Photograph of a loose, improperly installed, slanted plumbing vent above the building roof Backdrafting, Wet Weather & Causes of Sewage or Septic Odors

  • BACKDRAFTING & SEWER/SEPTIC ODORS - CONTENTS: What is backdrafting and how does backdrafting occur in buildings? Why is it dangerous? How backdrafting can cause sewer gases to enter a building. Leaks at bathroom fan cause sewer gas odors in building. Sewer gas backup to roof enters building through bath vent fan opening. How to diagnose sewer odors in wet or cold weather. Causes and cures for sewer gas odors related to wet or cold weather. How to find and cure bad smells in buildings
  • POST a QUESTION or READ FAQs about backdrafting and wet weather causes of sewage or septic odors and smells at or in buildings

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This article discusses the role of back drafting in buildings, exhaust fans, or other building air movement in causing sewer gas or septic odors (and other building smells and odors).

This article is part of our series SEWER GAS ODORS that helps track down and cure the source of sewer gas or septic smells in buildings.

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How Back-Drafting Can Cause Sewer Gas Odors in buildings

Bathroom ceiling vent fan, heater, light combination (C) Daniel FriedmanWhat is "Backdrafting" in buildings?

"Backdrafting" refers to indoor conditions that create sufficient negative air pressure inside a building such that gases may be drawn into the building from a plumbing drain system or such that heating appliances may lack adequate combustion air and may produce dangerous carbon monoxide.

Watch out: backdrafting in a building can be dangerous, causing intake of explosive methane gases from a sewer system, or potentially fatal carbon monoxide gas hazards from heating appliances.

In this article (below) we discuss the causes, effects, and cures for backdrafting in buildings that affects the building plumbing drain vent system. Unsafe heating appliance conditions caused by backdrafting are discussed at BACKDRAFTING HEATING EQUIPMENT where we discuss dangerous backdrafting conditions that can make heating appliances unsafe and can produce potentially fatal carbon monoxide gas hazards in buildings.

What Causes Building Backdrafting Hazards?

Any one or more of the factors listed below can create negative air pressure inside a building such that backdrafting occurs or may occur at building drains or plumbing vents:

Field Report: Example of Rooftop Plumbing Vent Backdrafting Causing Sewer Gases to Enter a Building - and The Cure

Rooftop vent fan hood (C) D Friedman

Sewer/Storm Drain Flooding:  indoor sewer gas odors have been tracked to a combination of wet weather, flooding sewer and storm drains, and backdrafting in the building due to improper bathroom exhaust fan vent installation.

Our photo (left) shows a poorly-installed rooftop bath vent fan outlet, patched to try to stop leaks, and likely to be blocked by snowfall on the roof surface. But you can see the "flapper" that closes this vent opening when the fan is not running.

Reader Jose Iturraspe provided the following sewer gas odor diagnosis and cure field report:

A customer complained of sewer smell in the home that occurred only when it rained, and only in his master bath.

Inside, at the exhaust fan housing in the ceiling, we found that there was no flapper. [The flapper is designed to stop indoor air movement up out through the exhaust fan system when the fan is not turned on.]

Outside, on the roof, the exhaust fan gooseneck had no flapper either. [The rooftop flapper is designed to keep birds, insects, and in some climates wind-blown rain or snow out of the exhaust fan system when the fan is not running.]

The plumbing vent was located about 2’6” from the exhaust fan gooseneck.

We replaced both flappers in the bathroom exhaust fan system, and the sewer gas odor issue has gone away.

My best guess is that during a rain, the storm drain/sewer had its sewer gas displaced with the rain water runoff, pushing the gas out the plumbing vents.

At the same time we think that the winds combined with the missing flappers on the bath exhaust fan created a situation where the gas was being sucked in thru the exhaust vent goosenecks.

See CORROSION & MOISTURE SOURCES in PANELS for examples of how backdrafting or negative building air pressure can cause unexpected air and moisture movement, condensation, and moisture-related problems such as rust, corrosion, or odors, or mold.

Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are welcome and are listed at REFERENCES.


Continue reading at TRACK SEPTIC / SEWER ODOR to SOURCE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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