Chimney flue separation (C) Carson Dunlop Associates Masonry Chimney Flue Separation Requirements

  • FLUE SEPARATION REQUIREMENTS - CONTENTS: Separation requirements for masonry flues in brick, concrete block, or stone chimneys. Code & Design Requirements for Masonry Chimney Thickness and Flue Separation Thickness. How much space is required between masonry flues? Horizontal and vertical separation requirements for chimney flues at the chimney top. How much solid masonry is necessary between masonry flues for fire and gas safety? Chimney inspection & photo guide to chimney diagnosis, & repair
  • POST a QUESTION or READ FAQs about chimney flue design specifications & codes: what is the requirement to separate individual flues passing through a single chimney structure
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Masonry chimney flue separation or spacing requirements:

This article explains the requirement for separation between individual 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.

Page top sketch courtesy of Carson Dunlop Associates. Contact Us by email to suggest content additions or corrections.

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What is the Requirement for Separation Between Masonry Flues in a Multi-Flue Chimney?

Inadequate separation between flues in a masonry chimney (C) Daniel FriedmanOur 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 Requirements for Chimney Flues At the Chimney Top

Chimney flue separation (C) Carson Dunlop Associates

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).

[Click to enlarge any image]

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.

Reader Question: how much vertical separation is needed between two flues passing out of the top of a masonry chimney?

2015/11/07 Hooman said:
We would like to place a clay flue for fireplace and a metal flue over stove in the same chimney. What are the required clearance between the flues and at the termination point at the roof?



Take a look at the sketch at the very top of this page: you'll see that even in a multi flue masonry chimney you don't run two flues through the same physical opening. Your local fire and building departments will probably require at least a wythe of masonry between the masonry flue and the opening through which you run a metal flue. You cannot run them together.

2015/11/09 Hooman said:
Thanks Dan. I was aware of the single wythe masonry between the 2 flues. My main concern is proper venting back draft. There are a lot of information in regard to flue height from adjacent roof line and so on, but I have not been able to find anything in regard to separation between 2 flues at the termination point. I guess a single wythe of masonry is all that is required.


Photo of a crowded chimney top with several issues (C) Daniel Friedman

Ah; if I've got this right, it's separation of the flue terminations vertically from one another - yes that has been discussed by various sources as a possible backdrafting issue.

Crowded multiple-flue chimney tops are illustrated just above and are discussed further at CHIMNEY "CAP" CROWN / TOP SEAL INSPECTION . You can see that all of the flues terminate close together and at about the same height. You are right that there is a risk of smoke downdrafting from one of these flues into another.

I've searched for an authoritative source on the dimensions of height separation among close-by chimney tops without success (and will continue to look).

In the ICC model code chapter 10 discussing chimneys uses the word height 8 times but none of them address the question of recommended vertical height separation of adjacent chimney flue terminations nor the downdrafting smoke issue. Within the code we find this excerpt that also appears in various U.S. state codes such as Ohio's Chapter 4101:8-10 Chimneys and Fireplaces:

1003.13 Multiple flues. When two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be at least 4 inches (102 mm) thick and bonded into the walls of the chimney.

Exception: When venting only one appliance, two flues may adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered at least 4 inches (102 mm). - Chapter 4101:8-10 Chimneys and Fireplaces, Ohio, retrieved 11/9/2015, original source:, retrieved 2015/11/09

This addresses what I thought was your original question.

The New York City chimney code adds a clear specification that the top of the chimney lining must extend the full height of the chimney from the base to 4 inches above the chimney walls - something none of the clay flue liners manage in the photos shown above. - "Table 15-4 Low Temperature Chimney Constructions", New York City Chimney Code, retrieved 11/9/2015 original source:

Metal chimney separation requirements - at least 16" (C) & Carson Dunlop Associates Toronto

My opinion is that if we install a proper rain-cap atop each flue (also protecting against downdrafts) then at a minimum the two adjacent flue top terminations should be spaced such that the uppermost surface of the actual flue opening of the uppermost flue should be six inches (or more) above the top of the rain cap atop the nearby lower flue.

But that's not code it's opinion. For masonry chimneys that are already built if you need to extend the height of one of the flues you may want to look into a flue stretcher chimney cap. A flue stretcher chimney cap can raise the flue height by as much as two feet depending on what's needed.

In a similar case: two adjacent metal chimneys illustrated by Carson Dunlop Associate's sketch just above (that is not two flues running within the same masonry chimney) we want also 16" of horizontal separation between the flues. The CDA sketch points out that smoke or even creosote damage and safety concerns may arise between two nearby chimneys even when their terminating heights differ.
See METAL CHIMNEY FLUE SEPARATION CODES for clearances inside the chimney chase or inside the masaonry flue,

Smoky oil burner chimney and close-flues at same height (C) Daniel Friedman

Watch out: if you actually see smoke coming out of an un-used flue or anywhere in the building through which that flue passes when its neighboring chimney flue is in operation there could be a second and dangerous situation: damage of the internal flues and flue liners that pass through the same chimney. A professional chimney sweep can check for and advise how to repair this problem - typically by re-lining one or both flues.


Examples of Unsafe Masonry Flues with No Separation or Insufficient Separation

Unlined unsafe brick chimney flue interior view (C) Daniel Friedman

Our photo 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:

At SHARED CHIMNEY & FLUE HAZARDS we illustrate in more detail an old chimney that lacked this flue divider and we discuss the hazards further.

Separated masonry flues but at same height (C) Daniel Friedman

Our photo above shows about 4" of separation between two clay flue tile lined flues in a masonry chimney. But errors at this chimney include

Metal Chimney Flue Clearances Within Common Chimney Chase or Masonry Chimney Relining

What separation is needed between multiple stainless steel chimney re-liners in a single masonry flue?

This topic is discussed separately at METAL CHIMNEY FLUE SEPARATION CODES


Continue reading at CHIMNEY HEIGHT & CLEARANCE CODE where we describe the importance of varying the height among adjacent or nearby chimney flues of all types, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see METAL CHIMNEY FLUE SEPARATION CODES for multiple metal flues in a single chase or inside a single masonry chimney during re-lining repair jobs

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FLUE SEPARATION REQUIREMENTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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