Chimney clearances (C) Daniel Friedman Height & Clearance Requirements for Chimneys
     


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Chimney height & horizontal clearance specifications:

This article describes the height requirements for chimneys, including rooftop clearances and overall chimney height necessary for proper chimney draft and function and for fire safety. We describe what can go wrong with chimneys that are not built to proper height or with proper clearances from other building features, including improper or unsafe heating appliance or fireplace or woodstove operation, odors, soot, draft issues, etc.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys as well as chimney construction & repair methods. A variation on a "too short" metal or masonry chimney is a chimney that may look tall-enough to some folks but which lacks adequate clearance from a nearby roof slope, as we illustrate and explain in the text below.

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Outdoor Chimney Height & Clearance Requirements

Photograph of a too short masonry chimney.

Chimney to Roof Clearance: from the ground you can guess and from on-roof access you can measure to determine if a chimney is simply too short for fire safety and code compliance.

The length of a horizontal line drawn from the top of the opening of the chimney flue to the point at which that line touches the roof surface should be ten feet or more (the blue line in our photo). And the top of the chimney flue must be at least 2 ft. above that point.

In our photo, our measuring tape is pulled to about 4 1/2 Ft. - this chimney top is too close to the roof surface (the horizontal blue line) and too short as well (the vertical green arrow). Detailed specifications for chimney vertical and horizontal chimney clearances are given below.

The height of the chimney above the roof surface, measured at the up-slope side of the chimney, from the roof surface to the chimney top, should be at least three feet - the green line in our photo. (This is a bit of an odd measurement and chimney clearance case because the chimney side is actually about 8" away from the roof edge. But the chimney-to-roof clearance rules still apply.)

This measuring tape location shown in this photo is slightly incorrect (I didn't want to lose my tape down the flue while taking the picture). Measure from the edge of the chimney flue closest to the roof, horizontally, until you touch the roof surface to check the ten-foot clearance rule distance. That's the light blue line in our photo.

Article Contents

Check the chimney top for damaged masonry (or rusted metal), a missing cap, damaged, cracked, or missing top seal or crown on the top of a masonry flue, and here, an important discovery (at least in some jurisdictions) is whether or not the chimney is single wythe or thicker masonry and whether or not the chimney has (or perhaps needs) a chimney liner.

Chimneys Too Short - What is "too short" and What Problems Occur?

Too Short Chimney (C) Daniel Friedman Short metal chimney (C) Daniel Friedman

A chimney that is too short is unlikely to vent properly and it may also be a serious fire hazard to the building, risking setting the roof on fire. The photo at above left is a too-short masonry chimney (with no cap and other worries). A horizontal line (blue in our photo) drawn from the chimney top to where it would touch the roof surface was just about one foot instead of the required ten feet OR two feet above the ridge.

A "Hidden" "too-short" Chimney Specification: Distance Above the Flue Collar - can lead to inadequate draft

NFPA-211 1-8.2 specifies that

Natural draft chimneys and vents shall not terminate at an elevation less than 5 ft (1.53m) above the flue collar or the highest connected draft hood outlet.

Inspecting from outdoors you may not be sure if a chimney violates this rule or not - you'll also need to look inside at the heating equipment and at the building structure, ceiling height distances etc. to make an actual measurement.

Too-Short Heating Flue Can Mean Sooty Oil Burners and Puffbacks or Dangerous CO Poisoning Hazards

Metal chimney too short (C) Daniel Friedman

This height requirement is to assure that the chimney will develop adequate draft. A "too short" chimney in this case won't have a tall-enough column of rising hot gases inside to develop a safe, adequate draft. Our photograph below shows a "too short" chimney through a flat roof on a one-story home.

This too-short chimney is more than a fire hazard. It can mean that heating equipment venting into the chimney won't work properly, is unsafe, or is "forced" to work by settings that waste your heating money.

The vertical distance from the top of this chimney to the top of the oil fired heating boiler it serves is less than six feet. Our vertical red line shows that we measured about 24" of chimney above the flat roof. This seems to meet the "two foot chimney rule" but it fails the three-foot rule and also the whole chimney height was just too short to produce adequate draft.

The oil fired boiler has blown soot into the utility room and garage throughout its' life, a constant source of annoyance that probably stems from inadequate total draft even when the oil burner, boiler, and chimney flue are up to full operating temperature.

Because a short chimney can mean bad draft and sooty oil burner operation, also see OIL BURNERS and OIL BURNER NOISE SMOKE ODORS.

The draft regulator on this installation was removed by an earlier service tech in an effort to improve draft in the flue. But even that step was not enough, and of course now the oil burner can never be properly tuned.


Stack temperature on oil burner (C) Daniel Friedman

In response to owner pleas that no one had been able to fix this trouble, an oil company's service technician who worked on this flue in 2010 "solved" the long standing oil burner soot problem by setting the draft up as high as he could at the oil burner. That slowed but did not stop the sooting problem, and for sure it increased the heating cost for this building.

Stack temperature was 600 degF (photo at left) - minus room temperature, making it about 530 - a bit high.

We could have addressed this short chimney with a draft inducer fan, but a taller flue would be smart anyway, to get the chimney top higher than the roof surface. We discuss examples of extending chimney height to improve draft, performance, and fire safety separately at CHIMNEY HEIGHT EXTENSIONS.

We discuss draft inducer or "draft boosting" fans for heating systems (and maybe for some fireplaces) in detail at DRAFT INDUCER FANS.

Just how short is "too short" - we discuss chimney height and roof clearance requirements beginning below.

Carbon Monoxide Poisoning Hazards from Too-Short Chimneys

Particularly with gas fired appliances, the lack of adequate draft for any reason, including a chimney flue that is too short, can result in improper combustion and the production of dangerous, potentially fatal carbon monoxide gases. And the same inadequate draft that affects combustion in the heater can increase the risk that the gas backdrafts out into the buildings.

NFPA 211 1.7.1 specifies:

... A chimney or vent shall be designed and constructed to develop a flow sufficient to completely remove all flue or vent gases to the outside atmosphere. The venting system shall satisfy the draft requirements of the connected appliance(s) in accordance with the equipment manufacturer's instructions or the chapter on Chimney, Gas Vent, and Fireplace Systems of the Equipment Volume of the ASHRAE Handbook.

Draft-Fixes for "Too Short" Chimneys

An obvious fix for a chimney that is too short to meet the height and fire safety clearances mentioned earlier is that the chimney height must be extended above the roof for fire safety. See CHIMNEY HEIGHT EXTENSIONS.

But in some installations, such as a heating appliance installed in a one story low ceiling structure, the chimney may meet the fire clearance specifications (NFPA 211 1-8: Termination Heights) but it not be tall enough to develop adequate draft (NFPA 211 1-7). In this case it is permitted (NFPA 211 1-7-2.) to use a draft inducer fan ("a mechanical draft system of either forced or induced draft design" - DRAFT INDUCER FANS) to meet the draft requirements.

Watch out: if you use a draft inducer system the installation must assure that the heating appliance won't run if the draft inducer is not running. Most if not all modern mechanical draft or draft inducer systems include this safety feature as do direct-vent or side-wall vented appliances that use no chimney at all (another solution to some chimney problems).

Also see COMBUSTION AIR for TIGHT BUILDINGS.

Bachrach, Field, & Tjernlund are examples of companies providing draft inducing or mechanical draft boosting equipment and direct-venting equipment that skips use of a chimney altogether. [14[15[16]

Required Chimney Height Above Roof Surfaces or Ridges

Summary of Vertical & Horizontal Clearance Distances for Chimneys

Chimney clearances (C) Daniel Friedman

Masonry Chimney Roof Clearance Requirements

The sketch at left shows the proper minimum chimney height and roof clearances for a masonry chimney.

Masonry chimneys must terminate at least 3 feet above the highest point of contact with the roof structure (the vertical green arrow in our sketch at left), and chimneys should extend 2 feet higher than any part of the structure within 10 feet horizontally (measurements as shown in our sketch).

Thanks to G. Howard for text clarification, and thanks to Thanks to Mark Wolff for pointing out a previous error in our chimney height illustration - now shown correctly. [click any image to see an enlarged, detailed version]

For chimneys that are 10 Ft (3.1 m) or MORE away from the roof ridge (measured at the up-roof side of the chimney):

  1. The top of the flue opening should be at least 10 Ft (3.1 m) away from any roof surface (or other structure) as measured by a horizontal line drawn from the top of the chimney flue opening to where it would touch the roof surface. (The horizontal red arrow in our sketch)
  2. The top of the flue opening should be at least 3 Ft (0.92 m) above any roof surface. (The vertical green arrow in the sketch) I.e. a vertical line drawn along the up-roof side of the chimney, from the roof surface to the top of the chimney flue opening shall be at least 3 Ft in height. This is for masonry chimneys. For a gas vent or Type L vent this height must be 2 Ft (0.61m) or more.
  3. The top of the flue opening (termination of the chimney) should be a minimum of 2 feet above any part of a structure within ten feet. (Vertial blue arrow in our sketch).

Notice that we use the words any roof surface or an part of a structure in this explanation. That means that if there are other nearby structures, say a dormer, the same clearance rules apply.

Roof Clearances for Chimneys Less than 10 Ft. (3.1m) from the Ridge

The photo at above right is a too-short metal chimney that is less than two feet above the ridge. This chimney fails both the ten-foot rule (the blue line) and the two foot rule (the red line). A horizontal line drawn from the chimney top just touches the roof ridge.

Roof chimney clearances near ridge (C) D FriedmanFor chimneys that are LESS than 10 feet from the roof ridge (measured at the up-roof side of the chimney)

- The top of the chimney should be at least 2 Ft (0.61m) above the ridge (the vertical red line at the left of our photo)

and

- The top of the chimney should be at least 3 Ft (0.92 m) above the roof surface. (This specification makes sure steep roof slopes are cleared.) For gas vents or a type L vent this distance must be at least 2 Ft (0.61m). (Vertical blue line along the left side of the chimney in our photo).

Watch out: We also notice that the chimney cap is discolored on this metal flue - possibly indicating an overheated appliance or other unsafe condition. Did you also notice that the sides of the chimney flashing are on top of rather than under the roof shingles?

We edited our "too-short chimney" photo to extend it up to meet the following roof clearance requirements (see edited photo at above left):

Where to Make Measurements When Checking Chimney Clearances

Chimney height clearance 2 3 10 foot rule illustrated (C) D FriedmanAnother question that has come up is "do we have to measure from the chimney cap or from the (lower) opening at the top of the flue?

In specifying clearances the NFPA 211 refers to the "Highest Point" of the chimney.

Click any image to see an enlarged, detailed version.

Our reading of the NFPA and codes is that

  • Vertical chimney to roof measurements are made from the top of the chimney flue opening, not the top of the chimney cap - reasoning that it is from the flue opening that an ash or spark might exit the chimney, making that the point of concern.

    Otherwise we could end up with an unsafe (too short) clearance of a low chimney on which someone stood a ridiculously tall "cap".
  • Horizontal chimney clearance distance measurements are made between the building surface (roof or nearby wall or other nearby roof surface) and the nearest facing surface of the chimney side.

Reader Question: Exactly where do we measure chimney roof clearances?

Refer to the sketch titled "Masonry Chimney Roof Clearance Requirements".
The line that is drawn that illustrates the 10 feet run is placed in the middle of the chimney. Depending on where you place this line has a HUGE impact on how high the chimney needs to be. If this line were drawn on the top of the chimney it would need to be substantially taller. Where am I supposed to take the measurement from?? - Jason Vetter 3/11/12

Reply:

Jason, you are dead right - the page top chimney to roof clearance sketch has long bothered me too, leading to other photos and sketches on this page that showed the required distances correctly. We have reviewed, edited, and adjusted all of the chimney clearance measurement drawings and photos in the article above to clarify these distances.

The original NFPA-211 drawings on which the page top sketch was based were equally confusing.

For chimneys that are 10 Ft (3.1 m) or MORE away from the roof ridge (measured at the up-roof side of the chimney):

  • The top of the flue opening should be at least 10 Ft (3.1 m) away from any roof surface as measured by a horizontal line drawn from the top of the chimney flue opening to where it would touch the roof surface.
  • The top of the flue opening should be 3 Ft (0.92 m) above any roof surface. I.e. a vertical line drawn along the up-roof side of the chimney, from the roof surface to the top of the chimney flue opening shall be at least 3 Ft in height. This is for masonry chimneys. For a gas vent or Type L vent this height must be 2 Ft (0.61m) or more.

Notice that I use the words "any roof surface" in this explanation. That means that if there are other nearby structures, say a dormer, the same clearance rules apply,

For chimneys that are LESS than 10 feet from the roof ridge (measured at the up-roof side of the chimney)

  • The top of the chimney should be at least 2 Ft (0.61m) above the ridge

and

  • The top of the chimney should be at least 3 Ft (0.92 m) above the roof surface. (This specification makes sure steep roof slopes are cleared.) For gas vents or a type L vent this distance must be at least 2 Ft (0.61m).

Another question that has come up is "do we have to measure from the chimney cap or from the (lower) opening at the top of the flue? In specifying clearances the NFPA 211 refers to the "Highest Point" of the chimney.

  • My opinion of the NFPA and codes is measurements are made from the top of the chimney flue opening, not the top of the chimney cap - reasoning that it is from the flue opening that an ash or spark might exit the chimney, making that the point of concern. Otherwise we could end up with an unsafe (too short) clearance of a low chimney on which someone stood a ridiculously tall "cap".

Thanks so much again for the reminder to fix this, Jason. We welcome reader questions and comments. We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone.

Reader Question: My masonry chimney extend 2' above the top of the pitch. Does it have to be extended to 3'

Chimney height clearance 2 3 10 foot rule illustrated (C) D FriedmanMy masonry chimney extend 2' above the top of the pitch. Does it have to be extended to 3' above to comply with the NBC? - Frank

Reply: here is a summary of chimney code and fire clearance rules: the two-foot, three-foot, ten-foot rule for chimney heights:

Click any image to see an enlarged, detailed version.

Frank,

Some building code officials, builders, and chimney professionals call this simple chimney fire clearance code the
"two-foot, three-foot, ten-foot rule for chimney heights" - which we explain as follows:

Your chimney top to roof clearance (for fire safety) is correct if the chimney height extends:

  • The two-foot chimney rule: The chimney top (shown at above right) must be two feet above the ridge of the roof (If the chimney is at or very close to the ridge) This is the vertical red arrow at the left side of our right-hand metal chimney height sketch above.
  • The three-foot chimney rule: The chimney top must be three feet (or more) above the roof surface (measured right next to the chimney, at the up-slope side of the chimney). This is the vertical green line in our chimney clearance sketch above.
  • The ten-foot / two-foot chimney rule: The top of a chimney must also extend at least two feet above a roof, ridge, or other structural component that is ten feet or less away in horizontal distance. This height is shown by the vertical blue arrow shown at the left hand chimney in the sketch above and the ten foot line is shown in black in the same drawing.

Take a look at the chimney height clearance requirements sketch at the top of this page. The only confusing detail in that sketch is that the "ten foot chimney height rule" horizontal line was shown as drawn mid way down the chimney - really that line is drawn from the chimney TOP to the nearest roof surface.

Reader Question: My A-Frame chimney is just one foot above the roof if measured at the ridge - do I need to extend it

I have an "A" frame cottage with a new metal roof. The woodstove has a 10" insulated metal chimney which extends above the roof line by approximately 7 ft. and is above the peak by maybe 1 ft.

Reply: details of the ten-foot rule for chimney distances from nearby roof surfaces

Bruce:

Ok, so technically your flue is one foot short; perhaps given the very steep roof and that it's metal, I'm not as worried as I'd be otherwise, but it's possible that the chimney is not fire safety and code compliant for a second reason: if you were to draw a horizontal line from your chimney top towards the ridge, and if the horizontal distance from the top to the ridge is less than 10 feet, then properly your chimney needs to go up another foot.

On the other hand if the horizontal distance from your chimney to the ridge is ten feet or more, you're ok as is.

In this article you can find details on required chimney heights above the roof line.

Reader Question: What is the chimney clearance from the chimney pipe to a second story window?

i have a two story home. i just built a single story room off this home with a wood stove. what is the clearance from the chimney pipe to 2nd story window? it is already 2 feet above the ridge and 12 feet from the 2nd story windows - Charlie

Reply:

Charlie I'm not sure I understand the picture and measurements of the addition you describe. If your chimney is two feet above the ridge of the upper roof it would certainly not be too close to any windows.

If your chimney is two feet above the ridge of a roof over the one story addition, if you can't get more than 12 feet of horizontal distance between chimney and windows of the upper floor, you probably need to extend the chimney above the upper roof, following the ten-foot horizontal line rule we describe above.

Otherwise in at least some conditions, someone is going to open an upper floor window and find smoke blowing into the room.

Reader Question: what is the minimum chimney height up through a flat roof near two windows?

What is the minimum height clearance needed for a wood burning stove pipe when exiting a flat roof. the roof is in front of two 1st floor windows
thanks - Paula Moss 10/2/12

Reply:

Paula:

The minimum chimney heights above roof are in the article above. For the case you describe, there are other distances like from windows, and depending on the total chimney height, even meeting the above-roof minimum distance (3 ft. on a flat roof) may not be tall enough.

Roof Clearances for Wood Burning Fireplaces

Wood burning Fireplace Roof Clearance Requirements

Minimum chimney height (C) Carson Dunlop Associates

Carson Dunlop's sketch above shows that the minimum chimney height above a wood burning fireplace is 15', and that a shorter minimum of 5' may be acceptable above the draft hood of a gas furnace is allowed in some jurisdictions.

[click any image to see an enlarged, detailed version]

Clearance specifications between Adjacent Metal Chimneys

Adjacent metal flue heights should vary (C) Carson Dunlop Associates

Adjacent Metal Chimney Separation Requirements

In addition to the requirement for safety fire clearance from rooftops and other building components, separate metal chimneys that are too close to one another may cause damage resulting in poor chimney performance or an unsafe chimney.

[Click to enlarge any image]

Carson Dunlop's sketch at left shows that metal chimneys should be at least 16" (that's 16 inches) apart to avoid damage.

See CLASS A CHIMNEYS, MetalBestos™

B-Vent Rooftop Clearance Specifications for Gas Fired Appliances & Heaters

Type B Gas Vent (C) Daniel FriedmanTable of Type B-Vent Rooftop Clearance Requirements

Vent caps larger than 12" must be located at least 2 feet above the highest point and 2 feet higher than any portion of the building within a horizontal distance of 10 feet.

Vent caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows:

Roof Pitch: rise/run in inches Minimum feet above roof surface
Flat to 7/12 1.0 foot above the roof surface
7/12 to 8/12 1.5 feet above the roof surface
8/12 to 9/12 2.0 feet above the roof surface
9/12 to 10/12 2.5 feet above the roof surface
10/12 to 11/12 3.25 feet above the roof surface
11/12 to 12/12 4.0 feet above the roof surface * Continues to 21/12 pitch at 8.0 feet

The 1992 Vent Sizing Tables require that all Type B gas vents terminate above the roof with a listed cap or listed roof assembly in accordance with the manufacturer's instructions.

For details about metal chimneys see METAL CHIMNEYS & FLUES

 

Continue reading at CHIMNEY HEIGHT EXTENSIONS or select a topic from the More Reading links shown below.

Or see CHIMNEY CLEARANCE to NEIGHBORS

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CHIMNEY HEIGHT & CLEARANCE CODE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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