Assembly
Ice and water shield: the layer that decides how winter ends
Every other layer of a roof assumes water is running downhill. Ice and water shield exists for the situations where it is not: backed up behind an ice dam, driven uphill by wind, or standing in a valley that cannot clear fast enough.
What it actually is
A sheet of asphalt modified with rubber polymers, with an adhesive back and a release film. It is rolled onto the bare deck and sticks permanently. Two properties make it different from anything else on the roof: it bonds continuously to the deck rather than being nailed over it, and it self-seals around a nail shank driven through it. So even where the shingles above it are fastened straight through the membrane, the penetrations do not leak.
Synthetic underlayment, by contrast, is a shedding layer. It is mechanically fastened, it sheds water that gets past the shingles, and every fastener through it is a hole. That is entirely adequate across the field of the roof and inadequate at the eave.
Where it goes on a roof in this climate
- The eaves
- The critical run. Code in cold-winter jurisdictions generally requires the membrane to extend from the eave edge to a line a set distance inside the exterior wall — the point being that it must continue past the wall plate, because that is how far water backs up behind an ice dam. On a low-pitch roof or a deep overhang that means more than one course.
- Valleys
- A valley concentrates the water from two planes into one line and it carries far more than any other part of the roof. Membrane the full length, wide, under whatever valley treatment is used.
- Penetrations
- Around every pipe, vent, skylight and chimney. These are the places where a shingle course has to be interrupted, and interruptions leak.
- Wall lines and headwalls
- Where a roof runs into a wall, behind the step flashing. Wind-driven rain moves horizontally at a wall junction and no shingle detail on its own handles that indefinitely.
- Low-slope sections
- Below about 4:12 the water moves slowly enough that shingles alone are marginal. Below about 2:12 shingles do not belong at all — see roof pitch.
On a Cincinnati roof the eave run is the one to ask about specifically, because it is the one an estimate leaves out. A quote that says "install underlayment" and nothing else may or may not include it, and after the roof is on nobody can tell.
Why the eave, specifically
The eave is the coldest part of a roof, because the overhang has no heated space beneath it. When heat escaping into the attic melts snow higher up the slope, the meltwater runs down and refreezes when it reaches that cold overhang. The ice builds into a dam, water ponds behind it, and ponded water on a shingle roof finds its way under the courses — shingles are laid to shed running water, not to hold standing water back.
The membrane is what stops that ponded water reaching the deck. It does not prevent the ice dam; nothing on the roof surface does. The prevention is inside the building, and it is covered in ice dams in Cincinnati and attic ventilation.
What to ask for, in the words that get it
- How far up from the eave the membrane runs, stated as a measurement or as courses, and whether it passes the inside face of the exterior wall.
- Whether valleys get it full length and full width, under the valley treatment.
- Whether penetrations and wall lines get it, or only the eaves.
- What goes over the rest of the deck, because a synthetic underlayment across the field is normal and appropriate — the membrane is not meant to cover the whole roof.
- Whether the deck is being cleaned and dried first. It is an adhesive product. It does not stick to sawdust, frost or a wet deck.
Full-deck membrane coverage is occasionally sold as the premium option. On a ventilated attic assembly it is usually unnecessary and it can trap moisture in the deck from below. The place for full coverage is a low-slope section or a specific problem, not the whole roof by default.
How it fails
| Problem | Why it happens | What it looks like later |
|---|---|---|
| It was never installed | Left off an older roof, or omitted from a cheap scope | Water into the soffit and down the wall every hard winter |
| It stops short of the wall plate | One course laid at the eave regardless of overhang depth or pitch | A roof that is fine most winters and leaks in the bad one |
| It was laid on a dirty or damp deck | Rushed sequencing, or roofing in the cold | It releases from the deck in sheets and stops sealing around fasteners |
| Laps run the wrong way | Upper course lapped under the lower one | Water runs into the lap rather than over it, which is the one thing the layer exists to prevent |
| It was used instead of flashing | Membrane wrapped around a chimney in place of metal | It works for a few years, degrades in sunlight where it is exposed, and then leaks |
Want to know what is actually under the shingles at your eaves?
Request a free inspectionIs ice and water shield required by code?
Cold-climate codes generally require an ice barrier at eaves on roofs in regions with a history of ice forming at the eaves, and local jurisdictions across this metro adopt and amend the model codes on their own schedules. The practical answer is to ask your roofer what the requirement is in your jurisdiction and have the answer written into the scope.
My roof does not have it. Do I need to replace the roof to add it?
Effectively, yes, for the eave run — it goes on the bare deck under everything else. That is why it is a question to settle before a replacement rather than after one. Targeted improvements are possible where a section is being reworked anyway.
Does it make the attic sweat?
The membrane itself is a vapor barrier, which is exactly why it belongs at the eaves and valleys rather than over the whole deck on a ventilated assembly. Adequate intake and exhaust ventilation is what keeps the deck dry.
Is one brand of membrane better than another?
The meaningful differences are between granular-surfaced and film-surfaced products, and in how they behave in cold-weather application. This site does not name manufacturers; what matters more than the product is the coverage, the sequence of the laps and the state of the deck it went onto.
All work is performed by licensed and insured local roofing professionals.