Ice Barrier Coverage Calculator
Ice and water barrier is the self-adhering membrane that goes on the deck at the eaves before the underlayment. Its job is to stay watertight when meltwater backs up behind an ice dam and sits on the roof instead of running off it. How far up the slope it has to reach is set by code, and the answer is less obvious than it sounds, because the requirement is horizontal and the membrane is laid along the slope.
Coverage from overhang and pitch
How far up the roof the ice and water membrane has to reach to clear the wall line, and how many courses that takes.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Courses of membrane at the eaves Rounded up, with the lap between courses already taken off.
- Horizontal distance to cover Overhang, plus the wall, plus the distance required inside it.
- Pitch multiplier
- Distance up the slope The figure that actually decides how many courses you need.
- Covered by those courses
- Membrane needed Eave length multiplied by the number of courses.
- Rolls to order
One course reaches past the wall line, which is the straightforward case. Make sure it goes on before the drip edge at the eaves and after it at the rakes, and that it is rolled down onto a clean deck rather than over old felt.
Two courses, so there is a lap to get right: the upper course laps over the lower one, never under it. This is the point where quantities quietly double, and it is worth checking that any estimate you have been given accounts for it rather than pricing a single run along the eaves.
Three courses or more, which comes from a deep overhang, a steep pitch, or a jurisdiction asking for more than 24 inches inside the wall. It is a real cost worth raising before work starts, and worth confirming against your local requirement rather than assuming the default.
Ice barrier is required where there is a history of ice forming along the eaves, which makes it a local question rather than a universal one — your building department gives the answer for your address, and in a warm climate none of this applies. The figures here cover the eaves only. Valleys are normally protected as well, and so are the areas around chimneys, skylights and any low-slope section, none of which are counted above. A membrane keeps meltwater out of the building; it does not stop ice dams forming, and a roof that dams badly usually has an insulation and ventilation problem underneath it.
The rule
The membrane must run from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line, measured horizontally. That wording is doing a lot of work. It means the distance you need is not 24 inches: it is the overhang, plus the thickness of the wall, plus the 24 inches — and then all of that stretched by the pitch, because the roof surface travels further than the ground beneath it.
A house with an 18 inch overhang and a 6 inch wall needs 48 horizontal inches. On a 6 in 12 roof that is 53.7 inches measured up the slope, which a single 36 inch roll does not reach. Two courses, and the quantity doubles.
Why the second course surprises people
Because the first one looks like plenty. Standing at the eaves, a three foot band of membrane feels generous, and it is only when you take the wall line into account that it turns out to stop short. Deep overhangs and steep pitches are what push it over, and both are common on exactly the kind of older house that dams in the first place.
It is worth checking against any estimate you have been given. An estimate that prices one run of membrane along the eaves on a house that needs two is not necessarily dishonest, but it will be short, and the shortfall is discovered on the day with the deck open.
Laps run the right way
Where there is more than one course, the upper course laps over the lower one. Water running down the roof then crosses the joint from above and stays on top of the membrane. Lapped the other way it runs straight into the seam, which defeats the entire point of the material. Each course also overlaps the one below by a few inches, which is why a 36 inch roll does not cover 36 inches of roof.
Where else it goes
The calculator covers the eaves, because that is what the 24 inch rule is about. Membrane normally goes in other places too, and none of them are counted above:
- Valleys, full length, because that is where the volume of water is.
- Around chimneys, skylights and dormers, where flashing has to be relied on.
- Low-slope sections, where water moves slowly enough to find its way back under the courses.
- Rake edges in some high-wind regions, though that is a local practice rather than a general rule.
This is a local question
Ice barrier is required where there is a history of ice forming along the eaves. That makes it a matter for your building department rather than a universal requirement, and in a warm climate none of it applies. Some cold-climate jurisdictions also ask for more than 24 inches inside the wall, which is why that figure is an input you can change rather than a constant. Confirm the number for your address before ordering anything.
A membrane is not a cure
It keeps meltwater out of the building. It does nothing to stop ice dams forming, because dams form when heat escaping from the house melts snow that then refreezes at the cold eaves. A roof that dams badly has an insulation and ventilation problem underneath it, and the membrane is there to limit the damage while that problem is fixed — not instead of fixing it.