Underlayment and Ice-and-Water Shield, Explained for Littleton
When a roof is quoted, the shingles get all the attention. The layer under them decides whether the roof leaks in March. In Littleton, that layer is two products: a synthetic underlayment over most of the deck, and a self-adhering ice-and-water membrane at the places where water sits, backs up, or concentrates. Here's what each does, why the code here requires the membrane, and how to tell whether a quote includes it.
The job of underlayment
Shingles shed water. They don't seal it out. Wind-driven rain and, more importantly here, meltwater from a snow load can get between and under shingles, and the underlayment is what keeps that water off the wood deck until it drains out at the eave. It also keeps the deck dry between tear-off and shingling, which on a two-day job in June means through an afternoon thunderstorm.
Felt versus synthetic
Asphalt-saturated felt (15-lb or 30-lb) is the paper-like product that's been under roofs for a century. It works. It also wrinkles when it gets wet, tears in wind before the shingles are on, and dries out and cracks under our UV if it's exposed for more than a few days. On a roof in Highlands Ranch in July, felt that goes on Monday and gets shingled Wednesday has already buckled.
Synthetic underlayment is a woven polypropylene sheet. It's lighter, stronger, doesn't wrinkle, doesn't absorb water, grips under a boot better on a steep pitch, and can sit exposed for weeks if a job gets interrupted by weather. It costs a bit more per square. Most quality replacements in the South Metro use it, and a quote that specifies "15-lb felt" in 2026 is a quote written to hit a price.
What ice-and-water membrane is
A sheet of rubberized asphalt with a peel-off backing that sticks directly to the deck and seals around every nail driven through it. Where felt or synthetic just lies on the deck, membrane bonds to it, so water that gets under the shingles can't get under the membrane. It's the product that makes a roof survive an ice dam.
Why Littleton's code requires it
The model residential code (IRC R905.1.2) requires an ice barrier "in areas where there has been a history of ice forming along the eaves causing a backup of water." Littleton and the surrounding jurisdictions — Jefferson, Douglas and Arapahoe counties, Centennial, Englewood — have adopted recent editions of the International Residential Code with that provision in force, because there absolutely is such a history here (confirm the adopted edition with your building department). A March upslope storm drops a foot of wet snow, the sun comes out at 5,400 feet the next day, the roof surface melts while the eave overhang stays frozen, and the water backs up under the shingles. Our post on ice dams covers the physics.
The requirement: membrane from the eave edge up the slope to a line at least 24 inches inside the exterior wall. On a house with a 2-foot overhang and a 6/12 pitch, that's roughly two courses of 36-inch membrane. The city or county inspector checks it at the mid-roof inspection, before the shingles go on.
Where else it belongs
Code sets the minimum at the eaves. Good practice in this climate puts membrane everywhere water concentrates:
| Location | Why |
|---|---|
| Eaves, to 24 inches inside the wall line | Code; ice dams |
| Valleys, full length, 36 inches wide | Most water and most sliding snow on the roof |
| Around chimneys, skylights and every pipe and vent | Flashing transitions; the leak points |
| Along walls and dormers under the step flashing | Wind-driven snow and rain |
| Rake edges on the west and northwest | Downslope wind drives water up under the edge |
| Low-slope sections under 4/12 | Water moves slowly and sits |
On a cut-up Southridge or Ken Caryl roof with four valleys, three skylights and two chimneys, that can be a third of the deck. It's the right third.
What it costs
Membrane runs a few times the price of synthetic underlayment per square, but it's a small line on a replacement — typically a few hundred dollars for the code-required eave coverage on a 26-square roof and a few hundred more for the valleys and penetrations. On a $16,000 roof, the difference between "eaves only" and "everywhere it should be" is under $500. A retrofit on an existing roof — stripping one eave, adding membrane, re-shingling — is $600–$1,400 per eave, which is why it's better done during a replacement.
What high-temperature membrane is for
Standard membrane softens on a very hot deck. Under a standing-seam metal roof, which runs hotter than shingles, or on a south-facing low-slope section, a high-temperature membrane rated to 240°F or more is the right product. If you're getting a metal roof quote, the underlayment line should say so.
How to read it on a quote
Look for these, in this order:
- "Ice-and-water membrane at eaves to 24 inches inside the wall line" — or a product name (Grace Ice & Water Shield, GAF WeatherWatch, Owens Corning WeatherLock) and a location. "Ice and water as required" means the minimum, maybe.
- "…and in all valleys and around all penetrations." The good-practice line.
- "Synthetic underlayment on the balance of the deck." Not felt.
- The permit. Because the membrane gets inspected.
If a quote doesn't mention membrane at all, ask. The roof replacement page lists what a full replacement here includes, and the cost page has the rest of the lines to compare.
Common mistakes on a re-roof
- Membrane tucked under the drip edge at the eaves. At the eave the metal goes on first and the membrane over it, so water running down the sheet lands on the metal and into the gutter. Reversed, water can run behind the drip edge.
- Too little coverage. One course at the eave on a house with a deep overhang may not reach 24 inches inside the wall line. Ask how many courses the crew plans.
- Membrane over wet or dusty decking. It won't bond properly. A good crew sweeps the deck and waits for the dew to burn off.
- Wrinkles and fishmouths. Folds in the sheet can channel water. They should be cut and patched, not left.
- Felt left exposed for days. In summer sun it buckles, and those ripples show through the shingles.
- Nothing in the valleys. Code may only require the eaves, but skipping the valleys saves little and risks a lot.
Questions to ask about the layer under your shingles
- What brand and type of synthetic will cover the deck?
- How far up from the eave will the membrane go, and how many courses is that on my house?
- Will valleys, walls, chimneys and skylights get membrane too?
- Under metal, is the membrane rated for high temperatures?
- Will I get photos of the deck after the membrane and synthetic are on, before shingles cover them?
Photos are worth asking for. Once the shingles are down, nobody sees this layer again until the next tear-off.
Membrane and the attic
Self-adhering membrane is a vapor barrier where it's installed, so the deck can't dry upward through it. That's fine at the eaves and valleys, but it's one more reason attic ventilation matters. Clear soffit vents and a working ridge vent carry moisture out from below. A deck covered entirely in membrane over a poorly vented attic can hold moisture in the wood, which is why most contractors use it only where it's needed.
The retrofit question
If your roof is 8 years old, dry, and built with felt and no membrane — common on the insurance-driven roofs put on after the 2009 and 2017 hail seasons — you don't need to tear it off. You need to keep the gutters clear, get the soffit vents unblocked, and put membrane on any north eave that has dammed before. The rest can wait for the next roof, which will have it.
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