Gap around furnace b-vent after roofing
Roofers often leave an open gap between the double-wall B-vent and the surrounding roof jack, which lets light and potentially water through. Standard caulk should not be used here because furnace vents get too hot; instead, you need a high-temperature rated storm collar and silicone flashing designed for gas vents to properly seal the opening without creating a fire hazard.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from October 2026.
- This comes up 14.2× more often in r/Roofing than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Installing a storm collar jammed down tight to the top of the roof jack and affixing a B-vent top directly to the pipe.Did this help you?
- 2applying new caulk to seal the gapsDid this help you?
- 3Using a curb adapter and a rain hoodDid this help you?
- 4Flashing it with torch applied modified bitumen, granulatedDid this help you?
- 5Using caulk or a piece of trimDid this help you?
- 6Google and YouTube the repair processDid this help you?
- 7Inspecting the attic and checking for water leaks or accumulated lintDid this help you?
- 8Using mastic sealant, vent tape, and new insulation wrap to DIY seal the gapsDid this help you?
- 9Using backer rod with fresh caulkingDid this help you?
- 10Applying furnace cementDid this help you?
- 11Using quick set mortar or siliconeDid this help you?
- 12Using fire caulkingDid this help you?
- 13Using foam or making a cover plateDid this help you?
- 14Using hydraulic cement or high temp spray foamDid this help you?
- 15Using a flat piece of sheet metal with sheet metal screws and foil or aluminum tapeDid this help you?
- 16Cutting tabs into a round cap, bending them out, and screwing them into the ductDid this help you?
- 17Using makeshift materials like a smashed coffee can or an old license plate with sealant and screwsDid this help you?
- 18Cleaning out old caulking, inserting a backer rod, and applying new caulk.Did this help you?
- 19Installing corner flashing to cover the gap cleanly.Did this help you?
- 20Hiring a structural engineer to check if the chimney is actively settling away from the house.Did this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“just got a house and there's a bunch of spots I need to redo the caulk on before it gets rainy again. I was wondering though, is there any reason why the tops of these vents would be left open like this?”source ↗
“Following a tech's advice, I removed two supply vents running off of my furnace. How do I cap this hole properly?”source ↗
“My first instinct is to make several ~2inch vertical cuts along the cap's edge, fan them outwards like flower petals, weave the pieces around the edge of the hole, and use duct tape (555 FlexFix) to seal it closed. How does my plan sound?”source ↗
“I couldn't find a cap with a flange in the store- do you happen to have a picture I can reference?”source ↗
“I noticed today while I was cleaning it that there’s a literal gap all around it where it’s supposed to be attached to the roof (I think?).”source ↗
“I am pretty sure it’s not supposed to be like this, especially since I can see some lint around it on the attic floor.”source ↗
“My question is - as a decent DIY-er, can I fix this? And if yes, any advice is appreciated.”source ↗
“New homeowner here working through inspection report findings. The water heater flue connects into the chimney but it wasn't properly sealed. I'm looking into the best way to seal this so CO can't leak out into the basement.”source ↗
“A handyman recommended I apply furnace cement around the opening but most brands I see need a heat cure. I'm not sure how I'd do that here since it's not inside a furnace or fireplace to achieve this and I don't know if not curing it means it won't make the proper seal to keep CO out.”source ↗
“Please call a plumber/gas fitter. Not aloud to repair without a chimney liner in my area. That is a very dangerous situation.”source ↗
“Hello first time home owner here, wondering what i can use to reseal this area?”source ↗
“It looks like multiple applications over the years and I can see gaps between the caulking and the brick- the home inspector pointed it out.”source ↗
“Hello, I am looking for advice for what to fill these gaps around the chimney near the wood stove”source ↗
“But as far as caulking goes for the chimney, this would take like 4+ tubes of caulking (both sides) wouldn't it? I just assumed that there was a wider application caulking?”source ↗
“Also any recommendations as to how to fix/fill the gap between the chimney and black wood cladding?”source ↗
“The old caulking cracked and fell apart. Removed all existing caulking and looking for any tips. Possibly backer rod with fresh caulking?”source ↗
“the B-vent (cylindrical double wall from the top of furnace or boiler) runs inside the roof jack (brown part that is roofed-in) but has a gap, and the top, that is secured by tabs through the roof jack, is letting in light around through that gap.”source ↗
“Hello I am replacing the duct work in my kitchen exhaust and I've managed to get the new duct in and taped up. But I don't know how to seal it.”source ↗
“The original duct penetration was uninsulated and exposed to outside air.”source ↗
“Normally this would be done from inside the attic... But this is a flat roof so I have no attic.”source ↗