Too-short metal chimney (C) D Friedman Chimney Top Extensions to Improve Draft & Chimney Performance

  • CHIMNEY HEIGHT EXTENSIONS - CONTENTS: Chimney top extensions, purposes - rooftop inspection. How to fix a chimney that is too short or that does not extend far enough above the rooftop
  • POST a QUESTION or READ FAQs about extensions to chimney height at the rooftop: how do we correct a chimney that is too short? What sorts of chimney top extensions are proper and what others are improper or even unsafe?

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This article describes methods of extending the height of a chimney above a rooftop to improve chimney draft and performance and also to improve fire safety. Our page top photo shows a too-short metal chimney serving an oil fired heating boiler.

Also see CHIMNEY HEIGHT & CLEARANCE CODE for details about the required height of a chimney above the building roof.

This website provides 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.

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Chimney Extensions for Improving Chimney Draft

Chimney Extender (C) Carson Dunlop AssociatesA chimney that is too short is unsafe, risking a building fire from sparks or heat, and it often is also a source of poor venting, resulting in bad draft, heating equipment malfunction and even unreliable or unsafe building heating.

[Click to enlarge any image]

Often the repair for a too-short chimney or flue, whether masonry or metal, is simply to extend the chimney to the proper height above the building rooftop.

But chimney height extensions must be properly designed, connected, or installed, or the result can be leaks into the structure or an ineffective chimney height extension that doesn't work or worse, is unsafe. Here we illustrate improper and unsafe chimney height extensions.


A chimney extension may be added, often combined with a rain cap that makes sure the chimney faces away from the direction of incoming wind, in order to improve chimney performance.

Sketch courtesy of Carson Dunlop.

Chimney cap and no crown (C) Daniel Friedman

Here is an odd case of a chimney rain cap on a metal flue sent through a single-brick wythe old unlined chimney which has no crown at its top. This is not a properly installed chimney extension nor a reliable chimney relining job.

Water continues to run down this flue in wet weather, and we're not going to have a very confident opinion about the safety of that old metal flue either as it's not stainless steel and is likely to be rust-damaged.

Also, unless we can see into this chimney, we don't know if the rusty, questionable metal flue extends fully down through the building.

Watch out.

Sometimes the metal "flue liner" you see projecting up out of a masonry chimney is a little shortcut: a metal flue extender may have been installed just at the chimney top.

Is this a safe "chimney extension or chimney relining job"?

When a masonry flue has been damaged and is unsafe, a chimney sweep or repair company may propose chimney re-lining to improve the chimney's safety and performance.

Actually from outside we're not sure what we're going to find at this property. Is this an attempt to "re-line" a bad chimney flue or was it an attempt to vent some other heating appliance up and out through an existing chimney? Is the chimney in use by two different heating systems or just one?

A professional chimney re-lining operation would have not left a chimney like the one shown here using easily rusted steel, no protection from weather or animals at the top, flimsy mounting of the metal flue, possibly an under-sized flue, possibly an unsafe sharing of a chimney between two different heating systems or sources.

The owner or inspector should be very nervous about this installation and should investigate it further.

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