Undershot Flashing on a Shingle Roof

What is an undershot?

In this video, I’m gonna show you how to properly waterproof the undershot of your shingle roof, one of the most difficult flashing details there is on a shingle roof. Now, different people may have different terminologies for this area, but what we call an undershot is when the fascia board, essentially an eve, hits the field of the roof and creates this weird pocket underneath. Now, every undershot is essentially different and this is why it’s so complicated to flash, because no two are like. You’ll never see a detail or manufacturing instructions how to properly flash this area, because the manufacturers don’t wanna risk taking a thousand options and condensing it into one. So the concepts that we’re gonna teach you in this video are pretty much gonna be standard concepts used in shingle roofs and waterproofing methods that can translate hopefully to what your project is. Just make sure to take your time, work slow, do it properly, install the correct flashings to allow the water to flow off your roof.

First method on how to flash an undershot of a roof

Alright, for this explanation and tutorial, I’m not gonna show you how to do step flashing on this side here, I’m gonna omit the step flashing. If you wanna know how to install step flashing, I’ve got another video on that. I’m just gonna focus right now on the detail, what we call the undershot. That’s this piece right here, where the fascia board meets a roof plane. And we’ve also got a valley up here. Every undershot is a little bit different. I’m gonna show you how to flash this one and also give you another option at the end. Now, one thing we wanna make sure is that our fascia board is cut off the roof deck. In California, an inch is fine since we don’t have any snow or sitting snow that pulls moisture in. So for right now, we’re gonna leave it at an inch. A lot of times when these fascia boards touch the roof, they’re a little rotted out at the bottom. So what we do is we just cut them up.

Now, the first option is pretty much just a… We’re gonna start from here. Again, you wanna make sure you have step flashing here, I’m not gonna show that in this video, but pretty much just to shingle underneath this. You’re gonna use the same nailing pattern as you usually would and slowly make your way up. You wanna make sure this is sitting nice and flush. Now we’re gonna continue up underneath the shingle. This next shingle you probably gonna have to cut and make a notch in.

Now we’ve got this cut out. It’s pretty much the depth and the width of this. So we’re gonna see if it’s the right size here. Alright, we’ve got a nice snuck fit, align it properly. And you should also get a nail underneath here if possible. Right here is a little bit tight, so we’re not gonna be able to fit a gun. You’ll probably have to end up doing that by hand. Now you can see here, the valley terminates here, and we’re still low. So we still have to put one row of shingles before we can continue onto our valley. We really wanna go as high as possible. So it’s better to cut less, then cut off a little bit more later if you need to.

That’s pretty much it for this type of under-shop flashing detail. You can see that our shingles are installed all the way up to where the valley will be. So once we install our next row of shingles, which would be done along with the Valley, this corner here is covered. You wanna make sure whatever it is that you’re at least an inch or two past up this edge, if not, you can always use step flashings to install, and cover this up. Regardless, when we install our valley, we always want our valley metal to come further down. I’m gonna use this piece of step flashing as an example. Imagine this was our valley. So you would not wanna end your valley right here. When we install our valley, you wanna come a little forward to make sure that any water that comes from this valley is not anywhere near this corner, and it’s actually rolling off onto the roof. Another thing we would do when installing the valley is seal this area, with a little bit of mastic just for that secondary layer of protection.

Second method on how to flash an undershot

Another way of doing this undershot detail is to use step flashing. Generally we use a step flashing detail if we’re not able to get into this undershot. So let’s say, for example, if this fascia board was going all the way, if this is a stucco under eave, if there’s a soffit built out, or pretty much something that prohibited us from sliding the shingles underneath, we can also flash up this wall, kind of similar to any other roof-to-wall joint, but I’m sure the detail is right here. So the first thing we wanna do is make sure we install our regular shingles all the way up to this corner point right here. This is where the exact point where the fascia board or the eave meets the roof. I’m just using some old scrap shingles here, just since it’s a tutorial, but of course, if you were to do this, make sure to use full and healthy shingles. Now, at this point, we’d be ready to install our step flashing. So imagine this being a wall, we’re just installing a roof to wall. You can watch our video on step flashing installation.

What I’m doing is marking this from behind to make sure I cut this at a correct angle so that I can slide this underneath here, and the step flashing will come all the way up to the bottom of this drip edge. Now what this is gonna allow us to do is slide this all the way up and go underneath this drip edge. Now we’ve got this in, you always wanna make sure that your drip edge is past your shingle right here. So if this is your single reveal, you wanna be an inch below, so any water that’s kicking out from here has no way of going up. Here it is actually traveling down, so we’re just gonna install this as we regularly would. Again, if you’ve watched my shingle, step flashing video, you’ll know that we always wanna leave about a half inch gap between the shingle and the edge of the step flashing to make sure that we allow a pocket here for water to flow. Now you’re not gonna have a lot of water here, so it’s not extremely crucial, but we’re gonna leave a small gap anyways. Alright, so we’ve got still room for another piece of step flashing. This is gonna be a smaller step flashing cut down to size. So let’s see how we’re gonna fabricate this here. Again, I’m gonna kind of measure it to go past this point right here, so I would probably say… let’s do this. Mark out of the back. Again, I’m putting this along the roof instead of measuring this angle. I’m actually just scoring it, and that’s why I’m not sure if you can see in the video here, we’ve got our score line across here. That gives me somewhere to cut. Alright, so I’ve got this angle cut out here and I’m actually gonna slide it in, even though we still have to do some further fabrication on this piece of metal right here. So now I can leave it super long, but I don’t wanna leave this much of a step flashing exposed, so I’m just sliding this into place, and I’m gonna be able to mark out my cut line right here. Again, you don’t wanna go up further than your reveal point, and in the detail like this, that is not too visible I would come little bit down, so I’m gonna give us a half inch exposure, so that once we’re done, we’ll see our step flashing about half an inch past our shingle. I’m just gonna use another piece of step flashing as a guide right here to cut a straight line. Alright, I’ve got this all cut out, let’s see how it fits. Push that up as far as we can, and nice and snug. I was actually able to push that up further than originally anticipated, so we don’t have that half inch reveal that I was originally talking about.

Actually, we might have a little bit. I’m just gonna put one nail on this for right now. We’re ready to install our final shingle before we go into our valley. And I’m gonna leave a small gap here, I’m just using a damaged single. Again, I’m gonna pretend that this step flashing right here is a valley. So we’ve got it flashed all the way up against here. When we install our valley, I wanna come down so that there’s no risk of water. This right here is your most vulnerable point. So what I would do is, before I install my valley, install a little bit of mastic here, come up and pass our valley…

Pass this point to make sure, again, no water gets in this corner right here. Now, one last detail that we wanna do is, you can see that the step flashing is installed against a fascia board here. Now, we have a risk of water getting in underneath here. What we wanna do is cut one less piece of step flashing, we can just use regular step flashing in a triangular shape to cover this all up, make it look a little nicer. It’ll essentially act as our counter flashing. I’m gonna mark out again these corners right here. Now I don’t really care about what this top edge looks like, because it’s gonna be hidden underneath this drip edge. And there you have it. Now you have this piece of step flashing covering up these tin shingles. What I would actually wanna do is lift this up or prior to installing this drip edge, install this piece of flashing, just so we can hide a nail underneath here, but in this case, we already have this installed. So you can just use a little bit of adhesive, or you can bend this up to install it now, so you can see we’ve got a nice piece of step flashing covering up that detail, and this is a water-proof installation that’ll last you years and years to come.