How to Install a Dormer Vent on a Shingle Roof

In this video, I’m gonna show you how to install a dormer vent on your shingle roof. Dormer vents have been around for many years and have been used to vent thousands of roofs over the years. Dormers are essentially half-round. They come in 18 and 24-inch sizes. So the width of a dormer is 18 to 24, and they do a great job in cross ventilating, as well as exhaust. They do protrude a little bit higher, so a lot of people do prefer O’Hagin’s or low-profile vent, but dormers are definitely a safe way to go. Another thing dormers have is a screen here on the back, which blocks any pests, bugs, and animals from getting into your attic. Dormers do only come in a galvanized finish. So you do have to paint them after you complete your roof. There’s a few extra steps that we like to take to make sure that the dormer flashings and dormer vents never leak. Watch the video and find out how.

How to locate where to install the dormer vent

The first thing we wanna do is align our dormer to find our center point, and cut the opening. Now we already have an opening cut. This was actually for an O’Hagin vent. So what we’re gonna do is re-align this and cut a new opening for the dormer. Essentially, you wanna match the opening at a minimum of the bottom of this flashing to make sure that you have proper airflow and you maximize your airflow. If your plywood is smaller than this and your opening is narrower, you’re gonna minimize your airflow and really ruin the whole idea, because we wanna get as much cubic inches of open venting area as we possibly can from one vent. Now, one thing you wanna make sure is you’re essentially gonna be installing your roof all the way up to the point that you want your dormer installed. You already have your underlayment here and it’s totally fine. You don’t have to pre-plan the dormer locations, you can decide that as you’re moving up.

Now as we’re aligning our dormer vent, we wanna make sure that just like any other flashing, similar to O’Hagin’s, we’re aligning this at a minimum flush or below our shingle exposure line. So in the same area that you would install your shingle is where we want to bring our flashing. You can come a little further down, however, we don’t like to come too far down as not to leave a large area of dormer exposed. Let’s leave it an inch below, align it pretty much on both sides. Then what I like to do is use a marker to mark the center of this dormer and align that. Now the easy way to do it is mark a line down here. Then I like to flip this around. That way I can measure my dormer and at the same time mark out my opening that I have to cut.

Alright, so we’ve got the bottom of our flashing marked out on our shingle. Now what I like to do is measure it so we’ve got 9 inches further up. So we’ve got 9 inches here, you can see we’ve already cut up below, which is fine. Having a bigger opening doesn’t necessarily hurt unless it’s too big. But I’m gonna come here and mark the 9 inches out on both sides here from our shingle line. There’s gonna be pretty much right there.

So this is gonna be the bottom of our opening. The centerline is gonna be right here. We’re gonna have a 16-inch width at the bottom. Now dormers taper up. So first off, we’re gonna measure the height of it – 12 inches, here’s the top. The top is another 12 and a half inches, we’re gonna round that up to 13. What I do is just use my tape measure essentially as a ruler, as a guide. You don’t need to be precise, you can go a little bit larger than necessary, and mark out your dormer opening.

Now keep in mind that generally, we wouldn’t have this cut out here. We will just have an opening in this area here. But we’re just gonna leave this here for now, it doesn’t hurt for demonstration purposes. But generally, you want to make the opening slightly larger than the dormer itself. First thing we gonna do is cut this paper back, then take a Skilsaw and cut the opening out.

Alright, now that we’ve got our opening cut out, we’re ready to install our dormer. Now, we like to take one small step that’s not necessarily recommended, or required, I should say, by manufacturers, but I think it’s a no-brainer. It’s gonna take you a few minutes and really give you a second and third layer of waterproofing for your dormer. That’s gonna be installing a bed of mastic around the back of the dormer, so in case there’s any water that gets in, it’s not going straight into your opening. Now, for dormers, we only generally install the mastic on the back, on the top side, and on the sides. We don’t like installing it on the bottom. The reason for that, in case any water gets in, we want it to be able to flow out of the bottom. Water does not travel up, it travels down. So we wanna leave a path for the water to flow.

Now that we’ve got our mastic aligned, we wanna make sure we pretty much just align with our opening there, place it down, and you press it down a bit to make sure that all the mastic gets properly bedded down.

Nailing pattern on a dormer vent

Now we’re ready to nail. The way we always wanna nail is, first off, we don’t wanna have any exposed nails on the bottom. We wanna just have nails along the sides. Even though dormers don’t have a specific specification, we use the same specification as used on other flashings, as it applies to this as well. Pretty much every 4 inches on the center, about 1/2 inch to 1 inch in from the edge of the roof. We use regular galvanized hot-dipped nails. The same nails that you’ve used for your shingle roof, we’ll use around the dormer.

Another thing we wanna do, you can do this before or after nailing down the flashing, is bend these down. These lips here are bent up in case you use this dormer for a tile roof. If you’re using it for a shingle roof, these need to be flat so to ensure that your shingle’s nailed flat. So it’s really not difficult. We’ve got that seam flat, our dormer aligned, and we’re ready to start nailing. So you can see nails every 4 inches on the center, about 1/2 inch to an inch in from the side.

Installing shingles over and around the dormer vent

Now we’re ready to install our shingles. The first thing we wanna do is put a bed of mastic right here at the edge. What that’s gonna do is give us a secondary layer of water protection. In case there’s any running water that goes diagonally or goes horizontally, it’s not gonna get past that flashing, and it’s gonna come straight down. And it’s also gonna cover up our nail holes right here. So there’s no need to go crazy, just a good 1/4 inch bed along that seam is what we’re looking to see.

Now, I’ve got our first piece of shingle pre-cut, and on the next one I’ll show you how to cut it, at least the way we like to do it. Now, similar to O’Hagin’s and other vents that we install, we always wanna leave 1/2 inch to 3/4 inch gap to ensure that water properly flows along the side. So we’re gonna align our shingle here, leave that 1/2 inch gap, and we’re ready to go.

Another thing we wanna be sure of is we don’t wanna nail too close to the center of this flashing. We wanna nail about 3 inches out along the edge of this flashing. So pretty much wherever this flashing line ends is where we wanna install our first nail. And then the rest of the nailing pattern is gonna be similar to regular shingles.

One thing we wanna do prior to installing our second shingle is cut the top of this first single off at a 45-degree angle. We call this dog-earing the shingle. What this is gonna do for us is, any water coming down this channel right here is not gonna get caught and travel horizontally. If you can imagine that corner still being here. So if this corner was still installed, we have a chance and risk water getting stuck right here and traveling this way along the shingle. But now that we’ve removed this corner, when the water hits, it’s gonna travel back down into this channel and go where we want it to go.

The way we usually cut our shingles… And this is a trick. You can do whatever you’d like, you can pre-cut your shingles, or install them, then cut them, but we like to actually install it. So we’re gonna align it right here, and we’re gonna come a little bit extra on this dormer. Again, we’re gonna make sure that we nail it pretty much flush with the flashing right here. We don’t wanna go too far in. Now we’re ready to cut this right here, and we have essentially a guide that we like to use kinda with our hand and cut that shingle.

Alright, now that we’ve got our shingles installed up here on the side, we’re ready to install our final shingle here at the top. The way I like to do it is align it pretty much on both sides and nail it again prior to cutting it. So it’s aligned here. We would use just regular shingle nailing pattern. Now that we’ve got it nailed, the last thing is to just open up and cut that opening and we’re ready to go.

That’s it. We’ve officially completed properly installing a dormer vent on this shingle roof. Now, the few extra steps that you still have to take, again, are not 100% necessary, but I definitely recommend them. It’s just gonna take you a few extra minutes, but it’ll ensure that this vent does not leak for years to come. The last thing I wanna just emphasize that’s really important is this channel right here. What this channel is made to do is take the water where we wanna go. You can see it’s rolling smoothly down this channel as opposed to risking going underneath the shingles. This concept will play throughout different installations and different flashings, so we always wanna guide the water in the direction that’s beneficial to us instead of having the water find its own way down.

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