8.1 Roof Sheathing: Panel Grades, Spans, Edge Support & Nailing

Why the Deck Matters

The deck is the base of the whole roof. It carries loads to the framing, acts as a diaphragm against wind, and holds the fasteners. R905.2.1 says asphalt shingles shall be fastened to solidly sheathed decks. PSI's outline includes installing deck sheathing and identifying and correcting unacceptable framing or sheathing.

Materials

Wood Structural Panels (R803.2)

Plywood and OSB panels must meet DOC PS 1, DOC PS 2, or the listed CSA standards. They must bear a grade mark showing grade, bond classification (for example, Exposure 1), and performance category. Panels permanently exposed outdoors must be exterior-grade. Roof sheathing exposed only on its underside may be Exposure 1. Allowable spans come from Table R503.2.1.1(1).

Lumber Sheathing (R803.1)

Solid lumber sheathing must meet Table R803.1. For 24-inch rafter spacing, the minimum net thickness is 5/8 inch. Spaced lumber sheathing is permitted only for wood shingle and shake roofs (R905.7, R905.8), so a shingle roof over spaced boards needs new solid sheathing.

Many older Michigan homes have 1-by board decks. Before shingling over boards, check for rot, large gaps, knotholes, and cupping. Many shingle manufacturers limit board width and gap size, and require panel overlay where boards are too wide, too open, or too uneven. Follow their instructions and the solid-deck rule.

Reading the Span Rating

A panel stamped 32/16 may span up to 32 inches as roof sheathing and 16 inches as subflooring, with its long dimension across the supports and continuous over two or more spans. Key roof spans from Table R503.2.1.1(1):

The table's loads assume a 10 psf dead load. If dead load exceeds that, as with tile or heavy snow design, the live load must be reduced accordingly. That is one reason heavy coverings and northern Michigan loads call for thicker panels or closer framing. The rafters or trusses themselves must also be designed for the snow load (Section 7.1).

Edge Support

"With edge support" means one of these (table footnote):

• lumber blocking;
• panel edge clips, one midway between each support, or two equally spaced where the span is 48 inches;
• tongue-and-groove panel edges;
• another approved type of edge support.

Without edge support, an unsupported panel edge can deflect under a worker's foot and crack shingle seal strips.

Fastening (Table R602.3(1))

The table's wind footnote:

• Where the ultimate design wind speed is 130 mph or less, nails attaching roof sheathing to gable end wall framing are spaced 6 inches on center.
• Where the ultimate design wind speed is greater than 130 mph, nails at intermediate supports are spaced 6 inches on center within 48 inches of ridges, eaves, and gable end walls, and 4 inches at gable end walls.

Michigan's design table lists a 90 mph basic wind speed, and most of the state is well under the 130 mph ultimate threshold. The usual Michigan schedule is therefore 6 inches at edges and 12 inches in the field, plus 6 inches along gable-end framing. Table R602.3(2) lists alternate staples and nails.

Installation Practice

1. Orientation: long dimension (strength axis) perpendicular to rafters or trusses, continuous over at least two spans.
2. Joints: end joints bear on framing. Stagger end joints between courses, following APA and panel-maker instructions.
3. Spacing: APA recommends a gap of about 1/8 inch at panel ends and edges to allow for moisture expansion, unless the manufacturer says otherwise. Butted panels can buckle and telegraph ridges through shingles.
4. Nailing: drive nails flush, about 3/8 inch from panel edges, into the framing. Overdriven nails reduce holding power. Nails that miss framing ("shiners") should be pulled and re-driven.
5. Moisture: protect panels from prolonged wetting. OSB edges that swell may not recover.
6. Matching thickness: when replacing decking, match the existing thickness so there is no step that will show through the roofing or cut the underlayment.

Replacing Damaged Deck During Tear-Off

• Cut back rotted or delaminated sheathing to the centers of framing members so new and old panels both bear on framing.
• Use pieces large enough to span at least two framing bays where practical, with blocking under any unsupported edges.
• Check framing under the rot. Soft rafter tails and fascia often come with deck rot at the eaves.
• Record the quantity for the unit-price change order (Sections 3.3 and 7.5).

Deck Problems to Correct Before Roofing