How to Install Underlayment on a Shingle Roof

What is underlayment?

Underlayment comes in different shapes, sizes, and different brands. I always recommend using the same brand that you’re installing your shingles. We’re Owens Corning Certified Platinum contractor. So we’re gonna be using Owens Corning Deck Defense. This is a synthetic underlayment. It’s similar to your old tar paper. It’s just stronger, doesn’t tear as much, and comes in bigger rolls. Each roll is 10 squares, so it covers 1,000 square feet, so it’s a lot easier to work on and safer when you’re on steeper roofs. This is probably the easiest component when installing a shingle roof, and we’re not relying on this for waterproofing. Even though we wanna make sure that it’s properly lapped and properly installed, your shingles are what really does your waterproofing. All this does is two things: in case of shingles blowing off or coming off, it makes sure that your roof is waterproof, and it also provides a separation layer between your shingles (which are asphalt-based) and wood, which can actually pull the asphalt into it. So it’s a separation sheet, as well as a temporary waterproofing. Let’s get going.

Should you install underlayment or drip edge first?

So when installing your underlayment, it’s very important to remember, you first install your drip edge at the eaves, then you install your underlayment on top of that. If you can imagine water rolling on your underlayment, you want it to come up and off your roof, as opposed to if your underlayment was installed underneath or flush with the drip edge, your water would come here and find its way underneath. We’re in Southern California. I know, in some states when they have ice and snow, you’ll have ice and water shield at the eaves. We’re in Southern California, we’re in December right now, it’s super sunny, we don’t have ice and snow that we’re worried about, so we’re gonna be installing our underlayment right up to the drip edge.

Tips for installing underlayment

You wanna make sure you have about an eighth of an inch back. You don’t wanna be too flush or overlapping ’cause you don’t wanna see that underlayment from underneath. We wanna come flush, come up with an eighth of an inch or so and start installing. The first thing that we’re gonna do is install three or four nails grouped together. What this is gonna allow us to do is, be able to pull our underlayment nice and tight. So we’re gonna roll it out. Make sure it’s snug, you don’t wanna over-tighten it, just smooth, you’re aligned at the bottom, and we can start installing.

Underlayment nailing pattern

Now, even though the nailing pattern on your underlayment is not very important because you’re gonna be installing your shingles on top of that, and all those nails are also gonna hold in your underlayment. There’s two reasons it’s important. If you’re leaving your underlayment overnight, you wanna make sure that you don’t have wind blowing it off. The second is, when you’re working on this, you wanna make sure that your underlayment is nice and snug so it doesn’t slip or rip underneath.

The underlayment that we have, we have a collaboration with Owens Corning and our logo on top. And what we’ve had them do is actually place dots everywhere we need underlayment. Your fields are 12 inches apart, and side laps at the ends are every six inches apart. So instead of getting measuring tape, we have it marked so that when we’re on a roof, it’s nice and easy and straightforward, and we know exactly where to nail.

How to install underlayment around a penetration, pipe, chimney

Anytime you come up to a penetration, whether it’s a skylight or a chimney, you don’t have to worry about being too snug. What we do is generally pull a little bit over, lay it loose, and now we’re able to work on these corners. You can either use a utility knife or scissors or whatever to cut this underlayment. And it’s okay for these corners to be a little bit exposed. We’ll show you why later.

How to overlap two pieces of underlayment

So when you’re installing, you don’t have to worry about having full pieces from end to end. This is a mock roof, so what I did was cut this short to show you the end laps. Now, end laps are anytime the end of your roll meets, and side laps are these top here. Your end laps, you wanna have an overlap about 10-12 inches, more is totally fine, but you wanna have at least 10 inches to 12 inches coming on your end laps. So we’re gonna again, just align this up, and essentially restart our roll. Again, we’re gonna place the three nails here in the middle. And the reason you wanna do three nails and not nail the entire side is for you to be able to align the roll as you’re working. So again, you pull it out, align it, and that allows you to pivot your roll.

How to install underlayment against a wall

So when we come up against the wall, now there’s a concrete wall, but imagine this is the side of your house. Instead of trying to work your roll in here, what I always like to do is, come out and give yourself some extra room. So you get rid of your roll, and now you’re able to work with a loose piece of underlayment pushing up against the corner, and now you’re able to cut properly up your rake walls. Generally, you want your underlayment to come up two, three inches and tuck underneath your flashing or Z-bar. We don’t have anything here on our mock roof.

What kind of nails should you use to install underlayment?

Now, the type of nails that we’re using for installing underlayment is called plastic round cap nails. These thick nails are unique and specifically used for underlayment. This plastic cap helps hold down the underlayment. If you can imagine, if we just add a small head, if we were just using this, this would rip right through. So this plastic holds it down. This nail is also a ring shank nail. So it has a special grip, and is able to fasten your plywood. Roofing hatches are also a very useful tool in this process. This actually has a magnet here, so instead of bumping your fingers when you’re installing, you just place a nail here, and that fastens with the magnet, and with one strike, we’re able to get that down.

If you’re ever working in tight corners, you don’t have to work with the entire roll. What we like to do in areas like this where we have a lot of penetrations is to pre-cut on the ground so that on the roof, we’re not struggling with that large roll. We’ve got our roll pre-cut here, and really the only thing to know, that obviously you’re starting from the bottom, you’re installing your first roll. And as you’re going up, your side laps, again, we’ve got a mark here, it’s three inches. Every manufacturer might be slightly different, but Owens Corning recommends three inches, we’ve got it marked out here. And always, just like anything else in shingles and in shingle roofs, we’re gonna install from bottom up.

Which comes first, ice and water shield or underlayment?

When you install ice and water shield in the valleys, and if you wanna know how to do that, you can reference our video after this one, “Installing Ice and Water Shield.” You wanna install the ice and water shield prior to your underlayment, that way underneath your underlayment, you have that secondary layer of waterproofing, then we will install our valley metal on top of that.

One thing I wanna really emphasize at this point is, always make sure to take safety precautions into account when roofing, whether you’re harnessing off or installing toe kick boards. In these demonstration videos we’re not doing those ’cause the point of these videos is to learn the proper way to install, but always make sure you’re following local codes, and most importantly, be safe on the roof.