1001.1.1 Support. Masonry chimneys shall be supported on foundations of solid masonry or concrete at least 12 inches (305 mm) thick and at least 6 inches (153 mm) beyond each side of the exterior dimensions of the chimney. Footings shall be founded on natural undisturbed earth below frostline. In areas not subject to freezing, footings shall be located a minimum of 12 inches (305 mm) below finished grade.

1001.2 Corbeling. Masonry chimneys shall not be corbeled more than half of the chimney's wall thickness from a wall or foundation, nor shall a chimney be corbeled from a wall or foundation which is less than 12 inches (305 mm) in thickness unless it projects equally on each side of the wall, except that on the second story of a two-story dwelling, corbeling of chimneys on the exterior of the enclosing walls may equal the wall thickness. The projection of a single course shall not exceed one-half the unit height or one-third of the unit bed depth, whichever is less.

1001.3 Changes in dimension. The chimney wall or chimney flue lining shall not change in size or shape within 6 inches (153 mm) above or below where the chimney passes through floor components, ceiling components or roof components.

1001.4 Offsets. Where a masonry chimney is constructed with a fireclay clay flue liner surrounded by one wythe of masonry, the maximum offset shall be such that the centerline of the flue above the offset does not extend beyond the center of the chimney wall below the offset. Where the chimney offset is supported by masonry below the offset in an approved manner, the maximum offset limitations shall not apply. Each individual corbeled masonry course of the offset shall not exceed the projection limitations specified in Section 1001.2.

1001.5 Additional load. Chimneys shall not support loads other than their own weight unless they are designed and constructed to support the additional load. Masonry chimneys shall be permitted to be constructed as part of the masonry walls or reinforced concrete walls of the building.

1001.6 Termination. Chimneys shall extend at least 2 feet (610 mm) higher than any portion of the building within 10 feet (3048 mm), but shall not be less than 3 feet (914 mm) above the point where the chimney passes through the roof.

1001.7 Wall thickness. Masonry chimney walls shall be constructed of solid masonry units or hollow masonry units grouted solid with not less than 4 inches (102 mm) nominal thickness.

1001.8 Flue lining (material). Masonry chimneys shall be lined. The lining material shall be appropriate for the type of appliance connected, according to the terms of the appliance listing and manufacturer's instructions.

1001.8.1 Residential-type appliances (general). Flue lining systems shall comply with one of the following:

1. Clay flue lining complying with the requirements of ASTM C 315, Specifications for Clay Flue Linings, or equivalent.

2. Listed chimney lining systems complying with UL 1777, Chimney Liners.

3. Factory-built chimneys or chimney units listed for installation within masonry chimneys.

1001.9 Flue lining (installation). Flue liners shall be installed in accordance with ASTM C-1283 and extend from a point not less than 8 inches (203 mm) below the lowest inlet or, in the case of fireplaces, from the top of the smoke chamber, to a point above the enclosing walls. The lining shall be carried up vertically, with a maximum slope no greater than 30 degrees from the vertical.

Fireclay flue liners shall be laid in medium-duty refractory mortar conforming to ASTM C-199 with tight mortar joints left smooth on the inside and installed to maintain an air space or insulation not to exceed the thickness of the flue liner separating the flue liners from the interior face of the chimney masonry walls. Flue lining shall be supported on all sides. Only enough mortar shall be placed to make the joint and hold the liners in position.

1001.9.1 Listed materials. Listed materials used as flue linings shall be installed in accordance with the terms of their listings and manufacturer's instructions.

1001.9.2 Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance.

Exception: This shall not prevent the instalation of a separate flue lining in accordance with the manufacturers installation instructions.

Exception: When venting only one appliance, two flues may adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered at least 4 inches (178 mm).

1001.12 Flue area (masonry fireplace). The net cross-sectional area of the chimney flue shall be determined in accordance with Figure 1001.12. A flue size providing at least the equivalent net cross-sectional area shall be used. Cross-sectional areas of clay flue linings are provided in Tables 1001.12a and 1001.12b or as provided by the manufacturer or as measured in the field. The height of the chimney shall be measured from the firebox floor to the top of the chimney flue.

Exceptions:

1. A round chimney flue which has a net cross-sectional area of at least 1/12 of the fireplace opening area shall be permitted for chimneys that are a minimum of 15 feet (4572 mm) in height.

2. A square or rectangular chimney flue which has a net cross-sectional area of at least 1/10 of the fireplace opening area shall be permitted for chimneys that are a minimum of 15 feet (4572 mm) in height.

FIGURE 1001.12
FLUE SIZES FOR MASONRY CHIMNEYS

1001.13 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

1001.14 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (153 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located at least 6 inches (153 mm) below the lowest chimney inlet opening. The height of the opening shall be at least 6 inches (153 mm). The cleanout shall be provided with a noncombustible cover.

Exception: Chimney flues serving masonry fireplaces, where cleaning is possible through the fireplace opening.

When masonry chimneys are constructed as part of masonry walls or reinforced concrete walls, combustible materials shall not be in contact with the masonry wall less than 12 inches (306 mm) from the inside surface of the nearest flue lining.

Exception: Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces. However, this shall not eliminate the requirement for noncombustible firestopping in accordance with Section 2111.16.

1001.17 Chimney crickets. Chimneys shall be provided with crickets when the dimension parallel to the ridgeline is greater than 30 inches (762 mm) and does not intersect the ridgeline. The intersection of the cricket and the chimney shall be flashed and counterflashed in the same manner as normal roof-chimney intersections. Crickets shall be constructed in conformity with Figure 2111.17 and Table 2111.17.

1003.2 Footings and foundations. Foundations for masonry fireplaces and their chimneys shall be constructed of concrete or solid masonry at least 12 inches (305 mm) thick and shall extend at least 6 inches (153 mm) beyond the face of the fireplace or support wall on all sides. Footings shall be founded on natural undisturbed earth or engineered fill below frost depth. In areas not subjected to freezing, footings shall be at least 12 inches (305 mm) below finished grade.

1003.3 Seismic reinforcing. Masonry or concrete chimneys in Seismic Zones 3 and 4 shall be reinforced. Reinforcing shall conform to the requirements set forth in Table 1003.1 and Section 607, Grouted Masonry.

1003.3.1 Vertical reinforcing. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars shall be placed between wythes of solid masonry, or within the cells of hollow unit masonry, and grouted in accordance with Section 607. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys greater than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be provided for each additional flue incorporated in the chimney or for each additional 40 inches (1016 mm) in width or fraction thereof.

1003.3.2 Horizontal reinforcing. Vertical reinforcment shall be placed within 1/4-inch (6.4 mm) ties, or other reinforcing of equivalent net cross-sectional area, placed in the bed joints according to Section 607, at a minimum of every 18 inches (457 mm) of vertical height. Two such ties shall be provided at each bend in the vertical bars.

1003.4 Seismic anchorage. Masonry and concrete chimneys in Seismic Zones 3 and 4 shall be anchored at each floor, ceiling or roof line more than 6 feet (1829 mm) above grade, except where constructed completely within the exterior walls. Anchorage shall conform to the following requirements:

1003.4.1 Anchorage. Two 3/16-inch by 1-inch (4.8 mm by 25 mm) straps shall be embedded a minimum of 12 inches (305 mm) into the chimney. Straps shall be hooked around the outer bars and extend 6 inches (153 mm) beyond the bend. Each strap shall be fastened to a minimum of four floor joists with two 1/2-inch (12.7 mm) bolts.

1003.5 Fireplace walls. Masonry fireplaces shall be constructed of solid masonry units, hollow masonry units grouted solid, stone, or reinforced concrete. When a lining of firebrick at least 2 inches (51 mm) in thickness or other approved lining is provided, the total minimum thickness of back and side walls shall be 8 inches (203 mm) of solid masonry, including the lining. The width of joints between firebricks shall not be greater than 1/4 inch (6.4 mm). When no lining is provided, the total minimum thickness of back and side walls shall be 10 inches (254 mm) of solid masonry. Firebrick shall conform to ASTM C 27 or C 1261 and shall be laid with medium-duty refractory mortar conforming to ASTM C-199.

1003.6 Steel fireplace units. Steel fireplace units incorporating a firebox liner of not less than 1/4 inch (6.4 mm) in thickness and an air chamber may be installed with masonry to provide a total thickness at the back and sides of not less than 8 inches (203 mm), of which not less than 4 inches (102 mm) shall be of solid masonry. Warm-air ducts employed with steel fireplace units of the circulating air type shall be constructed of metal or masonry.

1003.7 Lintel and throat. Masonry over a fireplace opening shall be supported by a lintel of noncombustible material. The minimum required bearing length on each end of the fireplace opening shall be 4 inches (102 mm). The fireplace throat or damper shall be located a minimum of 8 inches (203 mm) above the lintel.

1003.8 Smoke chamber. Smoke chamber walls shall be constructed of solid masonry units, stone or reinforced concrete. Corbelling of masonry units shall not leave unit cores exposed to the inside of the smoke chamber. When a lining of firebrick at least 2 inches (51 mm) thick, or a lining of vitrified clay at least 5/8 inch (16 mm) thick, is provided, the total minimum thickness of front, back and side walls shall be 6 inches (153 mm) of solid masonry, including the lining. Firebrick shall conform to ASTM C-27 or C-1261 and shall be laid with refractory mortar conforming to ASTM C-199. Where no lining is provided, the total minimum thickness of front, back and side walls shall be 8 inches (203 mm) of solid masonry.

1003.8.1 Smoke chamber dimensions. The inside height of the smoke chamber, from the fireplace throat to the beginning of the flue, shall not be greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees from vertical when prefabricated smoke chamber linings are used, or when the smoke chamber walls are rolled or sloped rather than corbeled. When the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees from vertical.

1003.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials and reinforced to carry their own weight and all imposed loads. No combustible material shall remain against the underside of hearths and hearth extensions after construction.

1003.9.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

1003.9.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

Exception: When the bottom of the firebox opening is raised at least 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than 3/8-inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

1003.11 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall not be less than 8 inches (203 mm) above the fireplace opening. The throat opening shall not be less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall not be less than the cross-sectional area of the flue.

Exception: Rumford fireplaces shall be permitted provided that the depth of the fireplace be at least 12 inches (305 mm) and at least one-third of the width of the fireplace opening, and that the throat be at least 12 inches (305 mm) above the lintel, and be at least 1/20 the cross-sectional area of the fireplace opening.

When masonry chimneys are constructed as part of masonry walls or reinforced concrete walls, combustible materials shall not be in contact with the masonry wall less than 12 inches (306 mm) from the inside surface of the nearest firebox, smoke chamber or flue lining.

1003.13 Mantel and trim. Woodwork or other combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material within 12 inches (305 mm) of the fireplace opening shall not project more than 1/8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening.

1003.14 Fireplace fireblocking. See Section 602.8.

1003.15 Ash dump cleanout. Cleanout openings, when provided, shall be equipped with ferrous metal doors and frames constructed to remain tightly closed, except when in use. Cleanouts shall be accessible and located so that ash removal will not create a hazard to combustible materials.

FIGURE 1003.1
FIREPLACE AND CHIMNEY DETAILS

1. The fire chamber assembly is installed to provide clearance to combustible materials not less than set forth in the listing.

2. The chimney sections are installed to provide clearance to combustible material not less than specified in the listing and if the fireplace chimney extends through floors and ceilings, factory-furnished firestops or firestop spacers shall be installed. Portions of chimneys which extend through rooms or closets are to be enclosed to avoid personal contact, contact of combustible material, and damage to the chimney.

3. Hearth extensions shall not be less than 3/8-inch-thick (9.5 mm) asbestos, hollow metal, stone, tile or other approved noncombustible material. Such hearth extensions may be placed on combustible subflooring or finish flooring. The hearth extension shall be readily distinguished from the surrounding floor.

4. Hearth extensions shall extend not less than 16 inches (406 mm) in front of and at least 8 inches (203 mm) beyond both sides of the fireplace opening.

5. Factory-built fireplaces shall be installed in accordance with their listing and the manufacturer's installation instructions.

6. The supporting structure for a hearth extension shall be at the same level as the supporting structure for the fireplace unit unless otherwise authorized by the listing.

1006.1.1 Factory-built fireplaces. Exterior combustion air ducts for factory-built fireplaces shall be a listed component of the fireplace and shall be installed according to the fireplace manufacturer's instructions.

1006.1.2 Masonry fireplaces. Listed combustion air ducts for masonry fireplaces shall be installed according to the terms of their listing and manufacturer's instructions.

1006.2 Exterior air intake. The exterior air intake shall be capable of providing all combustion air from the exterior of the dwelling. The exterior air intake shall not be located within the garage, attic, basement or crawl space of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of 1/4-inch (6.4 mm) mesh.

Reason: Exterior air ducts that are built with vertical portions rising to an exterior "intake" above the firebox can get warm and draw like a chimney and flow backwards adding to the risk of fire.

1006.3 Clearance. Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet.

1006.4 Passageway. The combustion air passageway shall be a minimum of 6 square inches (3870 mm2) and not more than 55 square inches (0.035 m2) except that combustion air systems for listed fireplaces or for fireplaces tested for emissions shall be constructed according to the fireplace manufacturer's instructions.

1006.5 Outlet. The exterior air outlet is permitted to be located in the back or sides of the firebox chamber, or within 24 inches (610 mm) of the firebox opening on or near the floor. The outlet shall be closable and designed to prevent burning material from dropping into concealed combustible spaces.

IRC 1997 version
IRC current draft version - 10/10/98
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