruction on site.

First, a severe exposure or a structural requirement can call for Type S mortar, which gives higher bond strength for wind and movement resistance. Second, soft or historic brick can call for a softer mortar, because a mortar much harder than the brick can damage the brick when the wall moves or when moisture freezes in the joint. For an old stack, the existing brick and mortar should be identified before any mortar is chosen. Third, a repair that must match existing brickwork can set the size and appearance, while the grade still has to suit the exposure. A visual match that drops the grade is a poor trade.

Brick size also belongs in the choice record. The measuring guide uses the Brick Industry Association wall-area method: modular brick at a 3/8 in joint gives 675 brick per 100 ft2, and standard brick at the same joint gives 655 per 100 ft2. The size on the order has to be the size the measurement used, or the count from the measuring step no longer holds.

What belongs in the firebox instead?

The firebox uses different materials from the outer stack. ASTM C1261 covers brick intended for use as the lining in the fireboxes of residential fireplaces. Firebox brick is typically installed using ground fire clay mortar or refractory mortar, with joints typically 1/8 in. (3.2 mm) thick. That thin joint and heat-resistant mortar suit direct contact with fire.

Do not carry the stack choice into the firebox. Facing brick and Type N or Type S mortar are chosen for weather exposure, not for direct flame contact. Likewise, firebox brick is a lining material, not a substitute for the facing brick of the outer stack. The flue liner and the crown or cap slab are also separate components with their own materials, and this guide does not choose them.

What should you record before ordering?

• Brick specification: clay facing brick, ASTM C216, grade and type, for example Grade SW, Type FBS.
• Brick size and colour, checked as a sample against the existing brickwork where the stack must match.
• Mortar specification: ASTM C270 type, for example Type N, with the bag or mix source named.
• Exposure and condition notes: freeze-thaw climate, any structural requirement, and whether the brick is soft or historic.
• Scope boundary: the outer stack only, with the firebox, flue liner, and crown or cap slab excluded from this choice.
• Project specification reference and the person who confirmed any change from the default choice.

Common choosing mistakes

• Ordering by colour alone, with no grade, type, or mortar type on the order.
• Using a negligible-weathering brick on a fully exposed stack.
• Choosing the strongest mortar on the table for an ordinary stack.
• Using stack mortar and facing brick inside the firebox.
• Using firebox brick as the facing brick of the outer stack.
• Matching colour while dropping the weathering grade.
• Changing the brick size after the stack has been measured, so the count no longer holds.
• Leaving a change from the default choice as a verbal instruction instead of a specification entry.

Safety and professional scope

Choosing brick and mortar 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.