The Membrane
Ice and water shield is the single most important material on a Toronto roof and the one most often skimped on, because nobody can see it afterwards.
Short answer
It is a self-adhering membrane that seals around fasteners and holds back standing water, which is exactly what an ice dam creates. It belongs at the eaves far enough up the slope to cover where a dam can form, in every valley for the full length, and around every penetration and flashing.
Where it has to go
At the eaves, extending up the slope far enough to cover the area where a dam can back water up — which on a low-pitch roof or a deep overhang means more than one course.
In every valley, full length, which is the second most important location.
Around every penetration: stacks, vents, skylights.
Up every wall and chimney junction behind the step flashing.
At the rake edges on an exposed or windy property.
And over the whole deck on a low-slope roof, which is a legitimate specification and is sometimes required.
How far up the eave
The Ontario code sets a minimum and the minimum is a minimum. On a house with a history of ice dams, a deep overhang, or a low pitch, going further up the slope than the code requires is cheap at install time and is the difference between a ceiling stain and no ceiling stain. It cannot be added later without opening the roof.
What it does and does not do
Seals around nails, so a fastener through it does not become a leak.
Holds back standing water, which ordinary underlayment does not.
Buys you the eave area, which is where an ice dam does its damage.
Does not stop an ice dam forming — that is an attic problem.
Does not help above the area it covers, so coverage is the whole question.
And does not substitute for proper flashing, drip edge and venting, which all still have to be right.
What to specify
The coverage at the eaves, in writing, by distance up the slope rather than by "to code".
Full length in every valley.
Every penetration and flashing junction.
The product, because there is a quality range and the cheap ones are harder to work with and less forgiving.
Deck preparation, because the membrane needs a clean dry deck to adhere and in cold weather that is a real constraint.
And synthetic underlayment rather than felt over the rest of the deck, which is now standard and performs better.
Common questions
Is ice and water shield necessary in Toronto?
Yes. At the eaves, in the valleys and at every penetration.
Does it stop ice dams?
No. It stops the water getting in. The dam itself is an attic heat-loss problem.
Can it be added later?
Not without opening the roof. Specify the coverage at install time.