Reuben's Home Inspection Blog


Sub-Zero Temps, Plumbing Vents Frosted Shut

When there's a long period of sub-zero temperatures in Minnesota, plumbing vents often stop working.  The problem is that plumbing vents can get completely blocked shut with frost when it's very cold outside for a long period of time.  I've seen this at a lot of home inspections this winter.  I wrote about why houses need plumbing vents last month, which should help to explain why this stuff matters.

This first photo shows how the frost begins to form.  This is a 3" steel vent pipe at an old house in Minneapolis.  Frost was just beginning to form at the top, and could actually be seen from the ground.

Plumbing vent frost forming - three inch steel

This next photo shows a 4" cast iron vent pipe at a multi-family home in Saint Paul.  It wasn't completely blocked with frost... yet.

Plumbing vent frost forming - four inch steel

As the frost keeps accumulating with very cold temperatures, eventually the vents get completely blocked with frost, which prevents them from 'venting'.   I've included several photos here to show that all types of vents can be subject to frost closure, given the right weather conditions.

Plumbing vent frosted shut - three inch ABS

This next one just looked like it could use a cherry on top.

Plumbing vent frosted shut - two inch PVC with a cherry on top

Plumbing vent frosted shut - two inch steel

Plumbing vent frosted shut - three inch steel

The way to prevent plumbing vents from getting covered over with frost is to increase the size of the vent before it protrudes through the roof.  Here in Minnesota, the smallest size vent that can penetrate the roof surface is 2", according to Minnesota Plumbing Code section 4715.2530, Subp.2.

Keep in mind, however, that this is the minimum requirement.  That's all the code is; a minimum requirement.  As you can see from the photos above, this isn't necessarily a formula for success.  Many old-school plumbers in Minneapolis and Saint Paul knew about this problem, and it was common for old houses to have vents that were significantly increased in size before penetrating the roof, just to help prevent the vents from getting blocked by frost.

Vent size increased before roof

The photos below show how this looks in real life.

Plumbing vent increased in size before roof

Plumbing vent increased in size 2

Another way to help prevent plumbing vents from getting covered over with frost is to keep them short.  The plumbing code requires plumbing vents to terminate at least 12" above the surface of the roof to help prevent the vents from getting blocked with snow, but the higher the vent the greater the potential for getting blocked with frost.

In other words, the best height for a plumbing vent is 12" above the surface of the roof, and the wider the pipe the better.  If you look up at your roof from the ground and you can see frost accumulation at the plumbing vents, don't get too worried about it.   The frost will go away as soon as it warms up a little - maybe later in the day, definitely by April.

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 19 commentsReuben Saltzman • February 20 2013 02:59AM


Tis the season for being ever watchful of this situation...and thank you for the remedy as well !

Posted by Sally K. & David L. Hanson, WI Realtors - Luxury - Divorce (EXP Realty 414-525-0563) about 6 years ago

Hi Reuben,

Where in the world would you find a cherry to add to the top of all that frost. I bet you stopped by a Sonic on the way to the inspection. :-) Really cool pictures

Have a good day in Minnesota.

Best, Clint McKie

Posted by Clint Mckie, Desert Sun Home, Comm. Inspection 1-575-706-5586 (Desert Sun Home, commercial Inspections) about 6 years ago

So you're telling me that it gets cold in Minnesota? Who knew? Great pictures for sure. Stay warm.

Posted by Scott Seaton Jr. Bourbonnais Kankakee IL Home Inspector, The Home Inspector With a Heart! (SLS Home Inspections-Bradley Bourbonnais Kankakee Manteno) about 6 years ago

Thanks for sharing the information and the pictures. Have a good day.

Posted by Gita Bantwal, REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel (RE/MAX Centre Realtors) about 6 years ago

Reuben, good information for homeowners in your area. Come April families will wonder why their plumbing works better.

Posted by Michael Setunsky, Your Commercial Real Estate Link to Northern VA about 6 years ago

That's fascinating Reubs.  I don't think I have ever heard of that happening here.  And I love a maraschino cherry on my plumbing vent.  It looks great seeing as it's so cold there.

Who talked that last photo into that foam stuff?

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) about 6 years ago

simple solution to a costly repair potential! Great info. Love the cherry on top picture!

Posted by Linda Edelwich, Glasotnbury Office's #1 Top Producing Agent-not on (William Raveis Real Estate) about 6 years ago

Amazing!  I've always wondered if something like this could happen.  Thanks for sharing!

Posted by Tom Jansson, Chicago Area Home Inspector - InterNACHI Certified (Acuity Home Inspections) about 6 years ago

One would hope that contractors would have this information.

Posted by Edward Gilmartin (CRE) about 6 years ago

Time to move to Arizona!  We don't have that problem in Phoenix!

Posted by Adam Tarr, PC -GRI, ABR, CDPE, RSPS, ePro - Designated Broker (MavRealty) about 6 years ago

Fascinating subject matter professionally presented ...good post and thank you with a cherry on top too

Posted by Richie Alan Naggar, agent & author (people first...then business Ran Right Realty ) about 6 years ago


Since we too spend a lot of time in certain areas around Steamboat Springs wayyyy below zero, your post was informative. Love the marachino cherry too-a little humor helps all posts!

Posted by Charlie Dresen, Steamboat Springs, CO e-Pro (Steamboat Sotheby's International Realty) about 6 years ago

Reuben -- it would seem that there might be more chance of the pipes icing over, if there is lack of snow cover - at least gauging by your photos.  Do you know if that is the case?

Posted by Steven Cook (No Longer Processing Mortgages.) about 6 years ago

Here, because of snow depth, the recommended height above the roof surface is 18". Don't always see it though.

I would have three questions about what is seen in the last photo.


Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) about 6 years ago

Reuben  Awesome post. Rarely do I see that type of frost build up on the vents out here but you guys get longer cold spells in your neck of the woods than I do. The cherry was a nice touch ; )

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) about 6 years ago

I have never seen that happen here. While it gets cold, almost never below zero and if it does, it doesn't lasts long. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) about 6 years ago

Sally & David - Thanks!

Clint - I had to leave the inspection, go to a Sonic, then come back to take the picture!  :-)

Scott - yeah, it's gets a little chilly around here... mostly between October and April.  It's not too bad for the other six months. 

Gita - thanks, you too.

Michael - definitely. 

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 6 years ago

Jay - yeah, the frosted shut plumbing vent thing is a super-cold-climate phenomenon, for sure.   As for the foam, that stuff is getting more and more popular around here.  You'll be seeing more of it :)

Linda - thanks.

Tom - not often, but it certainly happens.  

Edward - they know, but most don't do anything to fix it.

Adam - I don't want to hear about it ;-)


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 6 years ago

Richie - thanks!

Charlie - I couldn't help but add that cherry :)

Steven - I've never considered that possibility, but you're probably right.  It makes perfect sense.

Robert - what questions would you have with that last photo?  I might know who insulated that attic.  I could probably have them answer any questions you might have.

Donald - count your blessings.

James - see my comment above to Donald.  Same to you ;-)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 6 years ago