Ice and Water Shield: Where It Goes and How Far Up the Roof
Ice barrier membrane is the last line of defence against an ice dam. Getting the coverage right means understanding what you are measuring from — and it is not the eave.
Ice barrier membrane exists for one scenario. Heat escaping through a roof melts the snow sitting on it. The meltwater runs down the slope until it reaches the overhang — which is not heated from below and stays cold — where it refreezes. That ridge of ice grows, backs up the water behind it, and the water then sits on the roof rather than running off it. Water sitting on a roof will find every lap, every fastener, and every seam that was designed to shed running water rather than resist standing water. The membrane is what stops that becoming a ceiling stain.
The Measurement Everyone Gets Wrong
The rule of thumb the trade works to is that the membrane must extend from the roof edge up the slope to a point at least 300 mm — twelve inches — beyond the interior face of the exterior wall below. Note what that is measured from. It is not measured from the fascia, not from the drip edge, and not from the eavestrough. It is measured from the warm wall, because the warm wall is where the heated space stops and the cold overhang begins. Confirm the actual requirement with your local building department; what matters here is understanding what the dimension is referencing.
The practical consequence is that a house with a deep overhang needs more membrane than one with a shallow one, and a low-pitch roof needs more than a steep one, because a shallower slope covers less horizontal distance per foot of membrane run. On a 3:12 with a 24-inch overhang and a two-by-six wall, one 36-inch course does not get you there. Two courses do, with the lap. On a 10:12 with a 12-inch overhang, one course is often enough. Measure it out on the roof rather than defaulting to a single row because that is what came off the truck.
The Other Places It Belongs
Eaves get the attention, but the eave is only one of the locations where water stops behaving like running water. Anywhere flow concentrates, slows, or gets blocked deserves membrane.
- ✓Valleys, full length, centred on the valley line and wide — a valley carries the runoff of two roof planes through one narrow channel, and it fills with ice.
- ✓Around chimneys, skylights and curbs, running up onto the curb or the masonry face and out onto the deck well past the flashing.
- ✓At sidewalls and headwalls, turned up the wall and lapped correctly with the water-resistive barrier so it drains outward, not into the cavity.
- ✓Low-slope sections and porch roofs, where the whole plane may warrant full coverage.
- ✓Dead valleys and any pocket where two planes create a spot with little or no fall.
- ✓Roof-to-roof transitions and pitch changes, where sliding snow stalls and ice builds.
Sequencing at the Eave
The convention that has served the trade well is that at the eave the drip edge goes on the deck first and the membrane laps over its top flange, so any water that gets onto the membrane drains out over the metal rather than behind it. At the rake it is reversed: membrane first, then the trim over it. Metal roof systems often use a different eave detail — a starter cleat or an eave trim set over the membrane and sealed to it — so the panel manufacturer's detail governs. What never changes is the principle: each layer must lap so that water travelling downhill stays on top of the layer below.
Ice barrier membrane is a backstop, not a cure. If a house is damming every winter, the cause is heat loss into the roof assembly and inadequate ventilation, and the fix is air sealing at the ceiling plane, insulation depth, and balanced intake and exhaust. Membrane just buys you time before the water finds another way in.
Getting It Down Properly
Self-adhered membrane is unforgiving of a poor substrate and of a rushed installation. The deck has to be dry, swept clean, and free of frost — adhesion to a damp or dusty surface is not adhesion, it is just contact. Aged or rough sheathing may need a primer; check the product. Roll the sheet out, let it relax, then peel the release film progressively rather than all at once so the sheet lands flat. Fishmouths at laps and wrinkles in the field both become permanent once the panels are on, and a wrinkle under a metal pan is a visible defect as well as a water path.
Cold-weather application is the one to watch in Ontario. Below a certain temperature, which the manufacturer states, the adhesive will not wet out and the sheet will lift. Some products are formulated for low-temperature application; most are not. The options are to warm the material and the deck, to fasten mechanically as the manufacturer permits, or to wait.
Laps and Terminations
Side laps and end laps have stated minimums — commonly around 3 inches on the side and 6 inches on the end, but read the product. Lap uphill sheets over downhill sheets, always. At a valley, run the valley membrane first and lap the field membrane over it from both planes so the flow direction is respected. At a wall, terminate the membrane on the wall in a way that will be covered and drained by the water-resistive barrier and the cladding, not left as an exposed edge relying on sealant.
Should the Whole Roof Be Membraned?
Fully adhering a roof is the most protective option, and on a complex, low-slope or heavily shaded roof it is often the right call. The caution is that a fully adhered membrane is close to vapour-impermeable, so it only belongs on an assembly that can dry — a properly vented attic or a designed unvented assembly with the vapour control in the right place. On an assembly where the deck's only drying path is upward, sealing it completely can trap moisture in the sheathing. That is a design question, not a materials question, and it is worth asking before ordering.
Frequently Asked Questions
How many rows of membrane do I need at the eave?
As many as it takes to get past the interior face of the exterior wall by a sensible margin, measured along the slope. Deep overhangs and low pitches routinely need two, sometimes three. Measure the actual roof rather than defaulting to one row.
Does a metal roof still need ice barrier if metal sheds snow?
Yes. Snow does slide off a smooth metal surface more readily, but it does not always slide — snow bonds to a cold panel, sits through cold snaps, and the melt-and-refreeze cycle at the overhang happens on metal the same as anything else. Metal roofs in Ontario ice dam.
Can ice barrier be applied over an existing roof or a dirty deck?
No. It bonds to a clean, dry, sound deck and nothing else. Any dust, frost, moisture or loose material means the bond will not hold, and a membrane that is not bonded does not seal around fasteners — which is the entire reason for using it.
Is ice and water shield the same as high-temperature underlayment?
Not necessarily. Many ice barrier products are standard-temperature rubberized asphalt and are not suitable under metal panels. If a self-adhered membrane is going under metal, it needs to be a product rated for the temperature.
Where else besides the eave do leaks from ice actually show up?
Valleys and sidewalls, overwhelmingly. A valley carries two planes' worth of water and ice through one channel, and a sidewall running down to an eave will dump into the wall cavity if the kick-out and the membrane are not right.
Get a Free Metal Roofing Quote.
We measure your roof remotely by satellite and deliver a written, itemized quote — same day, no obligation.