Question 1

What is a roofing square and how many will my roof need?

Accepted Answer

A square is 100 square feet of roof surface. Roofers price in squares because bundles, underlayment and labour all scale with roof area, not with the footprint of the house. Your footprint is not your roof area. A 1,500 sq ft bungalow footprint at a 6/12 pitch is about 1,675 sq ft of roof before overhangs — roughly 17 squares. Add the overhangs and any dormers and you are near 18 or 19. Most Ottawa two-storey homes land between 15 and 25 squares. A bungalow with a simple gable is often 18 to 22, because a bungalow has the whole house under one roof. Order 10% over for waste on a simple roof and 15% on a cut-up roof with valleys and dormains.

Question 2

How does roof pitch change the cost?

Accepted Answer

Pitch is rise over 12 inches of run. A 6/12 roof rises 6 inches for every 12 across. Two things change with pitch. First, area: the multiplier on your footprint is about 1.03 at 3/12, 1.12 at 6/12, 1.25 at 9/12 and 1.41 at 12/12. A 12/12 roof has 41% more surface than the flat footprint under it. Second, labour. Anything over 6/12 needs roof jacks and planks and slows a crew down. Over 8/12 usually needs staging or harnesses and is often quoted at a premium of 15 to 30%. Below 4/12, standard asphalt shingles are not approved — that roof needs low-slope shingle detailing or a different system entirely.

Question 3

Why does my roof get ice dams, and will a new roof fix it?

Accepted Answer

An ice dam is heat escaping into the attic. Warm attic melts snow on the upper roof, the water runs down to the cold eave, refreezes, and backs up under the shingles. A new roof alone does not fix it. What fixes it is the three things together: air sealing the ceiling below, insulation to R-50 to R-60 in the attic, and ventilation that actually moves air from soffit to ridge. Ottawa gets freeze-thaw cycles all winter, which is why ice dams are common here and rare in colder, drier places. If you have had ice dams, say so before the roof is quoted — that is the moment the eave membrane, the ventilation and any bathroom fan ducted into the attic can all be dealt with at once. Afterwards it means opening the roof again.

Question 4

Is ice and water shield required in Ontario?

Accepted Answer

Yes, on shingle, shake and tile roofs. The Ontario Building Code requires eave protection extending from the roof edge a minimum of 900 mm up the slope, and to a line at least 300 mm inside the inner face of the exterior wall. On a house with a deep overhang, 900 mm may not reach that line — then you go further. The Code exempts it where the overhang itself exceeds 900 mm, on slopes of 1 in 1.5 (8/12) or steeper, over unheated garages, carports and porches, and for asphalt shingles installed to the specific subsection that covers them. In practice in Ottawa, most roofers run membrane well past the Code minimum — two courses at the eave, plus valleys, around chimneys and skylights, and up any wall the roof meets. The Code minimum is a floor written for the whole province. Our ice dams are not a provincial average.

Question 5

How much attic ventilation does my roof need?

Accepted Answer

The Code rule is 1 to 300: at least one unit of unobstructed vent area for every 300 units of insulated ceiling area. For a 1,200 sq ft ceiling that is 4 sq ft of net free vent area — about 576 square inches. For low-slope roofs the requirement tightens to 1 to 150. Balance matters as much as the total. At least 25% of the opening at the top and at least 25% at the bottom, so air enters at the soffits and leaves at the ridge. All the vents in one place moves no air. Two things that defeat it in real Ottawa attics: insulation stuffed into the soffits blocking the intake, and mixed vent types in one attic space — a ridge vent plus powered or gable vents will short-circuit and pull air from the nearest opening instead of from the eaves.

Question 6

Can new shingles go over the old ones?

Accepted Answer

Once. Ontario allows a single asphalt shingle overlay over existing shingles unless an engineering review supports more. A third layer is not permitted without one. Even when it is allowed, tearing off is usually the better call. You cannot inspect the deck under a layer you did not remove, the new shingles telegraph every unevenness underneath, the assembly runs hotter and ages faster, and most manufacturers reduce or void warranty coverage over an existing layer. Count your layers before quoting: lift a shingle at the rake edge, or look at the thickness at the eave.

Question 7

How do I know if the roof deck needs replacing?

Accepted Answer

You find out when the old roof comes off, which is why deck replacement belongs in the quote as a unit price rather than a guess. Ask for the per-sheet price before work starts — in Ottawa that is commonly quoted per 4x8 sheet supplied and installed. Then a surprise is a known number rather than a negotiation on the day. Signs before tear-off: a sagging line along the roof plane, stains or daylight visible in the attic, spongy feel underfoot, or a roof over 25 years old that has had a leak. Homes built before about 1970 in Ottawa may have plank decking rather than plywood, and plank decking with wide gaps needs sheathing over it before new shingles.

Question 8

How long should a roof in Ottawa last?

Accepted Answer

Architectural asphalt shingles are rated at 30 years and in Ottawa commonly deliver 20 to 25 because of freeze-thaw and sun on south-facing slopes. Three-tab runs 20 rated, often 15 to 18 here. Metal is 40 to 80. Cedar is 25. EPDM on a flat section is 15 to 25. What shortens it: poor ventilation, which cooks shingles from below and is the single biggest cause of early failure; ice dams; and overhanging trees that keep the surface damp and drop debris in valleys. Start planning at year 15 for asphalt. Start getting quotes when you see granules in the gutters, curled or cupped tabs, or bare patches on the south slope.

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