Notched Rafter and a PV Array on a Roof Already Near Its Span Limit
A single-story 1990 house has 2x8 rafters at 24 in. o.c. spanning 14 ft 6 in. at 6:12, with 1/2 in. 3-ply CDX sheathing and a 20-year-old three-tab roof. The owner is adding a 400 sq ft PV array (about 3 lb per sq ft of array area) and wants a reroof at the same time. While measuring you find one rafter with a clean 2 in. deep notch cut at midspan for a now-abandoned flue, wavy shingle tabs in two bays, and several sheets that feel spongy with fasteners that spin. Published span tables show this 2x8/24 in. o.c. roof is already close to its limit for the existing dead and live loads. The solar contractor proposes lagging rail feet "anywhere you can hit solid wood."
1. Assess the notched rafter. What do you require before the roof is closed up?
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- A 2x8 is 7.25 in. deep; a 2 in. notch leaves 5.25 in., a 28 percent loss of depth at the worst possible location - midspan, where bending moment peaks. Section modulus varies with the square of depth, so that notch removes roughly 45–50 percent of the rafter's bending capacity at that point.
- Even a notch at a bearing end is limited to a small fraction of the rafter depth; a midspan notch of this size is not something you can cover over. And this rafter was already near its span limit before it was cut.
- Required action: engineer-designed repair - normally a full-length sister of equal or greater depth, nailed/bolted per the engineer's detail, or rafter replacement. A short scab or a plywood plate is not acceptable.
- Do not install sheathing, underlayment or shingles over it. Photograph it, notify the owner and the solar contractor in writing, and get a signed change order for the repair before proceeding.
- Flag the location to the solar designer so array layout and attachment points don't load that bay until it is repaired.
2. How should the PV rail layout and attachments be designed for this roof, and what must be verified first?
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- Rails run perpendicular to the rafters so the array load is distributed across many rafters rather than concentrated on two or three.
- Attachment points must land on rafters, never on sheathing alone, and each penetration gets flashed - a sealant-only surface mount will leak within a season.
- Use the manufacturer's specified lag or structural screw with the required embedment (commonly about 2.5 in. into the rafter), centered in the member and clear of the bottom edge; minimum three rafters engaged per rail line.
- Verify capacity first. These rafters are already near their table limit; the array adds dead load plus wind uplift and seismic demand, and the roof may be asked to carry it all at the notch location. Have the engineer confirm, or require blocking/added structure before any foot is installed.
- Reject the "anywhere you can hit solid wood" approach: it produces point loads at bay centers, misses rafters, and puts holes where nobody flashed them.
- Coordinate inspection: the PV attachment and flashing should be inspected before modules go on, and the roof warranty should not be voided by unflashed penetrations.
3. Can you re-nail and shingle over the existing sheathing, or does it need more work?
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- Spongy sheets, wavy tabs and fasteners that spin are symptoms of delamination or fastener withdrawal, not a cosmetic issue. 1/2 in. 3-ply CDX is thin, and 20 years of it under a failing roof does not improve.
- Minimum approach: walk the deck, replace every sheet that is delaminated, crushed or water-damaged, and re-fasten the field to the rafters with ring-shank or screw-shank nails per the sheathing schedule. Tear off the old shingles rather than layering.
- If the existing fasteners will not hold in the remaining sheets, the realistic options are a structural overlay (new sheathing fastened through to the rafters) or full sheathing replacement. Do not shingle over a deck that will not hold a nail.
- Treat the decking as an unknown in the contract: quote a per-sheet unit price for replacement and a per-sheet price for overlay so the job doesn't stall on a handshake when you find more bad deck than expected.
- Verify the local jurisdiction's re-roof sheathing and ventilation requirements before you bid; they may force more replacement than the owner expects.