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Roof Structure

Structural Deterioration And Damage

Problem: Sagging Roof Sheathing.

Cause: Sheathing materials too thin to support concrete or clay tile roofing.

Most building codes for panel type sheathing require a thickness of 3/8 inch (9.5 mm) when supported spacing is 24 inches (600 mm). This thickness will support design dead and live loads but may allow deflections which cause a poor appearance.

Solutions: Reduce spacing of trusses and rafters.

  • To reduce deflection of roof sheathing, you may decrease the spacing of roof supports from 24 inches (600 mm) to 16 inches ( 400 mm) o.c.

Solutions: Increase thickness of roof sheathing.

  • You may also reduce deflection by increasing the sheathing thickness above minimum building code requirements as shown in Table 1.

Table 1 - Minimum Thickness Of Roof Sheathing In Inches (mm)

Maximum Spacing of Supports In Inches (mm)

Plywood & OSB

Grade 0-2

OSB, Grade 0-1

Waferboard Grade R-1

Lumber

Edges Supported

Edges Unsupported

Edges Supported

Edges Unsupported

12 (300)

16 (400)

24 (600)

5/16 (7.5)

5/16 (7.5)

3/8 (9.5)

5/16 (7.5)

3/8 (9.5)

1/2 (12.7)

3/8 (9.5)

3/8 (9.5)

7/16 (11.1)

3/8 (9.5)

7/16 (11.1)

1/2 (12.7)

11/16 (17)

11/16 (17)

3/4 (19)

 

Additional information on roofs and roofing

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