If you're asking where does ice and water shield go on a roof, you've probably seen the damage a winter ice dam or a heavy spring rain can do to a house. It's that thick, rubberized, peel-and-stick underlayment that acts as a second line of defense beneath your shingles. While you might want to cover your entire roof in the stuff—especially if you've dealt with leaks before—that's usually overkill and can actually cause ventilation issues if you aren't careful.
The short answer is that it goes in the most vulnerable spots where water likes to pool, back up, or flow in high volumes. Let's break down the specific areas where this material is a non-negotiable for a long-lasting roof.
Starting at the Eaves
The most common place you'll find ice and water shield is along the eaves, which are the bottom edges of your roof. This is where those dreaded ice dams form. When snow on your roof melts because of heat escaping from your attic, it runs down to the cold eave and freezes again. This creates a literal dam of ice that forces liquid water to back up under your shingles.
Standard felt paper or synthetic underlayment won't stop that standing water from getting into your plywood decking. That's why building codes in colder climates usually require ice and water shield to be installed here. Most pros will tell you to run it from the very edge of the roof up to a point that is at least 24 inches inside the heated wall line of the house.
Don't just measure two feet from the edge of the roof. If you have wide soffits or "overhangs," you might need two or even three rows of the material to reach that 24-inch mark past the interior wall. It's better to have too much than too little in this area.
Don't Forget the Valleys
Valleys are the "V" shaped channels where two roof planes meet. Think of these as the highways of your roof. During a heavy downpour, a massive amount of water funnels into these valleys. Because of the sheer volume and the way water can "splash" or bridge across the valley, shingles alone aren't always enough to keep things dry.
When we talk about where does ice and water shield go on a roof for maximum protection, the valleys are right at the top of the list. You want to center a full 36-inch wide strip right down the middle of the valley before any other underlayment or metal flashing goes down. This creates a waterproof "trough" that ensures even if water gets under the shingles during a torrential storm, it won't hit your wood deck.
Penetrations and Roof "Holes"
Every time a roofer cuts a hole in your roof for a vent, a chimney, or a pipe, they're creating a potential leak point. Gravity is usually on your side, but wind-driven rain can push water upward and around the flashing of these penetrations.
It's a smart move to "picture frame" these areas with ice and water shield. For a chimney, you'd wrap the material a few inches up the masonry and several inches onto the roof deck. The same goes for skylights. Skylights are notorious for leaking, but most of those leaks aren't the window itself; they're the result of poor flashing. Using a peel-and-stick shield around the perimeter of a skylight creates a gasket-like seal that's incredibly hard for water to penetrate.
Critical Wall Intersections and Dormers
Wherever a roof slope meets a vertical wall—like where a porch attaches to the house or around a dormer—you have a high-risk zone. Water running down the wall can easily find its way behind the shingles if the flashing isn't perfect.
In these spots, you should install the ice and water shield so it folds up the wall a few inches (behind the siding) and lays flat on the roof deck. This creates a continuous waterproof barrier. If the metal step flashing ever fails or gets clogged with debris, the shield is there to save your interior drywall.
Low-Slope Areas
If you have a section of your roof that is relatively flat—think of a sunroom or a porch with a very slight pitch—standard shingles might not even be recommended. However, if you are shingling a low-slope area (usually between a 2:12 and 4:12 pitch), you might actually want to cover that entire section with ice and water shield.
Because the water moves so slowly off a flat-ish roof, it has more time to "capillary" its way under the laps of the shingles. In these cases, the shield isn't just for the edges; it's the primary defense for the whole surface area.
The Great Drip Edge Debate
One detail that often gets overlooked is how the shield interacts with the metal drip edge at the eaves. There's a lot of back-and-forth on this, but the best practice for a "belt and suspenders" approach is to install the drip edge first, then put the ice and water shield over the top of the metal flange.
Why? Because if water backs up under the shingles and hits the shield, you want it to run over the top of the metal and into the gutter. If you put the shield under the drip edge, water could potentially find a gap between the metal and the wood deck. On the rakes (the sloping sides of the roof), it's usually the opposite—shield first, then drip edge—but at the eaves, keeping it on top of the metal is the way to go.
A Few Tips for Installation
It's worth mentioning that ice and water shield is incredibly sticky. Once it touches the roof deck, it's basically there forever. If you're doing this yourself, don't try to peel the whole backing off at once. It'll end up sticking to itself, your ladder, and your cat.
Instead, lay the roll out, get it straight, and peel the backing away gradually while you press it down. Also, make sure the roof deck is clean. If there's sawdust, old nails, or bits of the old roof left behind, the adhesive won't grab correctly. You want a nice, smooth bond to the wood to get that "self-sealing" effect around the nails.
Is More Always Better?
You might think, "Well, if it's so good, why not just do the whole roof?" While some people do this, it's usually not a great idea for a standard home. Ice and water shield is a total vapor barrier. Unlike some modern synthetic underlayments that "breathe" a little bit, this stuff lets absolutely nothing through.
If you seal your entire roof deck, you might trap moisture inside your attic. That moisture comes from your showers, cooking, and breathing. If it can't escape through the roof deck, it can condense on the underside of your plywood and lead to mold or rot. It's usually better to use the shield where it's needed and use a high-quality breathable underlayment for the rest of the "field" of the roof.
Wrapping It Up
Knowing where does ice and water shield go on a roof can save you thousands of dollars in interior repairs down the road. It's all about protecting the eaves from ice, the valleys from heavy flow, and the "holes" from wind-driven rain. If you hit those spots—the eaves, valleys, chimneys, and wall transitions—you've pretty much eliminated the most common failure points of a roofing system. It's one of those things where spending a little extra on material now prevents a massive headache ten winters from now.