Ice and Water Shield: Where Code Requires It and Where It Just Makes Sense

Felt and synthetic sheets get nailed down and shed water downhill. This membrane is sticky on the back. You peel a release film, press it onto bare decking, and it bonds. The property that matters is what happens around a nail driven through it: the rubberized asphalt grips the shank, so water sitting on top can't follow the fastener into the wood.

Across most of a roof, water is moving. At the eaves in January, or in a valley during a cloudburst, it sits or backs uphill. A membrane that seals itself around fasteners is the only underlayment built to handle standing water.

Under the IRC, the ice barrier starts at the lowest edge of the roof and climbs the slope until it's at least 24 inches past the inside face of the exterior wall. Picture the line on your attic floor where the wall stands. The membrane has to reach two feet beyond it, measured level.

On a house with a deep overhang or a shallow pitch, one standard 3-foot roll often falls short, so a second course goes on above the first. We measure overhang and pitch at each eave rather than assuming a single roll will do. Local building departments can amend the requirement, and some Midwest towns ask for more. We pull the permit and build to what the inspector in that town enforces.

Heat escaping from the living space warms the upper deck. Snow up there melts, trickles down to the cold overhang and refreezes. Repeat that for a week and there's a ridge of ice along the gutter line with a pool of meltwater behind it. That water works under the shingle laps, which were never designed to hold back a pond. With no membrane underneath, it ends up in the soffit, the wall cavity or the ceiling below.

A shingle with a fractured mat lets ponded water through faster. It's part of why a roof bruised in a May storm sometimes shows its first stain the next February. The cold-weather side gets more room on our winter roof damage page and the one on freeze-thaw shingle damage.

The code language is about eaves. On most Midwest jobs our crews put membrane in a few more places.

Valleys. Two slopes dump their runoff into one channel. A full-width strip down the valley before the metal or woven shingles go in costs little compared with tracing a valley leak later.

Chimneys and skylights. Flashing does the heavy lifting, but a membrane collar behind it catches whatever wind drives past. Our page on hail-damaged skylights and vents covers the flashing itself.

Sidewalls and dormers. Anywhere a roof plane runs into a vertical wall, rain gets shoved into the seam.

Low-slope tie-ins. A porch roof or a shallow section drains slowly, and slow water hunts for a way in.

Some outfits sell a full deck of ice and water membrane as a premium upgrade. On a typical Midwest asphalt roof with reasonable pitch, we don't recommend it. The membrane works as a vapor barrier, so moisture rising from the house can get trapped against the wood with no way to dry outward. It also makes the next tear-off miserable, because the membrane is bonded to the deck and often peels the top veneer off the plywood. Use it where water stands or backs up, and a breathable sheet everywhere else; our page on synthetic underlayment covers that sheet.

On a hail replacement, the ice barrier line is figured from total eave length and how many courses code calls for. It's one of the items most often shorted. An estimate may show one row where the overhang needs two, or leave the valleys off. Where the requirement is adopted, meeting it may fall under your policy's ordinance or law coverage, so check your declarations page for it. We photograph and measure overhang depth and send the difference in through a claim supplement.