Rotten Roof Decking: How It Is Found and What Replacing It Costs

Victoria's pre-1950 homes were sheathed with boards, shiplap or spaced planks rather than plywood, and after a century the boards at the eaves and around the chimneys of Fairfield, James Bay and Fernwood homes are often soft where moss and gutter overflow kept them wet. The 1960s and 70s plywood roofs of Gordon Head, Royal Oak and Colwood have a different weak spot: the bottom metre at the eaves, where water backed up behind clogged gutters and no ice-and-water membrane existed, and the plywood has delaminated in layers.

Moss is the third route. On the shaded north eaves of Highlands, Metchosin and Broadmead roofs, a mat of moss left for years holds water against the shingles until it soaks through to the deck, and roofers on those lots budget for sheets. Because no one can see the deck until the shingles are off, the difference between two replacement quotes in Greater Victoria is often simply whether one of them included a decking allowance and the other left it as a surprise.

Decking, or sheathing, is the structural skin the shingles are nailed to: plywood or OSB on anything built since the 1960s, boards or shiplap on older homes. It rots when it stays wet: under a leak that was never found, under moss that held water for years, at eaves where gutters overflowed, and around chimneys and skylights where flashing failed slowly. Attic condensation on the underside adds to it over damp winters. Wet plywood delaminates into layers and loses its ability to hold a nail; wet boards go soft and crumble.

Rot is silent. The shingles above can look ordinary, which is why decking is the most common surprise in a Greater Victoria re-roof.

You do not need to be on the roof to gather most of the evidence. From the ground and the attic hatch, look for:

• Dark staining, white fungal growth or peeling plies on the underside of the sheathing, seen with a flashlight from the hatch.
• Daylight around chimneys, vents or at the eaves.
• A wavy or dipped slope between rafters when you sight along the roof from the street.
• A history of leaks or ceiling stains in the same area, or years of thick moss on that slope.
• Plywood edges visible at the eave that look swollen or layered.
• A roofer's report that the roof felt soft underfoot during an inspection.

Even a careful inspection sees only the underside from the attic and feels the top through the shingles; the full extent shows on tear-off day. Good quotes handle this honestly in one of two ways. Where the condition is unknown, our pricing model carries an allowance of $500 to $2,500 and a written per-sheet or per-square-foot price for anything beyond it. Where rot is already visible, the quote prices a share of the roof for replacement: light rot around 5% of the area, moderate around 12%, heavy around 25%. A quote with no decking line at all is the one to question, because the surprise arrives as a change order with the roof already open.

In our pricing model replaced sheathing is $7 to $10 per sq ft installed at re-roof. On a typical 2,000 sq ft two-storey home with about 1,200 sq ft of roof, light rot is a few hundred dollars, moderate rot roughly $1,000 to $1,450, and heavy rot across a quarter of the deck about $2,100 to $3,000 on top of the shingle price. A standalone localized repair, where the shingles are lifted and one area of deck replaced under an otherwise sound roof, is $500 to $2,000. Board-sheathed heritage roofs are sometimes overlaid with new plywood rather than patched board by board, which is priced by the contractor per case.