How to Fix a Leaky Gas Flue Roof Vent – Part 4

By |Last updated on |Heating & Air, Roof|23 Comments

How to fix a leaky gas flue roof vent by replacing the flue vents, storm collars and rain caps. This repair is continued from How to Fix a Leaky Gas Flue Roof Vent – Part 3.

Disconnect the Old Gas Flue Pipe

The HVAC contractor began the work by disconnecting the old vent pipe at the first joint below the roof deck in the attic. To disconnect the pipe, just slide the retaining collar up and wiggle/lift the pipe to separate. The shiny new pipe sections are shown in this photo.

Furnace Vent Pipe in the Attic

Roof Gas Flue Vent Replacement

With one man working in the attic, the other HVAC technician went to the roof top to disassemble the vents. He began by lifting the retaining collar and wiggling off the top section of vent pipe. The top section with the rain cap is laying to the right in the photo below.

The storm collar is removed by cutting the caulk line with a utility knife and wiggling the collar up the pipe. Once clear of the flashing cone, the storm collar was cut in two with tin snips and pulled free of the vent pipe. Cutting the collar was quicker than working it off the length of the pipe.

Removing the Storm Collar from the Old Vent Pipe

After removing the storm collar, the caulk line at the flashing cone is cut and the old pipe is pulled down into the attic. The retaining collar at the bottom of the vent pipe inside the attic will not fit through the flashing cone, so the pipe must be pulled down.

The second vent stack is removed by the same procedure.

New Roof Gas Vent Installation

Flashing Cones

We originally planned to remove the old flashing cones. The HVAC technician advised against it, explaining the cone was in good shape and he was concerned about the shingles being brittle in the cool (mid-40’s) December weather. We agreed to leave the flashing cones in place.

The old caulk is trimmed and peeled away from the neck of the flashing cone with a utility knife. A clean bare metal surface is needed for a water tight caulk line.

Cleaning Up the Flashing Cone Neck

Flashing cone after cleaning the old caulk from the neck.

Roof Vent Flashing Cone Cleaned and Ready for New Pipe

The second man inside the attic pushes up the new section of vent pipe through the flashing cone. The man in the attic then attaches the vent pipe existing lower section inside the attic.

Pushing Up New Type B Vent Pipe

The joint between the vent pipe and flashing cone is caulked with Adaseal HVAC/R 100% RTV silicone high temperature caulk.

Caulking the Vent Pipe at the Flashing Cone

The storm collar is installed by sliding it over the pipe and down to the flashing cone. The storm collar is sealed with a generous bead of silicone and smoothed (radiused) with a finger.

Caulking the Storm Collar

The rain cap simply slides onto the vent pipe. The technician secured the rain cap with a short 1/4″ sheet metal screw to ensure it doesn’t blow off in a very strong wind. The screw is optional and must not penetrate inner section of the double wall pipe.

Installing the Rain Cap on the Roof Vent Pipe

The vertical seam on the flashing cone was caulked with silicone and the vent stacks are spray painted black for appearance. The two vent stacks are also now equal height for a better appearance.

New Gas Vent Stacks

Rusted Flue Vent Stacks

The old vent stacks were saved for inspection to see if there was an obvious source of the rain water leak. The black RTV sealant beneath the rain cap and on the storm collars is only one week old and did not stop the leak. The vents are a bit dented having been dropped off the roof onto the lawn.

Old Type B Gas Vents

The rust hasn’t corroded through the pipe wall, but the rust lines indicate rain water getting inside the pipe.

Rain Cap and Vent Pipe Rust

It’s really hard to say if this was the leak source, or maybe water was getting inside the vertical seam and channel past the storm collar.

Rusted Vent Pipe

No More Roof Vent Leaks!

The new Type B roof vents were installed only a day before a new cold front brought steady rain. The vents are water tight and no water trails are to be seen on the shiny new sections of pipe in the attic.

The job took about 2 hours and the final cost was $350 versus the $400 original quote because we decided not install new flashing cones.

Project Update

Several years after this repair the entire roof was replaced due to hail damage. I had the gas flue vents completely rebuilt, including the base flashing cones. You can read about that project Hail Damaged Roof Replacement – Gas Flue Vents.

Hope this helps,

Bob Jackson

Copyright © 2019   Reproduction strictly prohibited.


  1. Eric January 26, 2010 at 7:28 am - Reply

    Hi, Bob:
    This is a very detailed and descriptive article you have posted. I have the similar problem with water leaking inside the vent pipe. I had the vent cap replaced last year and resealed the storm collar. The leaking is still occurring. Even though there is not much water sipping through the pipe but it really bothers me when it drips on the attic. My roof is very steep and most HVAC guys will not want to do the job. Most likely, I will ask roofer to see if they can do this type work to replace vent stack, storm collar, and vent cap.

    • Bob Jackson January 26, 2010 at 7:40 am - Reply

      The leak could be along the vertical seam in the vent pipe. I tend to think this was my problem.

      Most roofers don’t normally have the gas vent parts in stock. Recommend calling HVAC companies until you find one that does roof work. HVAC companies that do “new construction” routinely install roof vents and can verify your vent pipe is correctly installed and condensation isn’t a factor. The HVAC guy that replaced my vent said he was a former roofer.

      Please post back with your repair and the cost.

      Bob Jackson

  2. Tom April 11, 2010 at 11:53 pm - Reply

    When they installed your new vent pipes did they put sealant on the vertical seams exposed above the storm collar or the cap?

    • Bob Jackson April 12, 2010 at 4:19 am - Reply

      The HVAC service tech only sealed the vertical seam on the cone flashing (because we kept the flashing) and the joint at storm collar. The vertical seam on the flue pipe shouldn’t need sealing. The rain cap slides over the flue pipe and doesn’t need sealing.

  3. Tom May 14, 2010 at 4:44 pm - Reply

    What kind of spray paint & Primer did the hvac tech use for your b-vent pipes?

  4. Tom May 18, 2010 at 10:29 pm - Reply

    Was it the Krylon Rust Tough Rust Preventative Enamel?

    • Bob Jackson May 19, 2010 at 10:15 pm - Reply

      Can’t say for certain. It was Krylon flat black – I think enamel would be unnecessary.

  5. James July 14, 2010 at 4:53 pm - Reply


    Great job detailing your experience. Just one question, because I noticed that the stacks are now shorter. Since this is an exhust, are they not supposed to be two feet higher that the highest point of your roof ten feet away? This case your ridge line?

    • Bob Jackson July 14, 2010 at 5:41 pm - Reply

      I’m aware of this rule and took notice of the vent stacks on homes in my neighborhood with 12/12 pitch roofs. Most stacks are no more than 2 to 4 feet high and badly fail the “2 feet higher than the highest point 10 feet away” rule. Here and there, a vent stack will be very tall that meets the “2 ten rule”, and it’s almost always crooked – probably from getting beaten up by the wind.

      Think about it: a 10 foot tall vent stack is only 10 feet away from the roof surface on 12/12 pitch roof (45 degree slope). The rule becomes impractical very quickly, especially if the vent isn’t close the ridge.

      The sample data gets more interesting on my drive to work. I’ve seen dozens of homes with steep roofs having short and tall stacks side-by-side at the same roof elevation. One stack cleared the ridge the other didn’t.

      All I can say for certain is I’ve had no problems with my vents since and I can’t see them from the street anymore.

  6. DAW August 27, 2011 at 12:42 am - Reply

    Thanks for this marvelous post. I’ve got a similar problem that I have just decided to tackle. I bought the house after new shingles were put on and it has flue always leaked during a hard rain. I’m going to start with new caulk around the flashing. Your post has helped me identify another possible solution… a storm collar.

    We’ll see how it goes and thanks for the very detailed info.

  7. Brian August 28, 2011 at 9:06 am - Reply

    I have a similar problem. It looks like I could caulk my gap from inside the attic. Any problem with me first attempting to seal from inside before calling a roofer / hvac tech?


    • Bob Jackson August 28, 2011 at 11:21 am - Reply

      I don’t recommend it because caulking from inside the attic would be temporary at best and make repair/replacement more difficult when working on the roof. If your flashing cone looks like mine, the angle between the flue pipe and flashing cone is deep and narrow when viewed from attic, making it difficult to caulk and inspect the seal. If you decide to apply caulk, use a non-adhesive 100% RTV silicone high temperature caulk designed for this type of work. You might embed a heavy construction string (yellow or orange string found at home improvement stores) in the joint before caulking and let the free end hang down a couple of feet. When the roofer arrives for a permanent repair job, you can pull the string and sever the caulk joint like a zipper.

  8. David April 23, 2014 at 1:05 pm - Reply


    Came across your postings recently and the replies. Very helpful. Hope you are still actively posting. I have a similar problem where I think wind-driven heavy rain is getting inside my attic furnace roof vent and then dripping onto the attic floor through an elbow joint. Other rains are not a problem. I thought it was because rain was getting past the rain cap (is this likely?) but now maybe it is because of a a vent seam leak. My roofer inspected and didn’t find anything wrong. My HVAC installer (house is 13 years old and I have had this problem for awhile) won’t handle this either but referred me to subcontractor who is coming out shortly. Sounds like the whole roof vent assembly and cap should probably be replaced to be sure the problem is addressed. Not an easy problem as the roof is quite steep at that location.

    • Bob Jackson April 23, 2014 at 1:27 pm - Reply

      Hi David,
      Wind driven rain could be getting inside the rain cap then pooling inside the duct elbow and leaking. The flue vent parts are not expensive and most of the repair/replacement cost is the labor, so I recommend replacing the entire assembly and be done with it.

      Did you see the follow-up project where I had everything replaced? That may be helpful because it shows the entire flue vent assembly: Hail Damaged Roof Replacement: Part 4 – Gas Flue Vents


  9. David April 23, 2014 at 1:43 pm - Reply


    Thanks for the immediate reply. I have seen all of your projects on this subject which have been very informative. I would agree that everything should be replaced (cap, vent pipe, collar and maybe the cone itself unless that looks Ok). I really think wind-driven rain is getting inside the vent pipe as I cannot see any water leaking down the pipe itself except where it comes through the elbow joint. Expect an inspection on this early next week and will post what I find out. Hopefully, the contractor is competent. He was recommended by current HVAC contractor who doesn’t do this type of work anymore. And my roofer was just not sure he would doing the right kind of repair. Thanks.

  10. David May 3, 2014 at 1:19 pm - Reply

    I had the service tech out on April 29th who seems highly experienced/knowledgeable.

    He thinks that the main problem is that the flashing for the vent was not sealed properly, is now showing some wear, and is letting water drip down the outside of the vent pipe inside my attic. I only saw water coming from the elbow and thought the water was mainly inside but there are some tell-tale white streaks on the outside. I couldn’t see that side all that well and there was no plywood flooring to stand at that spot to get a better look. And I didn’t want to go through the floor.

    He will also replace the storm collar and seal that. He doesn’t think there was really any significant water penetration past the rain cap which he says is in good condition as is the 3 ft long vent pipe which has no seam. He says that what he recommends will take care of any water either getting inside the vent past the cap or running down the outside.

    While I tend to agree that replacing fairly inexpensive parts during a follow-up service call seems prudent, I will go with his recommendation and see what happens.

    Of course, we had almost 5 inches on rain the next day and I had a lot of water running down the vent pipe because the repair is not until sometime this next week. But the bucket caught it all. Will keep you informed as to the outcome of the repair for this nagging problem.

  11. David May 16, 2014 at 4:24 pm - Reply

    The tech replaced the storm collar this week with one that has a significantly larger diameter and he also caulked extensively around the new collar and existing base flashing/nails. He said the caulking appeared old (it is the original caulking 13 years) but he didn’t notice anything unusual. We had some heavy rain earlier today for a couple of hours but there was no leak which is good news.

  12. David June 26, 2014 at 11:40 am - Reply

    Had a heavy storm earlier in June and there was some very minor water that again came down the flue pipe. The tech came out but didnt see anything wrong. I asked him to replace the vent cap anyway with one that has fewer slats. As one might predict, we had heavy storms yesterday and again early this morning, but NO WATER. So the new cap being put on today is probably not necessary but I will have it installed as planned.

  13. Jacob November 12, 2014 at 10:10 pm - Reply

    So im in the process of replacing my roof when a heavy snow storm comes out of nowhere and delays the roofers from finishing my roof. They lay plastic tarps over my unfinished roof. That night my hot water heater goes out in clouding the pilot. I can’t restart it. I have water coming down my flue stack every time my furnace kicks on. And to top it all off now i have a back draft and what i can only guess would be is a CO2 leak in the basement!! How can I have a backdraft in my flue stack if it appears to be un abstracted at the cap? could the cold weather have something to do with it? Or did the roofers mess up my flue stack when they removed the old shingles? I need some help here!

    • Bob Jackson November 13, 2014 at 8:11 am - Reply

      Anything could happen while replacing the roof. I suspect the cap or pipe is blocked even though it appears not to be from the ground. Call the roofer, explain the urgency of the problem and ask that they find the problem and get it working properly until the weather cooperates.

  14. Tony C April 27, 2016 at 11:42 am - Reply

    Excellent job, Bob! I can’t thank you enough for taking the time to put together such a thorough explanation of this project. I’m not a professional roofer/contractor. I’m just a normal handyman dad who needs to read up on how to do things like this. With your help, I was able to diagnose and repair the leak on our flue vent pipe.

    Getting the correct materials to use for any repair has always been half the battle for me. Explaining and taking pictures of the materials saved me a lot of time I would’ve wasted at Lowes and Home Depot and gave me peace of mind that I got it right.

    Thanks so much!

Leave A Comment