Separation requirements for masonry flues in brick, concrete block, or stone chimneys
How much space is required between masonry flues?
How much solid masonry is necessary between masonry flues for fire and gas safety?
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This article explains the requirement for separation between chimney flues in a masonry chimney for fire safety and performance. These articles describe and illustrate chimney inspection procedures and critical chimney defects which can be observed from outdoors at ground level. Readers should also see Chimney Height & Clearance where we discuss the importance of varying the height among adjacent or nearby chimney flues of all types. Page top sket5ch courtesy of Carson Dunlop.
What is the Requirement for Separation Between Masonry Flues in a Multi-Flue Chimney?
Our photo (left) shows three masonry flues in a common chimney, each separated by just an inch or so. The problems with this design include fire and gas safety hazards - an opening may develop between the flues causing draft interference between them and even inadvertent down-draft of dangerous flue gases from one chimney flue into another (and possibly out through a fireplace or heating appliance).
Code & Design Requirements for Masonry Chimney Thickness and Flue Separation Thickness
Good masonry chimney design will normally provide 8" of solid masonry between individual chimney flues in a multi-flue chimney. In Canada, single wythe brick flues are accepted. In many United States locales, single wythe brick flues remain in use but several standards require or recommend either re-lining (and other safety measures) or the confirmation that 8 inches of solid masonry exists - i.e. a double wythe or greater flue.
State building codes vary in how explicitly they address chimney design details. Further, there may be confusion between the general requirement for thickness of masonry chimney walls and the number of inches of masonry separating flues in a multi-flue masonry chimney.
For example in New York (in 1979) the code required by Para R-906 - Flue Lining (Material) Masonry chimneys shall be lined with fireclay flue liners not less than 5/8 of an inch in thickness or with other approved liner material that will resist, without cracking or softening, a temperature of 1800 deg. [F], but the code provided an
Exception: Masonry chimneys may be constructed without flue liners when walls are at least 8" in thickness.
This requirement was dropped when New York changed from an explicit specification code to a [stupid because it became vague] "performance" code in January 1984.
The International Residential Code® for chimneys and fireplaces requires normal masonry wall thickness of at least 4" of solid masonry units, but requires 8" of solid masonry for chimneys without liners.
This model building code specifies at least 4" of solid masonry between adjacent flues in a common chimney and specifies that the masonry wythes shall be bonded into the walls of the chimney. (An exception which probably applies only in rare cases is provided when only one appliance is being vented: two flues may adjoin each other in the same chimney with only the flue lining separating them. the joints of the adjacent flue linings must be staggered at least 4".)
See See UNLINED FLUE INSPECTIONS for additional details about building codes and the inspection, design, and safety of older chimneys.
Separation of Chimney Flues At Chimney Top
As Carson Dunlop's sketch shows, masonry flues should be separated inside the chimney structure for fire and gas leak safety.
Our photograph just above shows two clay tile flues that are well separated (though we had other issues with that chimney top).
We include chimney flue separation details as a rooftop chimney inspection item - this may be the easiest place to spot the absence of a needed flue divider.
Examples of Unsafe Masonry Flues with No Separation or Insufficient Separation
Our photo at left shows an older single-wythe brick chimney serving two fireplaces in a pre-1900 home. later one of the fireplaces was abandoned and its flue converted to use by a gas fired heating boiler.
But notice these defects and concerns with this flue:
Only a single brick thickness separates these two flues - risking holes, and gas leaks between flues,
Risk of down-draft of gas fired appliance flue exhaust back into the building through the fireplace - a potentially deadly carbon monoxide hazard
Fire hazards - inadequate masonry thickness
At Shared Chimney & Shared Flue Hazards we illustrate in more detail an old chimney that lacked this flue divider and we discuss the hazards further.
Our photo (left) shows about 4" of separation between two clay flue tile lined flues in a masonry chimney. But errors at this chimney include
Absence of a gasket or separation between the clay liner and the concrete top seal at the chimney (crown) leading to cracks and ultimate frost damage to the chimney and flue - a possible safety hazard.
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Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Thanks to Luke Barnes for suggesting that we add text regarding the hazards of shared chimney flues. USMA - Sept. 2008.
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
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 #211-3.1 1988 -
Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
NFPA # 54-7.1 1992 -
Specific to venting of equipment with fan-assisted combustion systems.
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.
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.
Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys,
refers to Chapters 23, 29, and 37 of the Building Code.
New York 1984 Uniform Fire
Prevention and Building Code, Article 10, Heating, Ventilating, and Air Conditioning Requirements
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:
"Top Ten Chimney (and related) Problems Encountered by One Chimney Sweep," Hudson Valley ASHI education seminar, 3 January 2000, contributed by Bob Hansen, ASHI
"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.
"Chimneys and Vents," Mark J. Reinmiller, P.E., ASHI Technical Journal, Vol. 1 No. 2 July 1991 p. 34-38.
"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.
Natural Gas Weekly Update: http://tonto.eia.doe.gov/oog/info/ngw/ngupdate.asp Official Energy Statistics from the U.S. Government
US Energy Administration: Electrical Energy Costs http://www.eia.doe.gov/fuelelectric.html
Books & Articles on Chimney Inspection, Testing, Diagnosis, & Repair
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)
Residential Masonry Fireplace and Chimney Handbook, James E. Amrhein, S.E., MIA, Masonry Institute of America, 2d. Ed., 1995, ISBN 0-940116-29-4. The MIA is in Los Angeles, CA 213-388-0472
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
NFPA 211 - 3-4 - Clearance from Combustible Material
NFPA 54 - 7-1 - Venting of Equipment
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, April, 1992
Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
National Chimney Sweep Guild - Yellow Pages of Suppliers
SaverSystems, Div. of Meredith - Water Repellent Technologies
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
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