Most homeowners never see the ice and water shield on their house. It goes down on the bare roof deck before the shingles, gets covered within hours, and then quietly does its job for decades. Yet when a roofer pulls apart a leaking eave or a rotted valley in the GTA, the story usually comes back to this one layer: it was missing, it was too short, or it was installed in a way that let water get behind it.
This guide explains what the membrane is, exactly where a professional roofer puts it, what Ontario’s building rules say about eave protection in plain terms, and why the weeks before the freeze-thaw season are when its quality matters most. It is written from the installer’s side of the job, so you know what to expect, and what to ask, when you are planning a new roof.
What Ice And Water Shield Actually Is
“Ice and water shield” started as one manufacturer’s product name and became the trade’s everyday term for any self-adhered, rubberized asphalt roofing membrane. Different brands call it by different names, but the product category is the same: a flexible sheet, usually around a metre wide, made of modified bitumen with a polymer or granular top surface and a sticky underside protected by a release film. The roofer peels the film and presses the sheet directly onto clean, dry sheathing.
Two properties make it different from ordinary synthetic underlayment:
- It bonds to the deck. Standard underlayment is fastened with nails or cap nails and simply lies over the wood. Water that gets under a shingle can track sideways under loose underlayment. A self-adhered sheet is stuck down across its whole surface, so there is no open path along the deck.
- It seals around fasteners. When a roofing nail is driven through the membrane, the rubberized asphalt grips the shank. That self-sealing behaviour is the whole point: the shingles above still need nails, and each of those nails would otherwise be a tiny hole in your only waterproof layer.
Why Sloped Roofs Need A Second Line Of Defence
Shingles shed water. They work because each course overlaps the one below, and gravity pulls water down and off the roof. Shingles are not watertight against water that sits still or flows backward. That is where trouble begins in an Ontario climate.
Three situations defeat shingles on their own:
- Ice dams at the eaves. Heat from the house melts snow on the upper roof. Meltwater runs down to the cold overhang, refreezes, and builds a ridge of ice. The next batch of meltwater pools behind the ridge and backs up under the shingles, sometimes well above the wall line.
- Concentrated flow. Valleys collect water from two roof planes. During a heavy autumn rain, a valley can carry far more water than any open field of shingles, and debris or ice in the valley can push it sideways.
- Interruptions in the roof. Chimneys, skylights, dormer walls and the junction where a roof meets a taller wall all break the simple overlap pattern. Metal flashing does the main work at those spots, and the membrane underneath is the insurance if wind-driven rain gets behind the metal.
In each case the membrane catches water that has already beaten the shingles and keeps it off the wood deck, out of the insulation and away from your ceilings. If you are planning a full roof replacement for your home, these are the areas where you want the specification spelled out in writing.
Where A Roofer Installs Ice And Water Shield
A well-detailed roof uses ice and water shield in specific, predictable places. The table below is a practical summary of where it goes and why.
| Location | Typical coverage | What it protects against |
|---|---|---|
| Eaves (lower roof edge) | From the drip edge up the slope, far enough to pass inside the heated wall line | Ice dam backup and wind-driven rain at the gutter line |
| Valleys | A full-width sheet centred in the valley, running from eave to ridge | Concentrated runoff, debris dams and ice in the valley |
| Chimneys | Around all four sides, turned up the masonry under the flashing | Water behind step flashing and at the upslope side |
| Skylights | Around the curb or frame, lapped up the sides | Leaks at corners and head of the skylight |
| Roof to wall junctions | Along the wall, turned up behind the cladding | Wind-driven rain behind step and counter flashing |
| Rake edges (optional) | A narrow strip along the gable edge | Wind-driven rain on exposed gables |
| Low-slope sections | Often the entire area, depending on the roofing product | Slow drainage and standing water on shallow pitches |
The eaves
The eave is the single most important location. On a typical Toronto-area house, the roof overhangs the exterior wall by 30 to 60 centimetres, and that overhang is the coldest part of the roof because there is no heated space below it. It is exactly where ice forms.
The membrane goes on the deck first, lapped over the metal drip edge at the bottom so any water on the membrane drains onto the metal and into the gutter. On many houses one course of membrane is not enough to reach inside the wall line, especially with a deep overhang or a low pitch. In that case a second course is run above the first, overlapping it by the amount the manufacturer specifies, usually several inches.
Valleys
In a valley, the roofer centres a full-width sheet so it covers both sides of the valley line. Getting the sheet to lie tight into the valley, without bridging, is a skill: a membrane that tents across the valley can puncture or crease when the shingles are nailed down. Whether the valley is finished as open metal, closed cut or woven shingles, the membrane goes underneath.
Chimneys, skylights and walls
At penetrations and walls, the membrane is cut and folded to wrap the corners and turned up the vertical surface a few inches. The flashing (step flashing, counter flashing, a chimney saddle or a skylight kit) then goes over it. Corners are where most leaks start, so an experienced installer adds small patches or target pieces at inside and outside corners before the main sheets go on. For chimneys in particular, the membrane only helps if the masonry and flashing above it are sound, which is why it is often paired with chimney flashing and masonry repairs during a reroof.
What The Ontario Building Code Says About Eave Protection
Ontario’s building rules for houses require eave protection on most shingled roofs. In plain terms, the requirement is that a waterproof layer runs from the edge of the roof up the slope for a set distance, and far enough that it extends past the inside face of the exterior wall. The commonly cited figures are at least 900 mm up the slope and at least 300 mm inside the inner face of the wall, and whichever is greater governs on a given house.
A few practical notes about how this plays out:
- The wall line is what matters. The goal is to carry protection over the heated part of the house, so meltwater backing up from an ice dam lands on membrane, not on bare wood above the wall plate.
- Different materials can satisfy the rule. Older roofs sometimes used heavy roll roofing or double layers of felt at the eaves. Today a self-adhered membrane is the standard choice because it is easier to install correctly and seals around nails.
- There are exceptions. Certain buildings, such as unheated structures, may not need it, and very steep or specific assemblies can be treated differently.
Codes change, and local building departments interpret details. Treat these figures as a general explanation, not a legal reference. Your roofer should know the current requirement, and if you are taking out a permit or working on an unusual roof, confirm the specifics with your municipality and with the membrane manufacturer’s installation instructions.
How Far Up The Roof Should It Run?
This is the question homeowners most often ask when comparing quotes, and the honest answer is: at least as far as the code requires, and often further.
A roofer decides the coverage by measuring from the roof edge to a point past the inside of the exterior wall, then adding a margin. On a steep roof with a short overhang, one width of membrane may be enough. On a shallow roof with a deep overhang, the horizontal distance to the wall is covered by more length along the slope, so two courses are common.
Situations that justify running it higher:
- A history of ice dams. If previous winters produced icicles and interior stains, meltwater has already shown it can back up high. Extra courses are cheap insurance compared with ceiling and insulation repairs.
- Low pitches. Water moves more slowly on a shallow roof, and ice dams can push water further upslope.
- Weak attic ventilation or insulation. A warm attic melts snow faster and feeds bigger ice dams. Membrane does not fix that, but it limits the damage while the root cause is being addressed through proper attic ventilation.
What you should not accept is a quote that simply says “ice and water shield at eaves” with no distance. Ask how many courses, how far past the wall line, and whether it is included in valleys and around every penetration.
The Full-Coverage Debate
Some homeowners ask whether the whole roof should be covered in ice and water shield. It sounds like the most protective option. In practice, full coverage on a normal sloped roof has real trade-offs, and most roofers reserve it for specific cases.
| Approach | Where it makes sense | Advantages | Drawbacks |
|---|---|---|---|
| Code-minimum eaves plus valleys and penetrations | Standard pitches, good attic, no ice dam history | Meets requirements, lower material cost, deck can dry upward | Less margin if ice dams become severe |
| Extended eaves (two or more courses) | Low pitches, deep overhangs, known ice dam problems | More protection where it counts, moderate added cost | Slightly more labour and material |
| Full coverage | Low-slope areas, some metal roofs, specific manufacturer systems | Continuous sealed layer across the deck | Traps moisture if ventilation is weak, higher cost, harder future tear-offs |
| Synthetic underlayment only in field | Most of the roof area above protected zones | Breathable enough to let the deck dry, easy to install | Not self-sealing around nails |
The main technical concern with full coverage is vapour. A self-adhered membrane is essentially a vapour barrier on the cold side of the roof deck. If warm, moist air leaks into the attic and condenses on the underside of the sheathing, the wood cannot dry through the top. On a well-ventilated attic this is manageable. On a house with air leaks from bathrooms, pot lights or an unsealed attic hatch, sealing the whole deck can contribute to damp sheathing and mould.
Installation Details That Decide Whether It Works
A roll of membrane is only as good as the way it is put down. These are the details a careful roofer pays attention to, and the ones that tend to show up when a failed roof is taken apart.
Deck condition and temperature
The membrane must stick to clean, dry, sound sheathing. Dust, sawdust, old granules or dampness reduce adhesion. In early autumn, mornings can bring dew on the deck, so crews often wait until the surface dries. Most membranes also have a minimum application temperature. As the weather cools later in the season, some crews switch to cold-weather formulations or take extra care to roll the sheets firmly so the adhesive bonds.
Laps in the right direction
Every overlap must shed water downhill. Upper sheets lap over lower sheets, side laps are staggered, and end laps meet the manufacturer’s minimum. A reversed lap, where the lower sheet sits on top, creates a small channel that directs water under the membrane instead of over it.
Drip edge sequencing
At the eave, the usual sequence is drip edge first, then membrane over the drip edge flange, so water on the membrane drains onto metal. At the rakes, the order is often reversed: membrane or underlayment first, then drip edge over it. Mixing these up is one of the most common errors a roofer finds on inspection.
Coordination with ventilation
The membrane at the eave should not cover soffit intake paths inside the attic, and at the ridge it should stop short of the vent slot so air can still move. Blocking ventilation undoes a lot of the benefit.
Warning Signs Your Current Roof May Be Missing It
You cannot see the membrane from the ground, and you should not go onto the roof to look. There are clues inside the house, though, that tell a roofer where to focus when they inspect.
| What you notice indoors | Likely cause at roof level | What the roofer checks |
|---|---|---|
| Stains on the ceiling near exterior walls after a thaw | Ice dam backup past short or missing eave protection | Eave membrane extent, attic insulation and ventilation |
| Damp or dark sheathing near the eaves seen from the attic | Repeated minor leaks at the overhang | Drip edge sequencing, membrane laps, gutter condition |
| Water on the wall or window trim below a valley | Valley without membrane or with debris damming | Valley construction and membrane under the valley |
| Drips or stains on the ceiling around the chimney breast | Flashing failure with no backup layer | Step and counter flashing, membrane at the chimney |
| Stains at skylight corners | Water behind skylight flashing | Skylight kit, corner patches, membrane wrap |
If any of these sound familiar, early autumn is the right time to have a professional inspection done, not in the middle of January when the roof is covered and nothing can be repaired properly. A roofer can often determine from the attic and from the roof edge whether eave protection is present and how far it extends. If ice dams are already a recurring issue, it is worth reading about safe ice dam removal so you know what should and should not be done when they form.
Why Early Autumn Is The Time To Get It Right
September and October are the practical window for most roof work in the GTA. Days are still mild enough for membranes to bond well, shingles lie flat and seal properly before cold weather, and there is time to fix problems found during tear-off before the first freeze.
The freeze-thaw season is also when ice and water shield earns its cost. Ontario winters swing above and below freezing repeatedly, and every cycle is a chance for meltwater to form and refreeze at the eaves. A roof replaced in late autumn or winter can still be done well, but the adhesive products are working against the temperature, and crews need more care and time.
Questions To Ask A Contractor About Ice And Water Shield
- How far up the slope will the ice and water shield run at the eaves, and how many courses will that take on my house?
- Will it go in every valley, around the chimney, around skylights and along roof-to-wall junctions?
- Which product are you using, and does it match the shingle manufacturer’s system?
- How do you handle drip edge sequencing at eaves and rakes?
- What happens if you find rotted sheathing at the eaves during tear-off, and how is that priced?
- How will the membrane layout work with my attic ventilation?
For typical houses, the added material and labour for extending eave protection by an extra course often falls somewhere in the range of a few hundred dollars to over a thousand, depending on roof length, pitch and access. That is an estimate only; the actual figure depends on the house and the scope. Compared with the cost of ceiling repairs and wet insulation, it is usually one of the best value upgrades in a roofing quote.
You can find more general answers on our roofing FAQ page, and if you want to hear from other homeowners, our customer reviews are a good place to start.
Is ice and water shield required on every roof in Ontario?
How far up the roof should the membrane go at the eaves?
Should I cover my whole roof with it?
Can it be added without replacing the shingles?
Does ice and water shield stop ice dams?
Can it be installed in cold weather?
Plan Your Ice And Water Shield Before The Freeze-Thaw Season
The membrane under your shingles is one of the least visible parts of a roof and one of the most important in an Ontario winter. At The Roof Technician, our roofers inspect the eaves, valleys and penetrations, explain exactly how much protection your roof needs, and install it to the manufacturer’s instructions before the cold arrives.
Call us today at (416) 826-0040 or request a free estimate.
Proudly serving homeowners across Toronto and the GTA.