Roof Underlayment and Ice & Water Shield: What Goes Under Your New Roof in Maryland
When you're comparing roof replacement estimates, almost all the attention goes to what you'll see from the street: the shingle line, the metal profile, the color. But some of the most important protection on your roof sits underneath all of that, in layers you'll never see again once the job is done. Underlayment and ice and water shield are what stand between a missing shingle, a hard sideways rain, or a winter ice dam and the inside of your home.
Here's what those layers actually are, how the main types compare, what changes depending on whether you choose asphalt, metal, synthetic cedar shake, or synthetic slate, and what to ask about them before you sign anything.
What Is Roof Underlayment?
Roof underlayment is a water-resistant layer installed directly on top of the roof deck (the plywood or OSB sheathing) and underneath the finished roofing material. It covers the entire deck and works as a secondary barrier, so if wind-driven rain gets past the outer layer, there's still something between that water and the wood.
It does a second job during installation, too. Between the moment the old roof comes off and the moment the new material goes on, underlayment is what keeps the deck dry. For a full breakdown of every layer in a roofing system, see our guide to the different parts of a roof.
The Three Main Types of Underlayment
Asphalt-Saturated Felt
Felt is the traditional option: a paper or fiberglass mat saturated with asphalt, usually sold as No. 15 or the thicker, tougher No. 30. Its main advantage is cost. The tradeoffs are that felt absorbs water rather than repelling it, can wrinkle once it gets wet, tears more easily in high wind, and shouldn't be left exposed for long before the roofing material goes on.
Synthetic Underlayment
Synthetic underlayment is made from woven or layered polymers like polypropylene or polyethylene. It's much lighter than felt (often around a quarter of the weight), has higher tear strength, repels water instead of absorbing it, and offers a more slip-resistant surface for the crew working on your roof. It also lies flatter after getting wet and holds up better to wind during installation. That combination is why synthetic has become the go-to choice across most of a modern roof deck.
Self-Adhered Membrane (Ice & Water Shield)
Ice and water shield is a peel-and-stick membrane, typically rubberized asphalt or butyl, that bonds directly to the roof deck and seals around the nails that pass through it. Rather than covering the whole roof, it's placed in the spots where water is most likely to find a way in: along the eaves, in valleys, and around chimneys, skylights, and vents.
Where Ice & Water Shield Goes, and Why It Matters in Maryland
Maryland winters swing above and below freezing, which is exactly the pattern that creates ice dams: snow melts on the warmer upper part of the roof, runs down, and refreezes at the colder edge. Water backs up behind that ridge of ice and can work its way under the roofing material. We cover how ice dams form, and how to prevent them, in 5 Tips for Preventing Ice Dams on Roofs.
The International Residential Code addresses this directly. In areas with a history of ice forming along the eaves, it calls for an ice barrier, either a self-adhering polymer-modified bitumen sheet or two layers of underlayment cemented together, running from the lowest edge of the roof to at least 24 inches inside the exterior wall line of the house, generally understood in the field as roughly three feet up from the gutter line. Maryland requires every jurisdiction to build on that code, and each county can modify it to suit local conditions, including whether an ice barrier is required.
Code is a minimum, though, not a finish line. On every roof we install, we go beyond that minimum to meet CertainTeed's own warranty requirements, which call for ice and water shield not just along the eaves, but at every rake, pipe penetration, roof-to-wall junction, valley, chimney, skylight, and other roof protrusion. Those are the spots where wind-driven rain and ice dams actually find their way in, and our team stays current on each county's code requirements on top of that and will walk you through exactly what applies to your home.
Should Ice & Water Shield Cover the Entire Roof?
Usually not. Because the membrane is fully sealed, covering the entire deck with it creates a vapor barrier, which can trap moisture if the attic isn't properly ventilated. For most homes, the stronger approach is ice and water shield in the high-risk areas and a quality synthetic underlayment everywhere else, paired with balanced attic ventilation.
Underlayment by Material: Asphalt, Metal, Synthetic Cedar Shake, and Synthetic Slate
Every roofing material we install sits on underlayment, but what goes underneath isn't one-size-fits-all. The finished material changes what the layers below need to handle.
Asphalt Shingles: On every asphalt shingle roof we install, we use CertainTeed products for the full underlayment system rather than mixing brands: CertainTeed RoofRunner synthetic underlayment across the deck, and CertainTeed WinterGuard ice and water shield at the eaves, rakes, valleys, chimneys, skylights, pipe penetrations, roof-to-wall junctions, and any other roof protrusion. Traditional paper felt is technically an option, but it's not what we recommend or typically use. Staying within one manufacturer's system matters for your warranty too: CertainTeed's extended SureStart PLUS coverage is tied to its Integrity Roof System, where the underlayment, starter shingles, hip and ridge caps, and ventilation are all installed together as one system by a credentialed contractor. Our breakdown of whether the CertainTeed 5-Star warranty is worth it explains how mixing brands on these components can affect coverage.
Metal Roofing: Metal roofs get hot. On architectural metal panels, in-service temperatures under the panels can reach as high as 240°F, which is enough to soften or damage some standard asphalt-based membranes. That's why, on every metal roof we install, we use a high-temperature ice and water shield along with a synthetic underlayment rated specifically for metal roofing, not the standard products used under shingles. High-heat membranes are also designed so the underlayment doesn't stick to the metal as the panels expand and contract with the seasons.
Synthetic Cedar Shake and Synthetic Slate: For synthetic cedar shake and synthetic slate, we install Brava and EcoStar, and both brands publish their own underlayment requirements: a synthetic underlayment across the entire deck, a self-adhered ice and water shield membrane in every valley, and coverage at the eaves and around roof projections wherever ice buildup is possible. Following those requirements, whichever of the two systems fits your home, is part of what keeps these premium materials performing, and warrantied, for the long haul.
Why Underlayment Is Replaced in Every Full Roof Replacement
In a full roof replacement, everything comes off down to the deck: the old roofing material, the nails, and the old underlayment. That gives the crew a clear look at the decking itself, so any compromised sections can be replaced before a single new layer goes down. Then new underlayment, ice and water shield, flashing, and the finished roofing material are installed as a complete system. You can see that full scope step by step in What Is Included in Roof Replacement Services?.
Timing matters here too. Ice and water shield generally needs to be covered by the finished roofing material within a limited window, often no more than about 30 days. Most of our replacements take one to two days from tear-off to final walkthrough, so your underlayment is never left sitting exposed to the weather.
Questions to Ask About Underlayment Before You Sign
Underlayment is easy to gloss over in an estimate because you'll never see it again. These questions help make sure you're comparing like for like:
What type of underlayment will cover the main roof deck, felt or synthetic?
Where exactly will ice and water shield go, and how far up from the eaves will it run?
Is the underlayment rated for the roofing material I'm choosing (for example, high-temperature products under metal)?
Is the underlayment part of the manufacturer's system required for the warranty I'm being offered?
Are the underlayment and ice and water shield listed by name in the written estimate and contract?
How Restoration Roofing Co. Builds What's Underneath
As a CertainTeed Premier ShingleMaster and the #3-rated CertainTeed installer in Maryland out of roughly 2,500 companies, we're held to CertainTeed's installation standards on every layer, not just the shingles you can see. We're licensed as a Maryland Home Improvement Contractor (MHIC 137778), every job is handled in-house by our team, and we serve homeowners throughout Howard County and across our Maryland service area.
FAQs About Roof Underlayment and Ice & Water Shield
Is roof underlayment really necessary?
Yes. Underlayment is the secondary barrier that keeps wind-driven rain off your roof deck if water gets past the outer roofing material, and it protects the deck during installation. Building codes and roofing manufacturers both call for it beneath steep-slope roofing materials.
Is synthetic underlayment better than felt?
For most homes, synthetic is the stronger choice. It's lighter, more tear-resistant, repels water instead of absorbing it, and lies flatter after getting wet. Felt costs less up front, but it's more prone to wrinkling and tearing and can't be left exposed as long.
Does ice and water shield go on the whole roof?
Usually not. It's typically installed along the eaves, in valleys, and around chimneys, skylights, and vents. Covering the entire deck creates a vapor barrier that can trap moisture unless the attic is very well ventilated.
Is the old underlayment removed during a roof replacement?
Yes. In a full roof replacement, the old roofing material and underlayment are removed down to the deck, the decking is inspected, and a completely new underlayment system is installed.
Does a metal roof need a different underlayment?
Yes. Metal roofs run much hotter than shingle roofs, so they need underlayment and ice and water shield rated for high temperatures, which also keeps the membrane from sticking to the panels as they expand and contract.
Does underlayment affect my roof warranty?
It can. Manufacturer system warranties, like CertainTeed's SureStart PLUS coverage, are tied to installing the manufacturer's full roof system, including the underlayment, so it's worth confirming exactly which products are going on your roof.
The layers under your roof decide how well it handles the next ice dam or sideways summer storm. Call us at 410-489-7663 or book your free roof replacement estimate today, and we'll show you exactly what goes under your new roof, layer by layer, no pressure, no pitch.
