How to Replace Rotten Roof Sheathing: Cutting Out, Sizing and Fastening New Panels

Step by Step

1. 1Step 1: Mark the damage and the rafter lines

   Mark the area to be removed in crayon, including a margin of sound wood. Locate the rafters from the old nail lines on adjacent panels and from the attic, then snap chalk lines over the rafter centres so your cut ends on the middle of a framing member. A rafter at 16 inches on centre is about 1-1/2 inches wide, so the cut must be accurate enough to leave about 3/4 inch on each side for the two panels to land.

   Decide on the new panel size. Whole-sheet replacement is stronger and easier to fasten than a patch, but cutting back to the nearest rafters on each side is fine.

2. 2Step 2: Set the saw depth and cut out the bad panel

   Set the blade depth to the thickness of the sheathing plus a hair. Cutting deeper nicks the rafters and can cut into the roofing or ceiling material below. Make cuts along the chalk lines, finishing corners with a reciprocating saw. Pry the piece out with a flat bar, working gradually so that adjacent panels are not damaged.

   Pull all nails out of the rafters. Clean off the framing with a scraper. Look at the rafters themselves. If the top edge is soft or rotten, repair or sister them before installing the new panel.

   Watch out: Do not cut over an area you cannot see the underside of. Know what is below the cut line, such as wiring, pipes or bath fans.

3. 3Step 3: Add blocking where the edges are not supported

   When you cut along a line that falls between rafters, the edge has nothing under it and will flex. Fit blocking of 2 by 4 between the rafters along the cut, flush with the top of the rafter, and nail through the rafters with two nails at each end. Alternatively, use H-clips on the new panel's unsupported edge.

   For thinner sheathing, the code and the panel manufacturer have rules about edge support. If the roof has tongue-and-groove panels, the joint between panels supports the edge. Standard panels are not tongue-and-groove, so clips or blocking are needed.

4. 4Step 4: Choose the right panel thickness and type

   Roof sheathing is sold as plywood (CSP, Douglas fir or CSA O121 and O151 grades) or OSB (CSA O325 or O437). Typical new roofs use 7/16 inch OSB or 1/2 inch plywood on rafters at 24 inches on centre, with 3/8 inch plywood or 7/16 OSB seen at 16 inches. Those are above the BC Building Code minimums, which for plywood and O-2 OSB are 9.5 mm (3/8 inch) at 24 inch spacing with the edges supported by H-clips or blocking, 12.5 mm (1/2 inch) with unsupported edges, and 9.5 mm at 16 inch spacing without edge support. Thicker panels such as 5/8 inch are used for long spans, heavy snow and some roofing types, including low-slope roofing, where 1/2 inch plywood at 16 inch and 5/8 inch at 24 inch spacing is a common rule. Confirm for your roof in BC Building Code Part 9 and with the shingle maker.

   Plywood holds nails well and dries faster, and it is less sensitive to wet edges. OSB is cheaper and common. Both are acceptable for shingles when rated for roof use. Whichever you choose, match the thickness of the neighbouring panels so the deck is flat, because a thin patch creates a dip and a thick patch creates a ridge.

   Tip: If the old deck is 3/8 inch and you fit 1/2 inch, shim the thinner area or replace the whole row. Level beats speed.

5. 5Step 5: Lay the panel across the rafters with staggered joints

   Set the panel's long dimension perpendicular to the rafters, with the face grain or strength axis across the supports. End joints must land on the centre of a rafter, and adjacent rows should be staggered so the joints do not line up. Stagger at least one rafter space between rows.

   Hold the panel tight to the old deck, then back it off slightly for the gap in the next step.

6. 6Step 6: Leave the expansion gap and set H-clips

   Wood panels swell and shrink with moisture. Panels butted tight buckle upward in humid weather, telegraphing through shingles as ridges. A gap of 1/8 inch at all edges and ends gives them room. A 16 penny nail is a fast spacer. Panels are usually manufactured about 1/8 inch under size to help.

   H-clips are small metal clips that slip over the edge of two panels between rafters, supporting the joint and keeping the gap. Use one per rafter span along unsupported long edges, and for 24 inch spacing, one between rafters. Use the clip size matching your panel thickness.

7. 7Step 7: Nail to the schedule

   Use 8d (2-1/2 inch) ring-shank or galvanized nails at 6 inches on centre at supported panel edges, and 12 inches on centre into intermediate rafters. The BC Building Code's basic schedule is 150 mm (6 inches) at the edges and 300 mm (12 inches) at intermediate supports with nails at least 51 mm (2 inches) long, and tighter schedules apply in high wind and seismic locations, so confirm it for your location and the thickness of the panel. Keep nails at least 3/8 inch from panel edges.

   Drive nails flush. Overdriven nails crush the surface and lose holding power, and underdriven nails stand proud and punch through underlayment. A pneumatic nailer with depth control makes this easier.

8. 8Step 8: Check the deck is flat and continuous

   A lip at a joint shows through shingles as a line. Plane down high edges, or shim low areas, or pull and reset a panel that is off. Check the new panels do not bounce. If they do, add blocking or fasteners. Then the deck is ready for ice and water shield and underlayment.

Plywood or OSB

Fraser Valley Notes

Common Mistakes

• Butting panels tight. With no gap, panels swell and buckle, and the ridge shows through the shingles.
• Using the wrong thickness. A thin patch dips, a thick one ridges, and either one is visible under the new roof.
• Ending cuts between rafters with no support. Unsupported edges flex and crack shingles and underlayment.
• Overdriving or underdriving nails. Crushed surfaces hold poorly and proud nail heads puncture the underlayment.
• Lining up end joints in adjacent rows. It makes a weak line across the deck.
• Patching over rotten rafters. New sheathing nailed to soft framing will not hold.