Asphalt shingle roofing

Table 9.26.3.1 of the BC Building Code sets the minimum slope for asphalt shingles at 1 in 3 for normal application and 1 in 6 for the low-slope method. Below 1 in 6 the covering has to change: modified bituminous membrane is permitted from 1 in 50 up to a maximum of 1 in 4.

Eave protection is the clause worth knowing. Article 9.26.5.1 requires it to run from the edge of the roof at least 900 mm up the slope, reaching a line not less than 300 mm inside the inner face of the exterior wall. The same article then lists what is exempt — including roofs of asphalt shingles installed in accordance with Subsection 9.26.8, and regions with 3,500 or fewer degree-days. Appendix C lists both Parksville and Qualicum Beach at 3,200 degree-days. On a shingle roof here, two separate exemptions apply.

That is a floor, not a recommendation. A self-adhered membrane at the eaves and through the valleys costs little against the price of the roof, and on an exposed lot it is the cheapest protection against wind-driven rain there is. The question worth asking any quote is whether it includes one, and how far up the slope it runs.

BC Building Code 9.19.1.2 sets unobstructed vent area at not less than 1/300 of the insulated ceiling area, rising to 1/150 where the slope is under 1 in 6 or the roof is framed with roof joists. Distribution counts as much as the total: vents on opposite sides of the building, with at least 25 per cent of the area at the top of the space and 25 per cent at the bottom.

Roofs fail this quietly. A ridge vent gets added, the soffits stay packed with blown-in insulation, and the new high vent has no low intake to draw from. A shingle roof over a poorly vented attic runs hot in summer and damp in winter, and neither is kind to the deck underneath.

Vent area gets calculated against the insulated ceiling area on every quote, and blocked soffits get reported whether or not the fix is in scope.

Going back to the sheathing is not optional on a re-roof worth doing. Laying new shingles over an existing layer hides the deck, adds dead load, and turns every flashing into a patch rather than a detail.

With the old material off, the things that actually cause leaks become visible: split sheathing at a valley, step flashing rusted through at a wall junction, a plumbing stack boot that perished years ago, chimney counterflashing that was never let into the mortar. Anything found is photographed, priced and reported before it gets covered up.

Article 9.26.6.1 lists asphalt-saturated sheathing paper or No. 15 felt as acceptable underlay, and 9.26.6.2 sets head and end laps at not less than 50 mm, overlapping eave protection by at least 100 mm. Synthetic underlays are common now and get installed to the manufacturer's written instructions.

The leak almost never starts in the field of the roof. It starts at a penetration, a valley, a wall junction, a skylight curb or a change of plane. Those are the lines a quote should name one by one.

• Full tear-off and shingle replacement

  Old covering and fasteners stripped to the sheathing so the deck can be seen and assessed.

• Underlay and eave protection

  Underlay to 9.26.6.1, with self-adhered membrane at eaves and valleys where the exposure justifies it.

• Ridge and intake ventilation

  Vent area calculated against the insulated ceiling area and split between high and low openings.

• Valley and step flashing

  Open or closed valleys detailed to the roof geometry, and step flashing renewed at every wall junction.

• Drip edge and gutter apron

  Metal edge at eaves and rakes so water leaves the roof rather than tracking back under it.

• Plumbing stack and vent boot replacement

  Perished boots and collars replaced during the re-roof rather than left as a future leak.

• Sheathing and deck repair

  Split, rotted or delaminated sheathing replaced span by span, photographed and priced before it is covered.

• Removal and disposal of old material

  Everything hauled off site, gutters cleared and the property magnet-swept for fasteners.

The north slope is where shingle roofs on this coast age fastest. Second-growth fir and cedar hold moisture against the surface for most of the winter, and moss lifts the tab edges before it does anything visible to the granules. On a lot with heavy canopy the two slopes of the same roof can be in genuinely different condition, and it is worth pricing them as two decisions rather than one.

Situation

A 1980s rancher on an acreage lot near Errington, original asphalt shingles still shedding water on the south slope, the north slope carpeted in moss under second-growth fir.

Complication

The soffit vents had been packed solid with blown-in insulation during an attic top-up, so the ridge vent added later had no low intake to draw from.

Resolution

Baffles went in at every rafter bay to reopen the intake before the new roof went down, bringing unobstructed vent area back to the 1/300 the code sets and splitting it top and bottom. The shingle specification was identical on both slopes. The ventilation is what changed.