How to Flash a Pipe on a Metal Roof: Boots, Ribs and Large Penetrations
Where the pipe falls relative to the seam or rib decides the method. Rubber boots suit small pipes, curbs and diverters suit the big ones.
- Advanced
- 30 minutes to 1 hour per pipe
- Updated
The short answer
Flash a small pipe on a metal roof with a rubber or silicone boot on a flexible flange that seals to the panel flat, fastened around its edge with washered screws and butyl tape beneath. Place it on the flat of the panel, never across a rib or seam. For larger penetrations, build a curb with a back pan and diverter so water flows around them and fasteners stay out of the water path.
On a shingle roof, a pipe boot sits under the upslope shingles and over the downslope ones. On a metal roof there is no shingle to tuck into, so the flashing must seal against the panel surface itself. That is a rubber or silicone boot with a flexible flange that conforms to the panel shape, stuck down with butyl tape and fastened around its edge.
The position of the pipe matters more than the boot. A pipe that lands on the flat of a panel takes a standard retrofit boot. A pipe that falls on a rib or seam gets problems: the boot cannot seal across a raised seam and the flange is stressed as the panel moves. The fix is to move the pipe, add a pan or cut the panel to fit a curb.
This guide covers small round pipes and the larger penetrations that need a curb. For chimneys, see chimney flashing. For walls, see metal roof wall flashing.
Tools
- Aviation snips or hole saw sized to the pipe
- Step drill bit or round file
- Impact driver
- Caulk gun
- Tape measure and marker
- Hand seamer
- Fall protection equipment
Materials
- Retrofit pipe flashing with a flexible flange, sized to the pipe (for example a Dektite type boot)
- Butyl sealant tape and a tube of polyurethane or silicone sealant
- Stainless or coated washered screws as the flashing maker specifies
- Stainless hose clamp if the boot calls for one
- Closure or curb materials for large penetrations
Safety first
- Penetrations are often near the ridge and on high, steep slopes. Tie off and avoid stepping on the boot or panel rib.
- Do not cut pipe or make hot work near vents without confirming what is inside. Bath fan and furnace vents can contain hot or toxic gas.
- WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury.
Step by Step
Step 1: Locate the pipe against the ribs and seams
Measure from the panel seams or ribs on the roof to the pipe position from the attic. If the pipe falls on the flat, a standard retrofit boot will seat well. If it falls on a rib, the boot will not sit flat and the flange will crack as the rib moves.
If the pipe lands on or near a rib, options are to offset the pipe with a bend in the attic, to cut the panel and fit a pan or small curb, or to use a boot designed for ribbed profiles. Seams on standing seam panels cannot be cut through at all, so the pipe should be moved.
Watch out: Never cut through a standing seam to pass a pipe. Re-route the pipe or build a curb that spans the seams.
Step 2: Choose the boot size and type
Retrofit boots are made of EPDM or silicone on a flexible aluminum or plastic flange. Pipe sizes are listed by range, for example 1 to 4 inches, and the boot stretches to seal. Choose one that fits snugly, not one that barely reaches, because a stretched boot will crack early.
EPDM is the common economy rubber and silicone lasts longer in UV. The flange should be at least 2 inches bigger than the panel flat it sits on. Match the boot to the panel profile where the manufacturer lists ribbed or flat-specific types.
Step 3: Cut the opening in the panel
Mark the centre of the pipe and cut the hole to the size in the boot's instructions. A step drill bit makes a clean round hole in thin steel. Finish with a file so there are no burrs. Use snips for a slot-shaped cut if you need to fit over an existing pipe and cut a slit to the edge, then seal and cover it with the flange.
Do not use an angle grinder on prepainted panels. See cutting metal roof panels. Clean off the swarf and touch up the cut edge with paint.
Step 4: Apply butyl tape and set the boot
Use a continuous ring of butyl tape in a continuous loop about 1/2 inch in from the flange edge. Slide the boot over the pipe, working from the top if the pipe is cut or from the bottom if you can reach. Press the flange flat to the panel so the tape spreads into a seal without gaps. Use a hose clamp or collar as specified on the boot to tighten it on the pipe.
On panels with raised ribs near the boot, press the flange so it follows the flat, and avoid stretching across the rib.
Step 5: Fasten the flange with washered screws
Screws go through the flange and into the panel, and in a solid deck or purlin where possible. They should be stainless or coated, and driven until the washer is slightly compressed. See metal roof screws.
Do not over-tighten. The flange is soft and the screws pull it into a dish, leaving gaps. Then run a thin bead of sealant around the outside edge of the flange, for a secondary seal.
Step 6: Check the lap direction and seal the top
Water runs over the boot from the upslope side, so the upper flange edge needs tape and sealant. The downslope edge should not be sealed so tight that water cannot drain. Use sealant on the upper half, tape on the whole ring.
Test with a garden hose, working from the ridge downward, and watch from the attic for moisture. Wipe off any squeezed-out sealant before it skins.
Step 7: Use a curb and diverter for large penetrations
A curb is a short frame around the opening with flashing on all four sides. The upslope side gets a back pan or saddle that diverts water round the curb. The panels are cut to the curb and lap onto the flashing so water never flows into an open joint. Keep screws on the upslope side out of the pan.
For a chimney, see step flashing a chimney and building a chimney cricket. For skylights, see flashing a skylight.
Boot Lifespan
Rubber boots are the wear item on a metal roof. EPDM can crack after 10 to 15 years of UV exposure, and silicone boots last longer. Inspect boots every few years and replace any that are cracked or hardened. Plan boot replacement as a maintenance item rather than waiting for a leak.
Fraser Valley Notes
Moss and needles build up above pipes on a wet roof, holding moisture against the flange. Clear debris gently without pulling on the boot. Frost and thaw cycles crack old sealant and aged rubber, so check boots in spring.
Common Mistakes
- Installing the boot across a rib or seam, where it cannot seal and cracks.
- Cutting through a standing seam for a pipe.
- Using a boot too small for the pipe, stretched until it splits.
- Skipping the butyl tape and relying on sealant alone.
- Overdriving screws so the flange dishes and leaks.
- Forgetting the back pan on a large penetration.
- Sealing only the top and trapping water under the flange.
When to Call a Professional
A single pipe on the flat of an exposed-fastener panel is a reasonable repair for a capable DIYer. A pipe on a standing seam roof, a pipe that lands on a seam, or any chimney, skylight or large vent is a roofer's job.
If you find a leak around a pipe or boot and it keeps returning after re-sealing, the cause may be the pipe's position or the panel profile, not the boot.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Metal Roofing service or the towns we cover.
How to Flash a Pipe on a Metal Roof: Questions
How do you flash a plumbing vent on a metal roof?
Use a retrofit rubber or silicone boot with a flexible flange. Cut a hole in the flat of the panel, put butyl tape under the flange, slide the boot over the pipe, press it flat and fasten around the edge with washered screws. Seal the outer edge with sealant. Keep the boot off ribs and seams.
Can you put a pipe boot over a rib on a metal roof?
Not reliably. A boot cannot seal flat across a raised rib or seam, and the flange is stressed as the panel moves. Offset the pipe so it lands on the flat, use a boot made for the specific profile, or build a small curb with a saddle. Never cut through a standing seam.
How long do pipe boots last on a metal roof?
EPDM rubber boots commonly last 10 to 15 years, sometimes longer in shade, and silicone boots usually last longer in UV. They are a wear item and should be inspected every few years. Cracks, hardening and gaps at the flange edge all mean replacement is due.
What goes around a large penetration on a metal roof?
A curb with flashing on all sides and a back pan or saddle on the upslope side to divert water around it. Panels are cut to the curb and lap onto the flashing. Screws are kept out of the water path. Chimneys, skylights and large vents all use this approach instead of a rubber boot.
Related Guides
All Metal Roofing 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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