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Fireplaces & Hearths: Design, Damage, Cracks, Settlement or Collapse Diagnosis & Repair
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Fireplaces & hearths, construction, inspection and repair: this article provides information about masonry fireplaces, including inspection for damage/hazards (cracks and gaps that appear at masonry fireplaces due to chimney or fireplace settlement or movement), fireplace chimney sizing requirements, draft problems, chimney safety, creosote problems, inserts, and other topics. Fireplace damage from chimney or fireplace settlement or movement may be a fire or gas hazard in a building
Fireplace hearth size specifications; How to add support below a settling fireplace hearth
Fireplace damper inspection, diagnosis, repair or replacement
Photo examples of cracks in on and around masonry fireplaces and a guide to their cause and remedy
Masonry fireplace chimney & flue size requirements
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Masonry fireplace chimney & flue size requirements
Readers of this article should also see FIREPLACE INSPECTIONS for a professional chimney sweep's fireplace and chimney hazard checklist. At FIREPLACE INSERTS we discuss fireplace inserts and zero-clearance fireplaces, both antique and modern, and their hazards and inspection limitations. Readers should also see Inaccessible Connections Fireplace or Woodstove. The inspection of chimney interiors is discussed at Chimney Inspection: Flue Interiors.
According to the Masonry Institute of America, the required flue size for masonry fireplaces (burning wood) is basically a function of the area of the fireplace opening. Typical standards require a cross-section area of the fireplace flue or chimney/vent to be equal to 1/10 of the area of the fireplace opening itself, for a conventional wood-burning installation and without considering the effects of a glass fire-door. The FHA requires using a 1/8 ratio instead of 1/10 for chimneys that are less than 15 feet high and the 1/10 ratio for chimneys that are 15 feet or more tall. [9]
How do we measure fireplace chimney height?
Note that "chimney height" for the purpose of determining fireplace requirements, is measured not from the ground outside nor from the "floor" of the fireplace hearth. Rather you should use the distance from the fireplace throat to the top of the chimney. Don't include the chimney cap in height measurements - that added distance does not develop draft in the flue.
Chimney Cross-Sectional Flue Shape Effects on Draft
At FLUE SIZE SPECIFICATIONS where we discuss chimney flue sizing for venting heating appliances such as boilers, furnaces and water heaters, we explain that in comparing two flues of exactly the same square inches of cross-sectional area, a round flue will have better draft than a rectangular one.
For this reason, fireplace and chimney guides offer a table of effective chimney vent area (measured as a cross-section of the flue opening) and flue sizes. You can find effective area and flue size tables in the Uniform Mechanical Code and in the MIA's Masonry Fireplace & Chimney Handbook and also in NFPA 211 - Standards for Chimneys & Fireplaces. [9][10][11]
Un-lined chimney flues are also specified as larger than lined installations and use the 1/8 ratio we explained above.
Table of Wood burning Fireplace Openings & Required Chimney Flue Sizes
To use the fireplace chimney flue sizing table below, calculate the area in square inches of the cross section of the inside of the fireplace opening. For a rectangular fireplace opening just multiply its width by its height in inches to calculate the value to look for in column A of the table. The dimensions given in columns B & C of the chimney sizing table present standard clay chimney flue tile dimensions and shapes.
| Type of Fireplace |
Square Inches of Fireplace Opening 1 |
Required Flue Size by the 1/10 rule |
Required flue size by the 1/8 rule |
A |
B |
C |
| Masonry hearth face surrounding single fireplace opening |
|
|
|
| |
672 sq .in. |
8 x 13 inches |
12 x 12 inches |
| |
720 |
8 x 13 |
8 x 17 |
| |
780 |
12 x 12 |
12 3/4 round |
| |
936 |
8 x 17 oval |
13 x 13 square |
| |
1008 |
9 x 19 oval |
10 x 21 |
| |
1120 |
10 x 21 |
13 x 17 |
| |
1536 |
13 x 20 |
17 x 17 oval 2 |
| |
1920 |
17 x 17 oval |
17 x 20 |
| |
2085 |
17 x 20 |
21 x 20 |
| |
|
|
|
| Fireplace hearth open at front and one side |
|
|
|
| |
1107 |
10 x 21 |
12 x 16 |
| |
1223 |
12 x 16 |
13 x 20 |
| |
1388 |
12 x 16 |
13 x 20 |
| |
1884 |
17 x 17 oval |
17 x 20 |
| |
2085 |
17 x 20 |
21 x 20 |
| |
|
|
|
| Fireplace hearth open at front only but with two wythe brick wall on one side |
|
|
|
| |
1344 |
10 x 21 |
13 x 20 |
| |
1470 |
13 x 17 |
17 x 17 oval |
| |
1764 |
13 x 20 |
17 x 20 |
| |
2016 |
17 x 17 |
17 x 21 oval |
| |
|
|
|
| Fireplace hearth open on three sides, one short side enclosed to form fireplace back |
|
|
|
| |
1638 |
13 x 20 |
17 x 17 |
| |
1932 |
17 x 17 oval |
17 x 20 oval |
| |
2184 |
17 x 20 oval |
21 x 20 oval |
| |
|
|
|
| Fireplace hearth open on three sides, one long side enclosed to form fire-back |
|
|
|
| |
1782 |
13 x 21 oval |
17 x 20 |
| |
1971 |
17 x 17 oval |
17 x 21 oval |
| |
2490 |
17 x 21 oval |
21 x 21 oval |
| |
2850 |
21 x 21 oval |
2/13 x 21 oval 3 |
| |
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Notes to the fireplace chimney flue sizing table:
1. These square inch areas were calculated by MIA based on common masonry fireplace widths and heights in inches; the effects of variations in fireplace depth, shape, and extent to which the fire-box follows optimum design principles were not considered.
2. We have not determined what MIA means by "oval" fireplace flues when both dimensions are equal
3. MIA notes that in a masonry wall some masons construct a single lined flue and leave the left flue unlined with 8" of space in the wall for added venting. Fire safety of this design relies on solid masonry construction and may not be code approved in all jurisdictions. |
Adapted from Masonry Fireplace and Chimney Handbook, 2nd Ed., James E. Armhein, S.E., M.I.A. Masonry Institute of America, Los Angeles, CA 213-388-0472 prepared to include requirements of the 1994 UBC and other codes. - MIA[9].
Fireplace Damage and Unsafe Hearths due to Chimney or Fireplace Settlement
Why are gaps at fireplace fireboxes, hearths, or other components a dangerous fire hazard? What should you do about them?
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 Do not use a fireplace that is in any doubt about safety before it has been inspected by a professional.
Our photo (above) shows a fireplace mantel that collapsed and fell into the room. Smoke stains revealed on the brick might point to a chimney draft problem too.
The fireplace schematic (at left) shows the basic components of a masonry fireplace and their names. This drawing is obsolete in that it is missing a combustion air supply for the fireplace.
These articles on chimneys 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. |
 Carson Dunlop's sketch shows a cross section of the basic components of a chimney where a fireplace is installed. (Click the image at left or photos below to enlarge them).
Beginning with the outside inspection of the chimneys and structure, and continuing indoors, we check for a wide range of possible chimney hazards. Among these are issues surrounding chimney movement, settlement, or separation from the building.
Too often we discover that a building owner was aware that a chimney has moved, s/he has patched the gap between the chimney and the building, but s/he has not realized that the movement causes cracks and gaps inside the chimney or fireplace which are very dangerous.
Below we provide three photographs showing how a fireplace can become a fire hazard due to chimney settlement or inadequate support of the fireplace itself.
We start with a look at the fireplace hearth for evidence of movement.
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 In an easy-to-spot case of movement and separation between a fireplace hearth and the building floor take a look at the white caulk installed in an open crack between the hearth face and the floor in our photo (left).
A bit more investigating was needed to determine whether the floor was sagging away from a stable masonry fireplace and chimney or whether the chimney and entire firebox were leaning away from the building.
In the next case, just below, the gaps and cracks made it obvious that the chimney and fireplace were tipping away from the building in a dangerous condition. |
First at above left we see a gap that has opened up between the fireplace floor and the hearth (above-left). Sparks may fall into this space, causing a building fire.
Second (above right) our photo shows a crack between the face of the fireplace and the fireplace box itself. We don't know without more analysis whether the brick facing has fallen away from a sound and safe fireplace or whether the fireplace has moved away from the facing.
Our third fireplace damage photograph (left) is the final nail in the coffin of this unfortunate fireplace.
A gap has opened in the fireplace below the chimney where the damper was cemented in place. There has been substantial movement of the fireplace itself (and probably the chimney too) - this is an unsafe fireplace that should not be used.
But not using the fireplace is not enough to be sure this home doesn't have another fire or glue gas hazard.
If a fireplace and chimney have settled and thus have become unsafe, we need to determine right away if any other building appliances such as a boiler, furnace, water heater, or woodstove are using other flues in the same chimney.
If the chimney has multiple users it is unsafe for all of them.
Also see these articles on chimney collapse hazards:
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Adding Support Below a Settling Fireplace Hearth
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 You may find a temporary supporting column such as this Lally column which was placed below a sagging fireplace.
Some diagnosis of just what caused settling or movement in a hearth is critical.
A gap appearing between the hearth and the edge of the firebox might be due to inadequate hearth support - not such an ugly repair - or it might be due to settlement of the entire chimney and fire chamber away from the building - a major repair and a dangerous condition.
See CHIMNEY COLLAPSE Risks, Repairs.
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Fireplace Hearth Size Requirements
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Our photo (left) shows a burned wooden floor in front of a fireplace hearth.
Hearth dimensions: A fireplace hearth should extend at least 16" (M.I.A.) past the front edge of the fireplace and at least 8" beyond each side of the fireplace opening.
Where the fireplace opening is 6 sq.ft. or bigger the front extension needs to be increased to at least 20" and the side extensions to at least 12" beyond the fireplace front.
The hearth for a masonry fireplace needs to be made of a brick, concrete, stone, or other (approved, listed) non-combustible material. The hearth slab needs to be at least 4" in thickness, it has to be supported by noncombustible materials or able to carry its own weight.
The "cribbing" or wood forms used to support a poured concrete hearth should be removed after construction is completed. We often find this wood material left in place - where sparks falling through a crack or gap can start a fire.
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Fireplace Fire Hazards: Carpeting too Close to Fireplace
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Carpets and other combustibles need to be kept away from the fireplace front and hearth.
Often where the hearth sits at floor level we find that someone has installed carpeting right up to the fireplace - a fire hazard as our client is remarking in our photo (left).
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Fireplace Damper Trouble Inspection, Diagnosis, Repair
Stuffing a pillow into the chimney throat of a fireplace (above left) might slow the loss of warm air from a home, but it's a dangerous substitute for a missing or broken fireplace damper. What if someone lights a fire without noticing this stuffing?
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 A normal cast iron fireplace damper is shown in closed position in our photo at above right. Closing the damper when the fireplace is not in use will make a significant reduction in heat loss from most buildings. \
Inspect fireplaces for a working damper. Where a steel insert fireplace is installed, inspect the upper portions of the fireplace at the chimney throat with great care. A rusted-out steel fireplace inset will be unsafe, wont' work properly, and will be costly to replace.
Our photograph (left) shows a steel fireplace insert at an incomplete fireplace installation in a basement. What about that plywood "face" nailed around the steel fire chamber? |
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Frequently Asked Questions (FAQs) about fireplace & hearth construction, inspection, defects, safety hazards & repair methods
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- 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
- [3] 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
- [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)
NFPA #211-3.1 1988 -
Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
- [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.
- [6a] 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.
- [7] 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] Masonry Fireplace and Chimney Handbook, 2nd Ed., James E. Armhein, S.E., M.I.A. Masonry Institute of America, 22815 Frampton Ave.
Torrance, CA 90501-5034
Toll free: 1-800-221-4000; the original text noted that mIA was prepared to include requirements of the 1994 UBC and other codes. Website ht.masonryinstitute.org,
- [10] Uniform Mechanical Code - UMC 2009, and UMC 1991, Sec 913 (a.) Masonry Chimneys,
refers to Chapters 23, 29, and 37 of the Building Code.
- [11] 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)
- 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
- Chimney Inspection Checklist, Carson Dunlop, Associates, Toronto, Ontario
- "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/chimneys.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
- Fireplace & Chimney Design, Repair Books - Fireplaces, Chimneys: design, repair
- Fireplace & Chimney Inspection Books - Inspecting and diagnosing chimney problems, fireplace problems, chimney & fireplace standards
- 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 Inspection Checklist, Carson Dunlop, Associates, Toronto, Ontario
- 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.
- Fireplaces, a Practical Design Guide, Jane Gitlin
- Fireplaces, Friend or Foe, Robert D. Mayo
- 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)
- Principles of Home Inspection: Chimneys & Wood Heating (Principles of Home Inspection), Carson Dunlop
- NFPA 211 - 3-1.10 - Relining guide for chimneys
- NFPA 211 - 3-2 - Construction of Masonry Chimneys
- NFPA 211 - 3-3 - Termination Height for chimneys
- 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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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