How to Build or Inspect Chimney Chases: Wood-Framed Enclosures for Metal Chimneys
InspectAPedia® -
Tips for inspecting wood-framed chimney surrounds
Common leak and failure points at wood chimney chaseways
Why leaks in chimney chases lead to insect damage, rot, mold growth in buildings
Specifications for wood framed chimney chase construction, codes, fire safety
Questions & answers about wood-framed metal chimney enclosures
How to build a wood-framed chimney chase: this article describes wood-enclosed metal chimneys, how a chimney chase should be constructed, and how those structures can be inspected for leaks, damage, or unsafe conditions.
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Wood Framed Chimney Chases around Metal & Insulated Metal Chimneys & Flues
This article series on chimneys, chimney construction, and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.
Chimney Chase Construction SNAFUs to Avoid
Our photographs show a typical wood-framed chimney chase constructed to house an insulated metal chimney and/or a Type-B gas vent. The photo at above left shows an incomplete chimney chase with poor workmanship - incomplete installation of housewrap, missing chimney flashing, and who knows what at the chimney top.
Our photo at above right shows vinyl siding has fallen off of the chimney chase, permitting leaks into the structure. Below we show common leaky conditions at the top of a wood-framed chimney enclosure.
The most common defect we find at wood-framed chimney chase ways is an improperly-constructed top flashing that is not sloped to drain properly (above left). Often people try to fix a leak at the the chimney top by smearing on roofing mastic (above right). We find that this is not a durable repair.
A close-look at the top of a mastic-coated wood-framed chimney chase may show that the top is still concave, sloping in towards the chimneys and that the combination of heat and sunlight has dried and cracked the sealant. (Photos below).
Because the top of chimneys is not readily accessible, leaks in this location may remain un-diagnosed for too long, risking insect damage, rot, and mold in the structure below.
The result is leaks inside of the chimney chaseway where water may lead to a damaged and unsafe fireplace insert or heating appliance, rot, and insect damage.
Our photo (left) shows how the interior of a wood-framed chimney chaseway may be constructed using common framing lumber and drywall. In this particular chase no chimney has (yet) been installed.
But leaks at the top of this structure wet the drywall sides leading to a (hard to see) mold contamination.
Building a Chimney Chase
Watch out: before buying materials and starting to hammer away at constructing a chimney chase for your home, be sure to check with your local building code officials. A building permit is required in most jurisdictions, and the applicable building codes and inspections are important fire safety checks on your chimney chase design and construction.
Common Specifications & Construction Details for a Wood-Framed Chimney Chase
Dimensions for a wood framed chimney chase may be as small as 24" deep x 36" wide, but are more typically 3ft x 5ft x the necessary height to meet roof clearance requirements. Some builders frame the entire chase, beginning inside the structure (where for example a zero-clearance fireplace is to be installed) as wide as six feet to accommodate the fireplace insertion and to support a mantle.
At our references section you will find referrals to chimney construction & fire building codes. Framing nailing and structure is typically the same as structural wood framed wall construction in dimensions and spacing of framing members.
Also see: Fire Clearances for Metal Chimneys. Properly framed and constructed, the portion of the chimney chase that extends above the roof surface at a height to meet the roof clearance distances will not require extra bracing.
Framing the chimney chase: typically uses 2x4 structural members sheathed with OSB or plywood exterior building sheathing. For wind resistance, if the chimney chase is passing through the roof structure (rather than abutting the roof structure end, it should be framed continuously through the roof structure, not simply tacked on top of the roof sheathing. Take a look at the photo above showing the inside of a wood-framed chimney chase.
Watch out: the chimney chase support at its base must be able to accommodate the weight of not only the structure and the chimney that may be routed through it (and its supporting brackets) but also the exterior siding (or stone) that may be applied. Extra supporting beams, headers, or a footing may be needed.
A chimney chase cricket is recommended between the chimney side facing the roof and the roof surface, otherwise you're asking for leaks in this location. In some jurisdictions (check with your local code officials) builders leave a one-inch space (see local and model building codes such as IRC code R1001.17) between the cricket edges and the chimney chase vertical side and similarly between the cricket and the roof deck.
Assembly: some builders construct the wood-frame of the chimney chase on the ground, perhaps in two sections depending on overall chimney height. By capping each section with solid plywood at each floor height, fire stopping has been added.
Chimney chase fire blocking: fire-blocking (perhaps 3/4" thick plywood or OSB (fire rated?)) is intended to slow or even prevent the movement of a chimney chase fire between building floors, and to prevent the increase of fire intensity and spread by the chimney effects that would occur inside the chimney chase without fires topping. Fire stopping must comply with local codes - also see IRC 602.8. Fire blocking is also referred to as fire stopping or "fire stops". Factory-built metal fireblock shields may be required by and provided by the manufacturer of the metal chimney to be installed through the chimney chase.
Chimney chase insulation - is not normally used; the fire rating and fire safety of the chimney will depend on proper selection and installation of the insulated metal flue that will pass through the structure. If for some reason you are insulating a chimney chase be sure that your insulation is kept back from the metal chimney itself in accordance with its instructions and fire codes.
Securing the chimney chase: The chimney chase must be adequately secured to the building to withstand storms and wind loads for your area. Builders use steel framing connectors such as Simpson Strong-Ties™ to secure the vertical framing of the chimney chase to the building structural wall. Connecting the chimney chase sheathing to the building framing adds stiffness and support.
Sealing the chimney chase against leaks: we find a lot of leaks into wood framed chimney chases, including at
The chimney chase top, (chimney chase cover) is usually metal, constructed of formed aluminum or stainless steel. We find leaks here especially where the top is framed and enclosed "flat" and the top flashing lacks adequate pitch and stiffness to avoid collapsing, even becoming concave" where there are snow loads. (See our chimney chase top photo at left).
Watch out: according to Dale Feb, some fireplace top shrouds or surrounds installed on some chimney chase tops may unsafe due to heat trapping at the chimney top. He points out that use of such shrouds may violate local and national building codes as well. [3]
At the flashing between the chimney chase box and the roof and roof cricket - these leaks are almost always because someone was not careful to follow normal roof -to-chimney flashing detailing about flashing size, using step flashing and counter flashing. Do not rely on roof sealants and mastics instead of proper flashing.
Through the chimney chase final covering: especially if faux stone or brick are used, but also if vinyl siding or other siding is not properly installed and sealed. Builders installing a stone or brick veneer often cover the entire chimney chase exterior with a stick-on flashing membrane such as WR Grace Ice and Water Shield.
Watch out: for animals entering the chimney chase. A bird or squirrel nesting in the chimney chase is likely to create a fire hazard. Building owners or maintenance staff (such as at condominium complexes where many wood framed chimney chases may be installed) should inspect for evidence of squirrel-chewed openings in the chase sides, lost or open combustion air screens, and blown-off siding or chimney top seals.
Questions & Answers regarding this article
Questions & answers about wood-framed metal chimney enclosures: construction, clearances, fire stopping, leaks, safety.
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Thanks to Luke Barnes for suggesting that we add text regarding the hazards of shared chimney flues. USMA - Sept. 2008.
[2] Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at ap@octoberhome.com
Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN, technical review by Roger Hankey, prior chairman, Standards Committee, American Society of Home Inspectors - ASHI. 952 829-0044 - hankeyandbrown.com
[3] [Chimney chase top shrouds & surrounds] "A Top Observation at Shrouds & Surrounds", Dale W, Feb, ICBO-certified building inspector, FIRE Associates, fireplace, chimney & venting consultants, F.I.R.E. Associates
Dale Warren Feb, MCI. Owner/Inspector., 5325 Commerce Avenue, Suite 5., Moorpark, CA 93021. Telephone: (805) 552-9954, Email: dale@fireassociates.org, web search 03/03/2012, original source: fireassociates.org/media/shrouds_surrounds.pdf
[4] NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
[5] NFPA #211-3.1 1988 - Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
[6] NFPA # 54-7.1 1992 - Specific to venting of equipment with fan-assisted combustion systems.
[7] GAMA - Gas Appliance Manufacturers' Association has prepared venting tables for Category I draft hood equipped central furnaces as well as fan-assisted
combustion system central furnaces.
National Fuel Gas Code, an American National Standard, 4th ed. 1988 (newer edition is available) Secretariats, American Gas Association (AGA), 1515 Wilson Blvd., Arlington VA22209, and National Fire Protection Association (NFPA), Batterymarch Park, Quincy MA 02269. ANSI Z223.1-1988 - NFPA 54-1988. WARNING: be sure to check clearances and other safety guidelines in the latest edition of these standards.
[8] Fire Inspector Guidebook, A Correlation of Fire Safety Requirements Contained in the 1987 BOCA National Codes, (newer edition available), Building Officials and Code Administrators International, Inc. (BOCA), Country Club HIlls, IL 60478 312-799-2300 4th ed. Note: this document is reissued every four years. Be sure to obtain the latest edition.
[9] Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys,
refers to Chapters 23, 29, and 37 of the Building Code.
[10] New York 1984 Uniform Fire
Prevention and Building Code, Article 10, Heating, Ventilating, and Air Conditioning Requirements
[11] New York 1979 Uniform Fire Prevention & Building Code, The "requirement" for 8" of solid masonry OR for use of a
flue liner was listed in the One and Two Family Dwelling Code for New
York, in 1979, in Chapter 9, Chimneys and Fireplaces, New York 1979
Building and Fire Prevention Code:
[12] "Top Ten Chimney (and related) Problems Encountered by One Chimney Sweep," Hudson Valley ASHI education seminar, 3 January 2000, contributed by Bob Hansen, ASHI
[14] "Rooftop View Turns to Darkness," Martine Costello, Josh Kovner, New Haven Register, 12 May 1992 p. 11: Catherine Murphy was sunning on a building roof when a chimney collapsed; she fell into and was trapped inside the chimney until rescued by emergency workers.
[15] "Chimneys and Vents," Mark J. Reinmiller, P.E., ASHI Technical Journal, Vol. 1 No. 2 July 1991 p. 34-38.
[16] "Chimney Inspection Procedures & Codes," Donald V. Cohen was to be published in the first volume of the 1994 ASHI Technical Journal by D. Friedman, then editor/publisher of that publication. The production of the ASHI Technical Journal and future editions was cancelled by ASHI President Patrick Porzio. Some of the content of Mr. Cohen's original submission has been included in this more complete chimney inspection article: InspectAPedia.com/chimneys/Chimney_Inspection.htm. Copies of earlier editions of the ASHI Technical Journal are available from ASHI, the American Society of Home Inspectors.
[17] Natural Gas Weekly Update: http://tonto.eia.doe.gov/oog/info/ngw/ngupdate.asp Official Energy Statistics from the U.S. Government
[18] US Energy Administration: Electrical Energy Costs http://www.eia.doe.gov/fuelelectric.html
[19] International Residential Code IRC R100.11 - Fireplace Clearance:
International Residential Code was recently amended:
R1001.11 Fireplace clearance. Combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The air space shall not be filled, except to provide fire blocking in accordance with Section R1003.12.
Exceptions:
1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. Combustible materials, including framing, wood siding, flooring and trim, shall be permitted to abut the sides and hearth extensions, but not the backs, of masonry fireplaces, in accordance with FIGURE R1003.12, provided such combustible materials are a minimum of 12 inches (306 mm) from the inside surface of the nearest firebox lining.
3. Exposed combustible mantels or trim may be placed directly on the masonry fireplace front surrounding the fireplace opening provided such 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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Ceramic Roofware, Hans Van Lemmen, Shire Library, 2008, ISBN-13: 978-0747805694 - Brick chimneys, chimney-pots and roof and ridge tiles have been a feature of the roofs of a wide range of buildings since the late Middle Ages. In the first instance this ceramic roofware was functional - to make the roof weatherproof and to provide an outlet for smoke - but it could also be very decorative.
The practical and ornamental aspects of ceramic roofware can still be seen throughout Britain, particularly on buildings of the Victorian and Edwardian periods. Not only do these often have ornate chimneys and roof tiles but they may also feature ornamental sculptures or highly decorative gable ends. This book charts the history of ceramic roofware from the Middle Ages to the present day, highlighting both practical and decorative applications, and giving information about manufacturers and on the styles and techniques of production and decoration.
Hans van Lemmen is an established author on the history of tiles and has lectured on the subject in Britain and elsewhere. He is founder member and presently publications editor of the British Tiles and Architectural Ceramics Society. Available at the InspectAPedia Bookstore.
Chimney & Stack Inspection Guidelines, American Society of Civil Engineers, 2003 - These guidelines address the inspection of chimneys and stacks. Each guideline assists owners in determining what level of inspection is appropriate to a particular chimney and provides common criteria so that all parties involved have a clear understanding of the scope of the inspection and the end product required. Each chimney or stack is a unique structure, subject to both aggressive operating and natural environments, and degradation over time. Such degradation may be managed via a prudent inspection program followed by maintenance work on any equipment or structure determined to be in need of attention. Sample inspection report specifications, sample field inspection data forms, and an example of a developed plan of a concrete chimney are included in the guidelines. This book provides a valuable guidance tool for chimney and stack inspections and also offers a set of references for these particular inspections.
NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
NFPA 211 - 3-4 - Clearance from Combustible Material
NFPA 54 - 7-1 - Venting of Equipment into chimneys
Brick Institute of America - Flashing Chimneys
Brick Institute of America - Proper Chimney Crowns
Brick Institute of America - Moisture Resistance of Brick
American Gas Association - New Vent Sizing Tables
Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
National Chimney Sweep Guild - Yellow Pages of Suppliers
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