How to Flash a B-Vent or Metal Chimney Pipe Through a Roof
Gas vents and factory-built chimneys need flashing made for their pipe, with clearances that belong to the venting manufacturer and the gas code.
- Advanced
- 1 to 2 hours
- Updated
The short answer
A B-vent or factory-built metal chimney passes through the roof inside a clearance-rated flashing made for that pipe. Use the flashing supplied or listed by the vent manufacturer, shingle the base over the shingles below and under the ones above, nail only the upper flange, then fit a storm collar over the pipe and seal it with high-temperature sealant. Never alter the vent or its clearance to combustibles.
A gas furnace or water heater vents through the roof in a double-wall metal pipe called a B-vent, and a factory-built fireplace or wood stove vents through an insulated metal chimney. Both must keep a specified clearance from combustible framing and roofing, and both need flashing that is made for the pipe, not a plumbing vent boot.
The roof work is the same idea as a pipe: shingle in a flange, cover the joint at the pipe with a storm collar, and keep water out. The difference is that the pipe gets hot, rubber boots melt, and the clearance rules are a safety matter. The vent manufacturer's installation instructions and the gas code decide what is allowed, and the roofer works within them.
For plumbing vents, see pipe boots and lead pipe flashing. For masonry chimneys, see chimney flashing parts.
Tools
- Flat pry bar
- Hook blade utility knife
- Roofing hammer
- Tin snips
- Tape measure
- Caulk gun
- Screwdriver or nut driver for the storm collar
Materials
- Vent flashing listed for the pipe, matching size and roof pitch (adjustable or pitch-specific)
- Storm collar sized to the pipe
- High-temperature sealant suited to the flashing and pipe, per the vent manufacturer
- Roofing nails, 1-1/4 inch
- Ice and water shield
- Replacement shingles as needed
Safety first
- Do not move, cut, re-angle or rework the vent pipe, its support or its roof cap. Gas venting is regulated, and a gas fitter or the venting manufacturer's instructions govern it. If the vent has to be disconnected or shortened, have a licensed gas fitter do it.
- Vent pipes carry hot combustion gases. Never work on a vent while the appliance is running, and do not block the vent or cap. Carbon monoxide is a hazard if a vent is disturbed.
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
Step by Step
Step 1: Identify the pipe and the roof pitch
B-vent is commonly double-wall and sized by the inner diameter, such as 3, 4, 5 or 6 inches. A metal chimney for a stove or fireplace has a different outer diameter. Read the label on the pipe. Flashings are sold for ranges of roof pitch, or as adjustable flashings that bend to fit, such as 0/12 to 6/12 and 7/12 to 12/12. Choose the one that matches.
Step 2: Check the clearance and the support before doing anything
Gas vents commonly need a clearance of about 1 inch to combustible material, and factory-built chimneys are usually larger, but this differs by product, so read the label and the installation instructions. If the clearance looks wrong, or framing or sheathing touches the pipe, that is a venting issue. Stop and call a gas fitter or venting installer.
Watch out: Do not pack insulation or spray foam around a vent pipe at the roof. Clearance space is required.
Step 3: Remove the old flashing
Loosen the storm collar and slide it up the pipe. Pull the nails and slide the old flashing out. Check the deck around the pipe for soft or stained wood. Clean the pipe and the deck. On low slopes, add ice and water shield around the pipe, sealed to it.
Step 4: Slide on the new flashing and shingle it in
This is the lapping rule again. See the lapping rule. Flashing bases are commonly 12 to 18 inches square or larger. Bend an adjustable flashing to the roof pitch so the base lies flat on the deck and the collar stands plumb around the pipe.
Step 5: Nail the upper flange and keep the lower flange clear
Exposed nail heads get a dab of sealant. Do not nail through the collar or near the pipe. Keep fasteners out of the clearance zone around the pipe.
Step 6: Fit the storm collar
The storm collar is a clamped band that sheds water running down the pipe away from the opening between the pipe and the flashing. It sits about 1/2 to 1 inch above the top of the flashing collar, or as the manufacturer specifies. Run a bead of sealant along its top edge where it meets the pipe. Use a sealant rated for the temperature of the pipe surface. For a B-vent, this is usually moderate, but for a wood stove chimney it is much higher. Follow the vent manufacturer's recommendation.
Step 7: Seal and check the flange
Replace shingle tabs over the flange's upper edges and re-nail them away from the flashing. Replace any cracked tabs. See how to replace a damaged shingle.
Step 8: Check from inside
Check that any firestop or attic shield is in place. Watch the spot after the next heavy rain. See how to trace a leak from the attic.
Why Not Use a Rubber Boot?
Standard rubber pipe boots are not rated for hot pipes. A B-vent surface is warm and a stove chimney can be very hot, which degrades rubber quickly and, in the worst case, is a fire hazard. Use the metal flashing with storm collar made for the vent. High-temperature silicone boots exist for some applications, but only when the vent manufacturer lists them.
Rain Caps and Terminations
The top of the vent has a cap or termination that belongs to the venting system. Do not remove, swap or modify it. If it is damaged, a gas fitter or vent installer replaces it.
Fraser Valley Notes
Wet winters keep vent flashing damp for months, and moss grows around chimney pipes on shaded roofs. Clear debris gently, and check the storm collar seal each fall, since sealant ages faster in constant sun and wet cycles.
Common Mistakes
- Using a rubber plumbing boot on a hot vent, which melts or cracks and leaves a hole.
- Altering the vent or its clearance to make the flashing fit, which is a fire and carbon monoxide risk.
- Forgetting the storm collar, letting water run down between the pipe and flashing.
- Reverse lapping the flange, with the lower half tucked under shingles.
- Using general-purpose sealant on a hot pipe, which cooks and fails.
- Choosing the wrong pitch range, so the collar leans and leaves a gap.
When to Call a Professional
If the vent needs to be moved, raised, shortened or re-angled, call a licensed gas fitter or the venting installer. If clearances look wrong, stop and get a professional to assess.
Flashing a hot vent on a steep or high roof is a job for a roofer. A fresh flashing on a B-vent or chimney is commonly replaced with a reroof.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Flash a B-Vent or Metal Chimney Pipe: Questions
How do you flash a B-vent through a roof?
Use a vent flashing matched to the pipe size and roof pitch, slide it down over the pipe with its upper half under the shingles above and its lower half over the shingles below, nail only the upper flange, then fit and seal a storm collar around the pipe just above the flashing collar. Follow the vent manufacturer's instructions for clearances and sealant.
What is a storm collar on a roof vent?
A storm collar is a clamped metal band that sits around a vent pipe just above the flashing collar. It sheds water running down the pipe away from the gap between the pipe and flashing. It is tightened and sealed along its top edge, and is a standard part of flashing for gas vents and metal chimneys.
Can I use a rubber pipe boot on a furnace vent?
Generally not. Rubber boots are made for cool plumbing vents, and a heated vent pipe can cause the rubber to harden, crack or melt. Use the metal flashing and storm collar made for that vent, or a high-temperature boot listed by the vent manufacturer. Check the instructions for the vent and do not modify its clearances.
What sealant is used around a chimney pipe flashing?
One rated for the temperature of the pipe surface and compatible with the flashing metal. For a gas B-vent, where the pipe surface is moderate, a high-quality exterior sealant may be allowed. For a wood stove chimney, use a high-temperature sealant. Follow the vent manufacturer's instructions, and use sealant as a backup, not in place of lapped metal.
Who can work on a gas vent?
A roofer can replace roof flashing around a vent, but any change to the vent itself, such as moving, cutting, re-angling or reconnecting it to an appliance, must be done by a licensed gas fitter or the venting installer, following the manufacturer's instructions and the gas code. If unsure, stop and call one.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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