Roof Replacement in Duncan

Every existing layer comes off down to the sheathing. That is the only way to see the deck, and on Cowichan Valley homes there is usually something to find — a soft patch below a valley, a rusted-out flashing at a chimney, or plywood delaminating along a north-facing slope that never dries out. Damaged sheathing is replaced before anything new goes down.

From there the system is rebuilt in order: ice-and-water membrane at eaves, valleys and every penetration, synthetic underlayment across the field, new metal flashing and drip edge rather than reusing old, then the shingle courses. Ridge and intake venting are installed or corrected as part of the same job. The old roof is loaded out and the site is magnet-rolled for nails before the crew leaves.

Roofing over an existing layer is cheaper on the day and more expensive over the life of the roof. It hides deck condition entirely, traps heat against the new shingles, and makes the next tear-off a two-layer job.

BC Building Code Section 9.19.1.2 sets the unobstructed vent area for a roof space at not less than 1/300 of the insulated ceiling area, tightening to 1/150 where the slope is under 1 in 6 or the roof is built with joists. A large share of older Cowichan Valley homes fall short of that, usually because intake venting at the soffits was blocked by insulation or never installed to begin with.

The consequence in this climate is specific. Warm moist air collects at the underside of the sheathing, condenses through the wet months, and the deck stays damp under a roof that looks fine from the street. Shingle life drops, plywood delaminates, and mould appears in the attic. Balanced intake at the eaves and exhaust at the ridge is a small line item during a re-roof and an expensive retrofit afterwards.

Roof size and pitch come first — steeper roofs need staging and slow the work. After that: how many existing layers have to come off, how much sheathing needs replacing once the roof is open, the number of penetrations and valleys to flash, access for a disposal bin, and whether venting has to be built rather than replaced. Two Duncan houses with the same footprint can sit $6,000 apart on those variables alone, which is why a figure quoted over the phone is not worth much. Full ranges and what drives each one are set out in the roofing cost guide.