Residential Chimneys – Design and Construction

Abstract: All residential chimneys. both for fireplaces and appliances, are designed and constructed to serve the
same basic functions. They must provide fire protection and safely convey combustion by-products to the exterior
of the structure at a rate that does not adversely affect the combustion process. Design, materials selection,
construction, and building code requirements all have a significant impact on the chimney’s potential to fulfill these
functions. Chimney height and flue area are the two most critical factors in chimney desire.

Key Words: bricks, building codes, chimneys, draft, flashing, flues, masonry, mortar.

INTRODUCTION

This Technical Notes addresses the design and construction of residential chimneys. Other Technical Notes in
this Series deal with residential fireplaces and commercial chimneys. The design of residential chimneys is
empirical and based on successful prototypes. The function of residential chimneys is to allow combustion byproducts to be conducted away from the structure safely.

GENERAL

Residential chimneys generally fall into two categories: 1) chimneys serving fireplaces, and 2) chimneys serving
appliances. While there are dissimilarities between the two types, they both serve the same basic functions. It is
worthwhile, therefore, to consider their similarities. Both are constructed of similar materials and must meet the
same building code requirements. Even though they may convey different combustion by-products at different
velocities, they both must be designed and constructed to discharge these by-products at a rate that does not
adversely affect the combustion process and to release the discharged material at a height and location that
provides fire safety.

Flues may slope to join with other flues so as to discharge through a common flue, or to achieve the desired
location of the chimney. The maximum allowable slope is 30 deg from vertical. When combining flues the main
discharge flue should be sized for the maximum combined flow from the smaller flues. Combining flues of
dissimilar systems or fuels. i.e., appliances and fireplaces, is not allowed by many building codes. Separate flues
may be incorporated into one chimney so long as minimum wall thickness requirements are met and a full wythe
of brick is laid between them and bonded to the chimney walls.

Building Code Requirements

Building code requirements for chimneys may vary on a local basis. There are, however, several that are
accepted nearly everywhere. They include:

1. Chimney wall thickness should be a nominal 4 in. (100 mm) unless no flue liner is used, in which case a
nominal 8 in. (200 mm) is required.

2. Neither chimney nor flue liner may change size or shape within 6 in. (150 mm) of either floor components,
ceiling components or rafters.

3. The minimum chimney height for fire safety is the greater of 3 ft (1.0 m) above the highest point where the
chimney penetrates the roofline, or 2 ft (600 mm) higher than any portion of the structure or adjoining structures
within 10 ft (3.0 m) of the chimney, see Fig. 1.

4. Chimney clearance from combustible material is a minimum of 2 in. (50 mm) except where the chimney is
located entirely outside the structure, in which case 1 in. (25 mm) is acceptable.

5. The spaces between a chimney and combustible material should be firestopped using a minimum of 1-in. (25
mm) thick noncombustible material.

6. All exterior spaces between the chimney and adjacent components should be sealed. This is most commonly
accomplished by flashing and caulking.

7. Masonry chimneys should not be corbeled more than 6 in. (150 mm) from a wall or foundation nor should a
chimney be corbeled from a wall or foundation which is less than 12 in. (300 mm) in thickness unless it projects
equally on each side of the wall, except that on the second story of two-story dwellings corbeling of chimneys or
the exterior of the enclosing walls may equal the wall thickness. Corbeling may not exceed 1-in. (25 mm)
projection for each course of brick protected.

Recommendations

In many situations it may be desirable to use the chimney as a structural element. This may be accomplished
within most building codes by maintaining the chimney wall thickness and adding a structural wall around the
chimney. This structural wall may be built integrally with the chimney wall. Most building codes require a minimum
of 4 in. (100 mm) of bearing. Considering all the building code dimensional requirements, the minimum wall
thickness of a lined chimney to be used as a structural component is 10 in. (250 mm) consisting of: 1) 4-in. (100
mm) chimney wall (brick), 2) 2-in. (50 mm) of noncombustible material (brick), and 3) 4-in. (100 mm) bearing
length (brick). An unlined chimney’s minimum wall thickness is 14-in. (350 mm) consisting of the same elements
as the lined chimney except that the chimney wall must be 8-in. (200 mm), see Fig. 2.

MATERIALS

Brick

The chimney, by the nature of its function, is at least partially exposed to weathering. The brick should conform to
ASTM C 216, Grade SW, or ASTM C 62, Grade SW, to assure sufficient durability. Paving brick should conform
to ASTM C 902, Class SX.

Mortar

To allow for both weathering and thermal considerations, Type N portland cement-lime mortar is recommended
for the chimney. Type S portland cement-lime mortar is acceptable, and may be necessary when the chimney is
subjected to high lateral forces such as wind loads in excess of 25 psf (1.2 kPa) or seismic loads. Where the
chimney is in contact with earth, Type M portland cement-lime mortar is recommended. The mortar used to bed
the flue liners should be able to perform well under high temperatures. Therefore, fireclay mortars are highly
recommended. Type N portland cement-lime mortar is an acceptable substitute. For a comprehensive discussion
of portland cement-lime mortar types and uses, see Technical Notes 8 Series.

Flue Liners

Flue liners should conform to ASTM C 315. They should be thoroughly inspected just prior to installation for
cracks or other damage that might contribute to smoke and flue gas leakage.

Flashing

Corrosion-resistant sheet metal flashing is required by most building codes. Quality materials should be specified
since replacement may be expensive and troublesome. See Technical Notes 7A Revised for selection of flashing
materials.

Chimney Caps

A prefabricated chimney cap similar to the one shown in Fig. 3 should be used. This type cap provides better
durability and is more easily made water-resistant than a cast-in-place cap. When a cast-in-place cap is used, it
should incorporate the same shape as the prefabricated. The thickened sides and overhangs will reduce the
potential for water penetration.

Rain Caps

Rain caps vary from sophisticated turbine type metal caps to simple slabs set above the termination point of the
flue liner. When specifying a manufactured rain cap, information regarding its effect on the gas flow through the
chimney should be obtained from the manufacturer. If the cap is metal, it should be corrosion-resistant.

Sealants

Caulking is frequently considered a means of correcting or hiding poor workmanship, rather than as an integral
part of construction. It should be detailed and installed with the same care as the other elements of the structure.
In all cases, the use of a good grade, polysulfide, butyl, or silicone rubber sealant is recommended. Oil-based
sealants should not be used. Regardless of the sealant used, proper priming and backing rope, are a must.

Ties and Reinforcement

Ties used in chimney construction should be corrosion-resistant metal ties. For a general discussion of ties and
their placement, refer to Technical Notes 28 Revised.
Reinforcing steel should conform to one of the following ASTM Standards:
1. Welded Wire-ASTM A 185
2. Steel Bar-ASTM A 615, ASTM A 616 or ASTM A 617
3. Wire-ASTM A 82

DESIGN

Design of fireplace and appliance chimneys is limited to the determination of height requirements that when used
in conjunction with proper flue sizes, detailing and construction will provide adequate draft. Building code
requirements for minimum chimney height remain in effect and must be met or exceeded.

Fireplace Chimneys

The design of residential fireplace chimneys is directly related to: 1) the area of the fireplace opening, 2) the area
of the flue liner, and 3) the height of the chimney. In most situations, the area of the fireplace opening is controlled
by considerations other than the performance of the system, such as aesthetics. The other components of the
system are usually designed based upon the desired fireplace opening.

A frontal face velocity of 0.80 ft per second (0.245 m/sec) at the fireplace opening has been accepted by the
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to be sufficient to prevent
smoke and gases from being discharged into habitable spaces. This is a minimum velocity and usually only
encountered while starting a fire. Flue liner size as a function of fireplace opening size may be obtained from
Technical Notes 19 Revised, Table 1.

Appliance Chimneys

Appliance chimneys are divided into two types those venting one appliance, see Fig. 4, and those venting two or
more appliances, see Fig. 5. The two variables that are most commonly known to the designer are the input rating
and configuration of the system. Typical design criteria are shown in Tables 2 and 3. Building code requirements
for chimney heights should be considered as minimum heights for fire safety and should be strictly adhered to.

 

General

Since both fireplace and appliance chimneys have an identical function, their construction methods and materials
are similar. Building code requirements insofar as construction is concerned are identical.
Fireplace Chimneys

General. The chimney of a fireplace is considered to be that portion of the fireplace from the base of the first flue
liner to the top of the last flue liner, or any rain cap above it.

Single-wythe chimneys should be attached to the structure. This is generally accomplished by using corrosionresistant metal ties spaced at a maximum of 24 in. (600 mm) on center. Multi-wythe chimneys that are not
masonry bonded should be bonded together using metal wire ties.

Racking. Chimneys are generally not as wide as the body of the fireplace below. When racking back to achieve
the desired dimensions or location of the chimney care must be exercised to insure that, since there is no
limitation on the distance each unit may be racked, cores of the units are not exposed. Preferred construction
consists of a setting bed over the racked face with uncored or paving brick set to provide a weather resistant
surface. Mortar washes may also be used. They may not, however, be as durable. When using a mortar wash it
should not bridge over the rack, but should fill each step individually. Both methods of racking are shown in Fig. 6.

Flue Liners. The first flue liner should be supported along its entire perimeter by masonry. The liner should be
bedded in mortar with the joints cut flush and smoothed on the interior and the exterior joint area parged. The flue
liners should be set one section ahead of the chimney brickwork.

Flashing. Base flashing and counter flashing are installed at the chimney/roof interface, see Fig. 7. The base
flashing is installed first on the faces of the chimney perpendicular to the ridgeline with tabs at each corner. The
flashing should extend a minimum of 4 in. (100 mm) up the face of the chimney and along the roof. Counter
flashing is then installed over the base flashing. It is inserted into a mortar joint for 3/4 to 1 in. (19.1 mm to 25 mm)
and mortared solidly into the joint. The counter flashing should lap the base flashing by at least 3 in. (75 mm). If
the flashing is installed in sections, the flashing higher up the roofline should lap over the lower flashing a
minimum of 2 in. (50 mm). All joints in the base flashing and counter flashing should be thoroughly sealed. The
unexposed side of any bends in the flashing should also be sealed.

Typical Section and Flashing Detail
FIG. 7

Cricket. If a cricket is desired, usually for chimneys whose dimension parallel to the ridgeline is greater than 30
in. (750 mm) and do not intersect the ridgeline, it should be constructed similar to the one shown in Fig. 8. The
dimensions of the cricket are based on the chimney measurements parallel to the ridgeline. The intersection of
the cricket and the chimney should be flashed and counter flashed in the same manner as a normal chimney roof
intersection. The flashing at the roofline should extend to at least 4 in. (100 mm) under the roofing material. For
dimensions and construction details, see Table 4, and Fig. 9.

Chimney Caps. There are, as discussed in the materials section, two options regarding chimney caps: 1)
prefabricated, and 2) cast-in-place. Prefabricated caps generally provide superior performance as compared to
the cast-in-place type. Regardless of which type cap is used, it should be thoroughly primed, backed, and sealed
at the cap and flue liner interface to reduce the potential for water penetration.

Prefabricated caps are set in place on a mortar bed. There should be a bond break between the brickwork and
the setting bed to allow the cap to respond to the differential movement it will encounter without distressing the
brickwork. Figure 3 depicts a typical prefabricated cap. From this figure, general configurations and waterproofing
methods may be obtained.

Cast-in-place caps should conform to the shape and minimum dimensions shown in Fig. 3. Feathering the cap to
the edge should be avoided since this substantially reduces the thickness at the edge and therefore the potential
for deterioration is increased. Waterproofing requirements are different since shrinkage of the concrete as it cures
is a certainty. Flashing is highly recommended for cast-in-place caps. The flashing may also be considered as the
bond break material. Adequate reinforcement should be placed in the cap to help control cracking due to
shrinkage and thermal movements. Additional reinforcement may be necessary in the portion of the cap that
overhangs the face of the chimney. Figure 3 shows one method of forming a cast-in-place chimney cap.

When using a chimney cap that does not overhang the face of the chimney, the last two courses of the chimney
brickwork should be corbeled out to form a drip to help reduce the amount of water allowed to run down the face
of the chimney. The flue liner should extend a minimum of 2 in. (50 mm) above the top of the cap, see Fig. 3.

Appliance Chimneys

General. Fireplace and appliance chimneys have few dissimilarities. The general recommendations for the
construction of fireplace chimneys and the proper consideration of three additional components should produce a
functional appliance chimney. The three components, either not present in fireplace chimneys or incorporated into
the body of the fireplace are: 1) the foundation, 2) the cleanout door, and 3) the thimble.
Foundation. The foundation supports the chimney and must be sized to carry all superimposed loads. However,
most building codes disallow using the chimney walls as structural elements to support other building
components. When designing the foundation, care should be taken to account for soil conditions and type.
Undisturbed or well-compacted soil will generally be sufficient, however, some types of soil conditions may
require additional analysis.

Building codes generally require that the foundation be at least 12 in. (300 mm) thick, and, in plan view, extend a
minimum of 6 in. (150 mm) beyond each face of the masonry bearing on it. It should also penetrate the frost line
to reduce the possibility of “heaving” of the foundation while the ground is freezing.

Cleanout Door. A cleanout door may not be necessary when venting appliances that use clean burning fuels
such as natural gas, however other fuels may produce combustion by-products that will accumulate at the bottom
of the chimney and require periodic removal. The cleanout door should be of ferrous metal and set to provide as
airtight a seal as possible. If desired, the cleanout door may be oriented toward the interior of the structure,
however. the prime consideration in sizing and locating the door is the ease with which it can be used.
Thimble. A thimble is the lined opening through the chimney wall that receives the smoke pipe connector, as
shown in Fig. 10. A thimble should be set in the chimney at the location of the entrance of the pipe connector. It
should be built integrally with the chimney and made as airtight as possible, by using either boiler putty or
asbestos cement. The thimble should be set flush with the interior face of the flue liners, and at least 18 in. (460
mm) below the ceiling. The thimble should have a minimum of 8 in. (200 mm) of flue liner extending below its
lowest point, see Fig. 10.

SUMMARY

This Technical Notes has given suggested design and construction methods for residential chimneys. Although,
there are differences, both appliance and fireplace chimneys use similar construction techniques and materials.
Since the prime function of a chimney is fire safety both quality workmanship and materials should be used.
The information and suggestions contained in this Technical Notes are based on the available data and the
experience of the Brick Industry Association’s technical staff. The recommendations and suggestions are offered
as a guide for consideration by the designers, specifiers, and owners of buildings when anticipating the design,
detailing and construction of residential chimneys. The final decision to use or not to use these recommendations
and materials in brick masonry chimneys is not within the purview of the Brick Industry Association and must rest
with the project designer, or owner.

REFERENCES

1. One and Two Family Dwelling Code, published by Building Officials and Code Administrators, Inc.,
Homewood, Illinois; International Conference of Building Officials, Whittier, California; and Southern
Building Code Congress, International, Inc., Birmingham, Alabama.
2. Book of Successful Fireplaces, How to Build, Decorate and Use Them, 20th Edition, by R. J. Lytle and
Marie-Jeanne Lytle, Structures Publishing Company, Farmington, Michigan, 1977.
3. How to Install a Fireplace, by Donald R. Brann, Direction Simplified, Inc., Briarcliff Manor, New York,
1976.
4. 1979 Equipment Volume, ASHRAE Handbook and Product Directory, by American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Inc., New York, New York, 1979.