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Roof Structure

Structural Deterioration And Damage

Problem: Sagging Roof Sheathing.

Cause: Inadequate support for the roof sheathing.

Roof sheathing that has inadequate edge support can have many shortcomings. It can lead to deflection from wind loading or people walking on the roof and to the distortion or separation of roofing materials.

h-clips for roof sheathing

Figure 17a - Roof sheathing H-clips

Solutions: Install plywood roof sheathing with the face grain perpendicular to the rafters or trusses. Support and secure panel edges adequately.

  • Install strandboard, waferboard and plywood with the face grain at right angles to the roof framing.
  • When using panel-type sheathing of less than 1/2 inch (12 mm) on rafters or trusses at 24 inches (600 mm) o.c., support panel edges between rafters or trusses with H-clips (Figure 17a) installed at the midpoint between rafters or trusses or with 2 inch X 2 inch (38 mm X 38 mm) blocking nailed between rafters or trusses as shown in Figure 17.

Correct Support Of Roof Sheathing

Figure 17 - Correct Support Of Roof Sheathing

Note 1: Allow a 1/8 inch (3mm) gap.

Note 2: Nail edges of sheathing at 6 inches (150 mm) o.c.

Note 3: Nail center portion of sheathing at 12 inches (300 mm) o.c.

  • Fasten all sheathing panels along rafters or trusses at 12 inches (300 mm) o.c., and edge fasten them at 6 inches (150 mm) o.c. where they are edge supported.
  • Boards used as roof sheathing cannot be more than 11 inches (286 mm) wide. Support all lumber ends on solid framing and stagger end joints.

Additional information on roofs and roofing