Roof Decking Replacement Cost

Decking failures in Greater Victoria follow moss and shade more than rain volume. A north-facing slope under Douglas fir in Prospect Lake, the Highlands or west Saanich stays damp from October to May, and moss roots lift the shingles enough for water to sit on the plywood beneath. Roofers here expect to replace some sheets on any roof that has carried heavy moss for a decade, and they expect the worst at the eaves, in valleys and around chimneys where water concentrates.

Older homes add a different decking problem. Pre-1950 houses in James Bay, Fairfield, Fernwood and Oak Bay were built with skip sheathing, spaced boards meant for cedar shingles, or with solid 1-by-8 shiplap. Asphalt cannot go on skip sheathing, so those roofs need a full plywood overlay at tear-off; shiplap can be patched board by board. A quote on a 1912 Fairfield home without a decking line has not thought about what is under the shingles.

Decking, or sheathing, is the plywood, OSB or boards nailed to the rafters that the shingles are fastened to. Shingles hide it completely; the only ways to assess it before tear-off are from the attic, by looking for staining, delamination and daylight, and by walking the roof and feeling for soft spots, which is the roofer's job, not yours. Because it cannot be seen, honest quotes handle it one of two ways: an allowance of $500 to $2,500 that is billed only if used, or a per-sheet price, typically stated for a 4-by-8 sheet, applied to whatever is found.

The two acceptable forms are a stated allowance with a per-sheet rate for anything beyond it, or a per-sheet rate alone with a note that you will be shown the rotten sheets before replacement. A quote with no decking line is not cheaper; it is incomplete, and the cost arrives as a change order on day one. Ask for photographs of every sheet replaced. In our model $7 to $10 per sq ft of sheathing works out to roughly $225 to $320 per sheet installed, which is the range to check a per-sheet price against.

• Allowance stated and refundable if unused
• Per-sheet or per-sq-ft rate for extra
• Photos of removed sheets before disposal
• Plywood thickness matched to rafter spacing

From the ground, a wavy or dipped roof plane between rafters, moss that has been thick for years, or shingles that look sunk along the eave may indicate soft decking. From the attic hatch, dark staining on the underside of the plywood, delaminated layers, or light at the eaves are consistent with moisture damage. None of these is a diagnosis; they are areas a professional may inspect. A long-standing leak at a chimney or valley almost always has some rot around it.

When the roof is otherwise sound, a localized patch, cutting out the rotten section, sistering in new plywood and re-shingling over it, is $500 to $2,000 in our repair menu. It makes sense on a roof under about 15 years old with one bad spot from a known leak. On an older roof with several soft areas the patches add up quickly and replacement with a decking allowance is usually the better spend; the repair-or-replace tool runs the comparison.