Fireplace chimney flue dimensions:

Table of required chimney flue size and flue dimensions for masonry fireplaces.

To provide adequate draft and safe venting of combustion products the chimney flue for a fireplace has to be adequate in size or cross-sectional area.

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 related fireplace and chimney topics.

Masonry Fireplace Chimney & Flue Size Requirements

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.

[Click to enlarge any image]

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.

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.

Notes to the tables

W = fireplace width measured across the front opening, in inches; H = fireplace opening height, mesured from hearth up to underside of top of fireplace opening; D = fireplace depth measured from the fireplace face to back, measured at the fireplace floor, not at a higher point where the sloping fireplace back will give a shorter measurement

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Notes to the tables above

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: Amrhein, James E., S.E., Masonry Fireplace and Chimney Handbook, 2d Ed., (1995), Masonry Institute of America, 2550 Beverly Blvd., Los Angeles CA 90057, Tel: 213-388-0472. Table 5B, pp. 104-105.

Note: This manual has been prepared to include the requirements of the 1994 Uniform Building Code, Energy Conservation REquirements of the State of California, and the U.S. Department of Housing and Urban Development.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-09-21 - by (mod) -

Mike, you will need a flue liner. Concrete block is not intended to serve by itself as both the chimney structure and its flue.

On 2020-09-20 by mike

I am planning to build an outdoor fireplace out of concrete.blocks. The hearth will be lined with fire brick. My question is must the flue be lined with fire brick
as well or can I just leave it concrete block?

On 2020-07-25 by Tomo

I have outdoor fireplace for barbeque with opening 170cm x 100cm, chimney dimensions 40cm x 30cm and chimney is 50cm tall. Every time it send smoke back to opening. I see perfect ratio is 1:8 and mine is 1:14.2. Do you think i can fix this problem with extending chimney for 1 meter or resizing opening on 140-150cm x 80-90cm?

On 2020-05-12 - by (mod) -

Installing the proper Chimney Cap can actually sometimes improve draft, and in more extreme cases it may be necessary to install a draft inducer fan.

On 2020-05-11 by john

I have a 8 inch terracotta flue supporting 42 inch hearth fireplace opening, what can I do aside from replacing chimney to get a good draw up chimney? The chimney itself was belo wthe peak of house - I'm raising it to 3 feet above peak.
-

On 2019-10-20 - by (mod) -

Bob, calculate the total area.

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]

On 2019-10-20 by Bob

I have a fireplace inside my house and it's opened on all 4 sides and the clear opening is 39"x36" on each side. My flue height is 30 to 32 ft and asking what size should my round stainless steel flue be. Thanks

On 2019-10-17 - by (mod) -

Yes Alfred - we repeat your question and give a detailed reply at

FIREPLACES & HEARTHS - home - https://inspectapedia.com/chimneys/Fireplace_Design_Specifications.php in our discussion titled

Benjamin Thompson Count Rumford & the Rumford Fireplace Design, Specifications, Proportions

On 2019-10-17 by Alfred

Is there an effect if the fire box is narrowed from front to back.

I have a round masonry flue size of 7.5 inc diameter. What should be its throat dimension and opening dimension

On 2019-09-10 by Henry

What size fireplace flue tile is necessary for a 4 ft by 4 ft opening?

On 2018-12-09 - by (mod) -

Jeremy

Thank you for an interesting fireplace flue safety question. The bottom line is I don't know.

When we mix materials using two completely different types of material I'm not sure how anyone can assert but the end result would be a listed are classified standard solution.

It might be fine but I just don't understand the repair. Perhaps you could identify the manufacturer of the extension product that you used and we could ask them what they think of your repair.

On 2018-12-07 by Jeremy

I had to tear the top 10' of my decrepid masonary structure down and rebuild. I tied in to the 2 existing 12x12 clay tiles with class A adapter plates then ran 10" class A solid pack flue back up to code height replacing the compromised 12x12 tiles.
Is this a proper repair .

Question:

(Aug 14, 2011) Robert said:
When adding support brace under hearth should new footing be poured to support brace and weight

Reply:

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?

Reply:

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.

Question:

(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.

Reply:

Dee

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?

Question:

(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?

Reply:

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?

Reply:

Mary

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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Continue reading at FIREPLACE HEARTH DIMENSIONS & SUPPORT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• FIREPLACES & HEARTHS - home
• CHIMNEY FIRE ACTION / PREVENTION
• CHIMNEY FLUE INSPECTION CAMERA
• CHIMNEY DRAFT & PERFORMANCE
• COMBUSTION AIR DEFECTS
• CREOSOTE DEPOSITS, FIRE HAZARD
• FIREPLACE CHIMNEY ROOF CLEARANCE
• FIREPLACE DAMPER DEFECTS, REPAIRS
• FIREPLACE FLUE SIZE SPECIFICATIONS
• FIREPLACE HEARTH DIMENSIONS & SUPPORT
• FIREPLACE INSERTS INSTALL, INSPECT, REPAIR
• FIREPLACE INSPECTIONS
• FIREPLACE INSPECTION PRE-FAB
• FIREPLACE DAMAGE & UNSAFE HEARTHS
• GAS FIREPLACE VENT CLEARANCE REQUIREMENTS
• RUMFORD FIREPLACE DESIGN RULES
• WOOD BURNING FIREPLACE ROOF CLEARANCE

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FIREPLACE FLUE SIZE SPECIFICATIONS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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