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Roof Framing. A Quick Primer The National Association of Certified Home Inspectors www.NACHI.org. 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

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Roof Framing

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Roof framing l.jpg

Roof Framing

A Quick Primer

The National Association of Certified

Home Inspectors

www.NACHI.org

Copyright 2006 The National Association of Certified Home Inspectors


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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


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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


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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


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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


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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


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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


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Roof Framing

  • Determining rafter lengths – There are two distances needed for rafter layout

    • Ridge cut to birdsmouth

    • Ridge cut to tail cut

      Determining rafter length can be done using calculator, builder’s calculator, or look-up tables

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • To calculate the rafter length, the “rafter square” contains tables that are inscribed in the square. The carpenter can use this information to avoid the need to work with trigonometric functions. The square includes info for common rafters, hips, valleys and jacks.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • For example:

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Hips and Valley rafters can also be calculated and laid out using the rafter square, with some important differences;

    • The unit run is 17, not 12

    • The ridge, birdsmouth, and tail need cheek cuts, or some modification or the top surface needs to be beveled

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • The Hip (or Valley) rafter forms a diagonal on the roof, and the length of that diagonal is 17” for each 12” of run of the common rafters.

Copyright 2006 The National Association of Certified Home Inspectors


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The ridge cut is modified to fit into the space between the common rafters

Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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The Birds mouth must be dropped, or the top of the rafter beveled to account for the centerline being lower than the edges of the rafter

Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • The tail cuts will be beveled for solid nailing at the outside corner. For Hip rafters this is an outside corner, and for valley rafters this is an inside corner.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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Jack rafters have a cheek cut where they meet the hip or valley. Each one is shorter than the last by a “common difference”.

Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • The concept of “common difference” will also be applied when cutting gable end studs.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

Copyright 2006 The National Association of Certified Home Inspectors


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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


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Roof Framing

  • Where shed or gable dormers meet the main roof, the rafters must be cut to create either a valley or break.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Gable end overhangs – Both rafter and truss roofs commonly use gable end overhangs. However the overhangs are framed differently for trusses than for rafters.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • The vast majority of new construction uses trusses for the roof framing. Each truss is designed for the individual characteristics of the job, and delivered to the site ready to be erected. It is very rare that anyone site builds a truss today.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • There are a number of important points in building a truss roof:

    • Proper handling

    • Proper lifting and setting

    • Proper temporary bracing

    • Proper permanent bracing

    • These are explained in notes will be found on the paper that comes with the trusses

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Proper Handling –Trusses are made of small dimension lumber connected by metal plates. Side loading, heat, shock loading can damage metal plates and greatly weaken truss.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Proper lifting and setting – A truss erection plan will show the location of each numbered truss.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Proper temporary bracing – The most common cause of truss collapse is insufficient or improper temporary bracing. Temporary bracing stays in place until the roof is sheathed and the permanent bracing is installed.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • The result of not bracing trusses.

  • 47 MPH wind speed for a period of 1 minute.

Copyright 2006 The National Association of Certified Home Inspectors


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Roof Framing

  • Permanent Bracing – This is usually shown on the truss erection diagram. Compression members will buckle easily (and truss will not develop its design strength) if not properly braced. This can be done with continuous lateral or individual T bracing

Copyright 2006 The National Association of Certified Home Inspectors


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