Shingles & materials
Ice and water shield: where the code wants it and how much
One sentence of code decides how much membrane a roof needs, and the pitch decides how it reads on the deck. Here is the sentence and the arithmetic.
Why ice dams need a membrane
In cold weather, heat leaking from the house warms the roof deck and melts snow on it. The water runs down to the eave, which sits outside the heated walls and stays cold, and freezes there. The ice builds into a dam, and the next meltwater pools behind it. Shingles shed running water; they do not hold back standing water, which backs up under them and into the house. A self-adhering ice barrier membrane, rubberised asphalt that seals around every nail driven through it, keeps the deck dry where the water pools.
Ventilation and insulation reduce ice dams by keeping the deck cold; the membrane is the defence for the ones that still form. Attic ventilation and the 1:150 rule covers the first half.
The code sentence
The International Residential Code, section 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”, extending from the lowest edges of all roof surfaces to a point at least 24 inches inside the exterior wall line of the building. Local amendments decide which areas count: most of the northern United States and all of Canada apply it, and many jurisdictions mark it on a map or in a table. Detached accessory buildings without conditioned space are exempt.
Two details matter in that sentence. The membrane starts at the lowest edge of the roof, the drip edge, not at the wall, so the overhang is inside the distance. And the 24 inches is measured horizontally inside the wall, because what it protects is the heated building behind the wall.
From the code to courses on the deck
| Overhang | Wall | Pitch | Horizontal | Up the slope | 36 in courses (6 in lap) |
|---|---|---|---|---|---|
| 0 in | 6 in | 4/12 | 30 in | 31.6 in | 1 |
| 12 in | 6 in | 4/12 | 42 in | 44.3 in | 2 |
| 12 in | 6 in | 6/12 | 42 in | 47 in | 2 |
| 18 in | 6 in | 4/12 | 48 in | 50.6 in | 2 |
| 24 in | 8 in | 8/12 | 56 in | 67.3 in | 3 |
| 24 in | 8 in | 12/12 | 56 in | 79.2 in | 3 |
On almost every house with an overhang the answer is two courses of a 36-inch roll. Wide overhangs on steep roofs need three. The pitch calculator gives the factor for your roof.
An 80-foot eave on a 6/12 roof
12-inch overhang, 6-inch wall, 24 inches inside, 36 in × 66.7 ft rolls with a 6-inch lap, 10% waste.
- Horizontal: 42 in; up the slope: 42 × 1.118 = 47 in, so 2 courses (66 in covered)
- Eave membrane: 80 ft × 2 × 3 ft = 480 sq ft; with waste 528 sq ft
- Rolls: 528 ÷ 200 = 2.6, so 3 rolls
The ice and water shield calculator adds valleys and penetrations to the same count.
Valleys
Valleys collect water from two planes and hold snow. The code requires a valley lining under closed valleys (IRC R905.2.8.2): 36-inch mineral-surfaced roll roofing or a self-adhering membrane. Most roofers use the membrane, a full 36-inch strip centred on the valley line for its whole length plus a foot of run-out at the eave, which also serves as the ice barrier the valley needs. Open metal valleys get the same membrane under the metal. The valley flashing calculator counts it with the metal.
Penetrations and low slopes
Around chimneys, skylights and large vents, membrane wrapped up the sides and under the flashing adds protection where water slows. About 20 square feet per penetration is a fair allowance.
On roofs between 2/12 and 4/12, many shingle makers recommend membrane well beyond the code minimum, sometimes over the whole deck, with double-coverage underlayment above. Some manufacturers also ask for 36 inches inside the wall rather than 24 on low slopes, which often turns two courses into three.
Order of installation
- Drip edge on the bare deck at the eaves.
- Ice barrier membrane over the drip edge flange, down to its outer edge, starting at the eave; the second course laps the first by the lap line.
- Valley membrane, centred, before the field underlayment.
- Underlayment lapping over the top edge of the membrane.
- Drip edge at the rakes, over the underlayment.
Drip edge, step flashing and valleys draws the sequence, and underlayment: felt vs synthetic covers the layer above the membrane.
Installing it so it sticks
The membrane bonds to a clean, dry deck above about 40 °F; below that, a primer or a warm afternoon is needed or the laps lift. It is not UV-stable and must be covered by roofing within the exposure limit on the roll, usually 30 to 90 days. Install it on the deck, never over old shingles or felt. And because it does not breathe, a deck fully covered in membrane depends on good attic ventilation to dry from below.
The full-deck question
Some homeowners ask for membrane over the whole roof. It is permitted and effective against wind-driven rain, but it costs several times as much as synthetic underlayment, and it turns the deck into a vapour barrier that can trap moisture from a poorly ventilated attic. Most roofers keep it to the eaves, valleys, penetrations and low slopes, which is where the water is. The underlayment calculator handles the rest of the deck.