1. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Roof types
Gable – Most common, built with “common” rafters
Hip – Provides overhang on all four sides
Gambrel – Provides more space on second floor
Mansard – Combination of Hip and Gambrel
Shed- Frequently used to attach one structure to another
Copyright 2006 The National Association of Certified Home Inspectors

2. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Definitions
Common rafter – Runs from top plate to ridgeboard of a gable roof
Hip rafter – Runs from corner of top plates to ridgeboard on a hip roof
Jack rafter – any rafter which does not run the full length from plate to ridge ( e.g. – Hip jack, Valley jack)
Copyright 2006 The National Association of Certified Home Inspectors

3. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Roof Geometry Definitions
Span – Measurement from outside of wall to outside of opposite wall
Run – One half of span (for symmetric roofs)
Rise – The total vertical distance that the roof projects above the top plate
Slope – The rise divided by the run, always given in terms of 12” of run (e.g. 3 on 12 written 3/12)
Pitch – The rise over the span
Copyright 2006 The National Association of Certified Home Inspectors

4. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Overhang – The section of the rafter extending past the edge of the wall
Projection – The horizontal distance that the overhang covers
Rafter tail cuts – Cuts made to form the overhang
Birdsmouth – Cuts made to sit on the top plate
Ridge cut – Cut made to attach to the ridgeboard
Copyright 2006 The National Association of Certified Home Inspectors

5. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Rafters vs. Trusses
Rafters used frequently for remodeling, for cathedral ceilings, for shed roof additions, for full 2nd floor storage, and spans up to 24’
Trusses used in most new construction, for spans 24’-60’, and most commonly for lower sloped roofs
Copyright 2006 The National Association of Certified Home Inspectors

6. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Sizing Rafters – Rafter size (like span tables for floor joists) depends on spacing, species, load, and span. Sizing of rafters typically based on snow load in Northeast. The specific loads come from the International building code
Copyright 2006 The National Association of Certified Home Inspectors

7. Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Dormers – Most dormers are either shed or gable dormers. They are framed with common rafters.
Copyright 2006 The National Association of Certified Home Inspectors

8. Roof Trusses

9. Definition
Load
Transferred
Load not
Transferred
Load
Transferred

10. Truss Types
Trusses are categorised into 3 groups depending on the shape of the top chord
Triangular Roof Trusses
Crescent roof Trusses
Other Types

11. Triangular Roof Trusses
Simple Triangular geometric shape
Web Bracing
Straight Top Chord

12. Triangular Roof Trusses

13. Crescent Roof Trusses
Top Chord is manufactured with a curved top chord
The Harbour bridge is a good example

14. Other Types Top Chords may be parallel – such as floor joist trusses
Or they may be nearly parallel – such as bridges

16. Roof Truss Members

17. Roof Truss Stress Types

18. Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
Lighter
Larger Spans
Allow for easy access for services
Disadvantages
Cannot be site modified

19. Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
Lighter
Larger Spans
Allow for easy access for services
Disadvantages
Cannot be site modified
Generally Deeper

20. Truss Manufacture Designed by Structural Engineer
No Site Modification or repair without engineer supervision
Trusses manufactured in controlled factory conditions to ensure design is strictly followed

21. Camber Trusses are manufactured with camber in the bottom chord
To allow for calculated deflection while dead loads such as Roof Covering & Ceiling Linings
Bottom chord should not be supported between supports, unless specifically designed

22. Transfer of Loads
Click to show load flow on correctly installed trusses
Tensile Load to
Counteract Compressive Load
Compression Load
Internal Wall
Min 12 Clear
No load in this Area

23. Transfer of Loads
Click to show load flow on incorrectly installed trusses
Bottom Chord is not designed to take horizontal load and will fail
Bottom Chord bearing on Internal Wall
Load transmits Horizontally to wall.

24. Complex Roofs
Trusses can be made to suit just about any roof shape

25. Complex Roofs
Trusses can be made to suit just about any roof shape

26. Truss Layout

27. Trussed Hip Roofs
Jack End Truss

28. Hip Truss

29. Truncated Girder Truss
Placed at position determined by Engineer
Will take load of Jack Truss, Hip Truss & Creeper Truss
Designed to take more load than other trusses

30. Truncated Truss Placed between girder truss and gathering point
Important to be placed at specified spacing at they will increase in height to match the hip end
Made to similar specification to standard truss as they take no extra load