How To | Metal Roof

How to Install a Roof Vent Flashing

So you’re installing a metal roof and you need to flash those vent pipes poking out here and there? This basic step by step tutorial shows that it is an easy and relatively painless task!

Step 1. Before Installing Metal Roof Panels

Make sure that the vent pipe is not going to come up in the middle of a roof panel seam. If it is, the best course of action is move the vent pipe over a few inches and try to center it up in the flat of the roof panel. Believe me, it is far less trouble to relocate the vent pipe now than it will be to relocate it later! If you fail to do this, you will have a roof leak that can only be temporarily stopped. The only solution after the metal roof is installed is to remove the roof panels, relocated the vent pipe and reinstall new roof panels.

Step 2. Before Installing Metal Roof Panels

Cut the decking back. You need to have clearance around the vent pipe so that the screws holding the pipe boot down are not fastened into the plywood. The screws should only fasten the pipe boot to the metal roof panel. If you skip this step , thermal movement of the metal roof panel will cause oil-canning (wrinkling) in the roof panel at the very least. Use your pipe boot as a template to decide how big the hole in the plywood needs to be.

Step 3. Install Roof Panels

Follow the roof panel manufacturer’s recommendations for installation. When you are ready to install the roof panel that straddles the vent pipe, carefully measure the location, mark it on the roof panel and cut a hole large enough to allow at least 1″ of clearance around the vent pipe. This will allow the roof panel to expand and contract without damaging the pipe. For very long roof panels (more than 30′ in length) you may want to consider elongating the hole.

Step 4. Select and Prep the Pipe Boot Flashing

Select an appropriately sized EPDM or silicone pipe boot for your vent pipe. 

DO NOT use the smallest boot available for the pipe you need to flash! The most common pipe sizes are 2-4″. A No-3 Pipe will fit all of these, though I would suggest going to a No 4 pipe boot for a 4″ pipe. Use one that is barely big enough and you lose all of the extra flexibility you might need to accommodate the roof slope and movement of the roof panels!

The boot on the left is a standard No 3 pipe flashing. The one on the right is the same boot cut for a 3″ diameter pipe.

Note that for “hot” pipes, such as wood burning stoves and furnaces, you should use silicone pipe boots. EPDM is pretty good for constant temperatures up to around 212° F, and can even withstand temps up to 275° F or so for short periods of time, but these hot temperatures will cause the EPDM to break down prematurely. Silicone pipe boots are good for temperatures up to around 437° continuously!
Once you’ve selected the right size, cut the boot so that the opening is just a little smaller than the outside diameter of the pipe. this will ensure a snug, watertight fit. You can cut the hole witha razor knife, scissors or even a good pair of aviation snips. Cut the hole to big? Throw the boot away and use another one. You will never get it to seal properly. It is a very good idea to do a trial fit now.

Step 5. Caulk the Base of the Boot

Using a high quality roofing sealant, caulk the base. Don’t get cheap here. You want to use a sealant that is designed to adhere to the paint, will not degrade over time and is preferably recommended by the metal roof manufacturer.

Step 6. Slide the Pipe Boot Over the Pipe

Taking care not to get your sealant all over everything, slide the boot down the pipe to the metal panel.  If the boot is difficult to slide, trying misting the pipe with a soapy water solution. Once you get close to the panel surface, make sure the pipe boot is centered well and then bed it on the panel. If the panel has raised ribs or striations, press the aluminum base around them to get the boot to conform as closely to the shape of the roof panel as possible

Step 7. Fasten the Pipe Boot to the Roof Panel

Taking care not to get your sealant all over everything, slide the boot down the pipe to the metal panel.  If the boot is difficult to slide, trying misting the pipe with a soapy water solution. Once you get cUse screws with bonded EPDM washers. Install the screws with a staggered pattern, in other words, one on one side, go to the opposite side, then somewhere between. Sort of like tightening the lug nuts on your car. Try to keep the screws centered in the aluminum base flange. Take care not to over-tighten them. There isn’t really a set number of screws required per pipe flashing, but I recommend a screw spacing of about 1″ all of the way around the boot. After the boot is installed, you may also consider running a bead of sealant around the interface between the pipe boot and the roof panel, especially in colder climates where freezing water on the roof is likely.
lose to the panel surface, make sure the pipe boot is centered well and then bed it on the panel. If the panel has raised ribs or striations, press the aluminum base around them to get the boot to conform as closely to the shape of the roof panel as possible

Step 8. Inspect Your Work

Step back and admire your work. Seriously, check to make sure that all fasteners are snug (not overtightened), the base flange isn’t buckled up. If it looks good, it probably is.


If you have questions about this process, feel free to call us at 770-405-1060 Monday-Friday, 8am to 5pm Eastern time, or email us at tech@rapidmaterials.com. Emails even get answered on the weekends and evenings!

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