Roof Underlayment and Ice and Water Shield: What the Ontario Building Code Requires, Where Good Roofers Put More, and What It Costs

The two products

What the Ontario Building Code requires

The Code requires eave protection on shingle, shake and tile roofs: a membrane at the eaves extending from the edge of the roof at least 900 mm (about 3 ft) up the slope, and to a line at least 300 mm (about 1 ft) inside the inner face of the exterior wall. On a house with a 2 ft overhang and a 2×6 wall, that is about 3 ft of eave protection measured along the slope, more on a low pitch where 300 mm inside the wall is further up the roof. There are exemptions: roofs over unheated garages, carports and porches; roofs steeper than 1 in 1.5 (about 8/12, where water runs off before it can back up); and a few others that a roofer will know. Check the current edition for your municipality, but in practice every Ontario roofer runs a 3 ft (one roll width) or 6 ft (two) band of ice and water at every eave and does not argue about it.

Where a good roofer puts it anyway

• Valleys. A 3 ft strip centred on every valley, under the metal or under the woven shingles. Valleys carry the water of two roof planes and are where wind-driven rain goes uphill.
• Around everything that pokes through. Chimneys, skylights, plumbing stacks, vents: a square of membrane under the flashing, lapped so water runs onto, not under, the membrane below.
• Along walls. Where the roof meets a sidewall or a dormer, a strip turned up the wall behind the step flashing.
• Rakes. The gable edges, in windy sites; not required, but it is $30 a roll.
• Low slopes. From 2/12 to 4/12, where shingles are marginal, full coverage of the whole deck is cheap insurance and what most manufacturers require for their warranty.
• North-facing cottage roofs, and any roof that has had an ice dam. Full coverage. The dam will come back (the fix for that is in the attic, on the ice dam page), and full coverage means the backed-up water cannot get into the house while you fix the cause.

The order it goes on, which is where mistakes are made

1. Drip edge at the eaves, nailed to the deck, first.
2. Ice and water shield at the eaves, over the drip edge, so water on the membrane runs onto the metal and off.
3. Ice and water in the valleys and around penetrations.
4. Synthetic underlayment over the rest, lapped over the top edge of the eave membrane, each course lapping the one below by the manufacturer’s line, cap-nailed.
5. Drip edge at the rakes, over the underlayment, so wind-driven rain at the gable runs onto the underlayment and not under it.
6. Starter strip, then shingles.

Drip edge under at the eaves and over at the rakes is the detail that tells you whether the crew knows what the layers are for. Backwards, the eave membrane sheds water behind the fascia and the rakes let it under the underlayment. The installation checklist has you look at the eave edge before the shingles cover it.

Felt, if someone offers it

Fifteen-pound asphalt felt was the underlayment for a century and still works. It also wrinkles when it gets wet before the shingles go on (the wrinkles telegraph through), tears in wind, and degrades under shingles over 20 years to something you can poke a finger through. Synthetic costs a few dollars a square more and is better in every way; a quote with felt on it is a quote from a roofer who has not changed a habit, which is a small thing on its own and a reason to look at the rest of the quote.

Sources and further reading

• Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
• O. Reg. 163/24, the Building Code regulation: the full text if you want to read 9.26 and 9.19 yourself.
• IKO: Canadian-made shingles; the warranty and the technical data sheets are on the site.