All the plumbing fixtures in your home need air supplied to the drain pipes for the liquid to flow properly, and the pipes poking through the roof are there to provide it. Every home is required to have at least one plumbing vent above the roof, and most have several.

   To understand why air supply is is so important, think of opening a 2-liter bottle of soda pop and turning the bottle completely upside-down to pour. It will drain slowly, and make a gurgling sound as air bubbles fight their way up to the top of bottle to displace the falling liquid. But, when you turn the bottle horizontally and allow some space above the stream of soda for air to enter the bottle, it flows smoothly. The same principle applies to a home’s plumbing system, with a poorly vented or unvented drain making similar gurgling, slurping noises.

 As the liquid drains out of a plumbing fixture, it flows through a U-shaped pipe loop—called a P-trap—that holds a reservoir of liquid that acts as a plug to keep any sewer gases from rising out of the drain. The vent is always located behind the P-trap, which allows the P-trap reservoir to remain intact because air is not sucked through it for drainage.

   The hole in the roof where the vent pipe penetrates is usually weatherproofed with a jacket made of lead, called a “boot.” The malleable lead is bent over the pipe rim at the top and slipped under the roofing at the base. Unfortunately, because lead has a sweet taste, squirrels love to chew them up and one the defects we regularly in our home inspection reports is a “squirrel-damaged pipe boot,” like in the photo below. When the wrap of lead over the rim is chewed away, rain can come down the outside of the pipe into the attic. It’s usually not enough to do major damage, but will cause staining and rot at the roof sheathing over time.

    Some roofers tie aluminum around the the lead boot for protection from squirrels, and we know homeowners that surround them in chicken-wire. But the most elegant solution is the one shown below, which we are seeing more often now on new roofs in our area.

    By the way, the flue and combustion air intake pipes for a high efficiency gas furnace look similar to a plumbing vent pipe, but are taller and do not terminate vertically open at top, like in the photo below.

    Also, see our blog posts What is plumbing venting? When was it first required for plumbing drain systems? and What are those metal boxes on the roof?

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  To learn more about roofs and attics, see these other blog posts:

• Why is my roof sheathing sagging between the trusses?

• Why is granule loss a problem for an asphalt shingle roof? 

• What are the mistakes to avoid when doing attic improvements?

• What causes roof shingles to curl up at corners?  

• What causes shingles to buckle along a line on the roof?

• What causes leaks at a fake roof dormer? 

• What causes a sagging roof ridge line?

• What causes bubble-like blisters in a built-up and gravel roof?  

• Why does it cost so much more to replace a steep roof than a low slope roof? 

• What is "ponding" on a flat roof?

• Is an attic required to have a light by the building code? 

• How can I inspect my roof for hurricane damage?

• Why is premature curl of roof shingles a problem?

• How can I tell if a roof has more than one layer of shingles? 

• What are the common problems with attic insulation? 

• If my roof is not leaking, why does it need to be replaced?

• How can I be sure my roofing contractor got a permit?

• How many layers of roofing are allowed on a home? 

• What are the dark lines running parallel to shingles on my roof?

• Can metal roofing be used on a low slope/pitch roof? 

• How can I make my roof last longer?  

• What are the warning signs of a dangerous attic pull-down ladder?

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• How can I tell if the house needs a new roof?  

• Why does my homeowner's insurance want a roof inspection?

• What are the hazards to avoid when going into an attic? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.