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Fireplace hearth size, specifications, & support:
This article series 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.
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.
Reader Question: hearth extension is 16" inches minimum not 18" inches
2016/07/13 Manku said:
These days hearth extension is 16" inches minimum not 18" inches as mentioned in the sketch, am i right ?
Above: a masonry fireplace with no hearth extension in a home in San Miguel de Allende, Guanajuato, Mexico. This fireplace originally burned wood but has been converted to gas.
Thanks Manku. You're right - mostly. The required specifications for a masonry fireplace, including hearth dimensions depend on both what building codes apply (where you live, country, state or province) and the fireplace overall opening size as well as other features. The 18" hearth requirement to which you refer was a design recommendation in a fireplace sketch, not a code citation - apologies. The U.S. hearth depth dimensions are 16" or 20" as we explain in more detail here.
Please note that the hearth extension to front of a fireplace, given as 16" or 20" is a minimum dimension not a maximum. More is safer.
In the United States and referring to the ICC, a widely-used model building code, the code specifies a 16" hearth extension to front of the fireplace for fireplaces <6 sq.ft. of opening size, and 20" hearth extension to front of fireplace for fireplaces = or larger than 6 sq. ft.
R1001.10 hearth extension dimensions. hearth extensions
shall extend at least 16 inches (406 mm)in front of and at least 8
inches (203 mm) beyond each side of the fireplace opening.
where the fireplace opening is 6 square feet (0.6 m2) or larger,
the hearth extension shall extend at least 20 inches (508 mm) in
front of and at least 12 inches (305 mm) beyond each side of the
fireplace opening. - ICC, Chapter 10, Chimneys and Fireplaces, Section R1001 Masonry Fireplaces, excerpted from the 2006 Virginia Residential Code.
Here's another quote from a typical residential building code:
There shall be a minimum distance of 36
inches from the back of the firebox to the
end of the hearth extension. hearth
extension shall extend at least 16 inches in
front of, and 8 inches beyond each side of
the fireplace opening. when the fireplace
opening is 6 sq. ft. or larger, the hearth
extension shall extend at least 20 inches in
front of, and 12 inches beyond, each side. - "TB 21, residential building code masonry, chimneys and fireplaces - areas frequently misunderstood." Allegheny county division of permits and land development services residential information sheet # 21 revised january 14, 2003 )
At left in the photo above you can see the remains of a fireplace built into the stone walls of Goodrich Castle, a Norman midieval castle north of the village of Goodrich, Ross on Wye, Herefordshire, U.K. By the time the original wooden fortification was replaced with a stone keep and then expanded, this 13th century fireplace was very shallow, wide and tall, with a projecting hearth, now mostly fallen away. You can place the height of the floor relative to the fireplace hearth by noting the openings for floor beams in the opposing wall at the right in this photograph.
[Click to enlarge any image] Search InspectApedia.com for "Goodrich Castle" to see more cameo appearances of this historic old, interesting structure.
Adding Support Below a Settling Fireplace Hearth
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.
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(Aug 14, 2011) Robert said:
When adding support brace under hearth should new footing be poured to support brace and weight
Good question about hearth support, Robert.
If your floor slab is 4" or thicker, you should be OK with placing supporting columns below the hearth right onto the slab surface.
If the floor slab is thinner than 4", if it already shows signs of settlement, cracks, damage, the best repair is to cut a hole in the slab, excavate and pour a footing or pier for your columns, typically using a cardboard tube or form for the pier.
An alternative that I used in many building renovations of older homes where we needed to support an additional Lally column in a basement where the floor slab looked "OK" but was of unknown thickness, was to bed a solid 4" concrete block in concrete right onto the floor surface. The block served as a footing or pier for the column and helped spread the load out onto a wider area than otherwise had we just put the Lally column base right onto the floor itself.
Question: heatilator not working after a chimney fire
(Nov 4, 2011) Jim Jackson said:
I had a chimney fire last wintwer and immediately putit out. After that I have noticed that the heatalator that is built in does not work...do I need an inspection and is it covered under my home owners insurance?
Jim I don't know what your homeowner's policy covers - you'll need to call your insurance company to ask. But it makes sense to have an inspection of the system for two reasons
- the chimney may be very unsafe. Frankly, after a chimney fire I would never use that chimney again without first having an expert and thorough inspection of the entire assembly
- the damage, if traced to the fire, may be covered by insurance.
(Nov 21, 2012) Dee said:
I have a glass plate hearth on top of an oak floor and under my log burning stove. It is about 18 months old now. Over the last few months the wood floor underneath the clear glass hearth has started to darken and grow mold. Now I have droplets of water under the glass and this is clearly where the mold is coming from. What can be causing this? The rest of the wooden flooring is fine, with no mold or dampness even under rugs.
I'm not sure where it's coming from, but if you are seeing mold growth there has to be a moisture source. Are you sure it's mold?
(Feb 23, 2013) KLynne said:
Inspection of the fire box in the house I am renting revealed cracks and chips. The tech said they are small and adv I could burn occasional fires but not too much wood and no more than 2 hours. Owner will not replace fire box. I don't want to use it at all due to the cracks and chips but was researching online and saw mention of fireclay mortar that can be used to repair a fire box. Is this a practical and safe way to repair the fire box and be able to use the fireplace?
(Apr 24, 2014) Roger said:
If the firebox is elevated 12" or more off the floor do you still need a non combustible hearth extension or con the hardwood flooring be installed up to the wall that the firebox is located in?
Roger I don't have the full picture of your installation, but a general answer is yes you need a non-combustible hearth even for an elevated fireplace. Depending on the type of fireplace and fuel, at some height the worry about radiated heat damaging the floor would of course diminish, or fall to nil. There remain spark issues.
Question: safety of shared fireplace flues?
(Jan 24, 2015) Mary Siegel said:
Have a fireplace with 2 masonry flues. One serves a first floor wood burning fireplace and the second was built to serve a lower level wood burning fireplace. Since we never used the lower level fireplace, we built an outdoor fireplace on the back side of the lower level place and used that flue to vent it. Both the first floor and outside fireplaces work fine with this configuration. However, this year we decided to put a vent free log system in the lower level (closed off) fireplace. This fireplace had a damper with a space above it. Question: is there a way to vent our vent free logs to get rid of the gas smell. Could we install a vent that goes from the space above the damper to the outside? I believe there is plenty of depth in the masonry to fit a vent but would that work and is this safe?
Sharing a fireplace flue is asking for trouble in draft as well as raising safety and fire spread concerns (which is why it's a code violation).
For a gas fireplace insert you'll want to provide both combustion air and venting as per the manufacturer's specs. You MIGHT be able to do that by building a direct vent to the outdoors for each purpose. I've done that using a small positive vent fan to assure no backdrafting. You'll be required to and should want to also ask for a building permit and inspections.
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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 email@example.com
 Roger Hankeyis 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 #211-3.1 1988 -
Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
 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.
[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.
 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.
 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,
 Uniform Mechanical Code - UMC 2009, and 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_Repair.php. 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
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.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. 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 extensive home inspection education and report writing material.
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.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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.
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.
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