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Roof FramingA Quick Primer
The National Association of Certified Home Inspectorswww.NACHI.org
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingRoof typesGable Most common, built with common raftersHip Provides overhang on all four sidesGambrel Provides more space on second floorMansard Combination of Hip and GambrelShed- Frequently used to attach one structure to another
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingDefinitionsCommon rafter Runs from top plate to ridgeboard of a gable roofHip rafter Runs from corner of top plates to ridgeboard on a hip roofJack 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
Roof FramingRoof Geometry DefinitionsSpan Measurement from outside of wall to outside of opposite wallRun One half of span (for symmetric roofs)Rise The total vertical distance that the roof projects above the top plateSlope 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
Roof FramingOverhang The section of the rafter extending past the edge of the wallProjection The horizontal distance that the overhang coversRafter tail cuts Cuts made to form the overhangBirdsmouth Cuts made to sit on the top plateRidge cut Cut made to attach to the ridgeboard
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingRafters vs. TrussesRafters used frequently for remodeling, for cathedral ceilings, for shed roof additions, for full 2nd floor storage, and spans up to 24Trusses 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
Roof FramingSizing 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
Roof FramingDetermining rafter lengths There are two distances needed for rafter layoutRidge cut to birdsmouthRidge cut to tail cutDetermining rafter length can be done using calculator, builders calculator, or look-up tables
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingTo 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
Roof FramingFor example:
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingHips and Valley rafters can also be calculated and laid out using the rafter square, with some important differences;The unit run is 17, not 12The 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
Roof FramingThe 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
Roof FramingThe ridge cut is modified to fit into the space between the common rafters
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingThe 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
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingThe 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
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingJack rafters have a cheek cut where they meet the hip or valley. Each one is shorter than the last by a common difference.
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingThe concept of common difference will also be applied when cutting gable end studs.
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingDormers Most dormers are either shed or gable dormers. They are framed with common rafters.
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingWhere 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
Roof FramingGable 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
Roof FramingThe 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
Roof FramingThere are a number of important points in building a truss roof:Proper handlingProper lifting and settingProper temporary bracingProper permanent bracingThese are explained in notes will be found on the paper that comes with the trusses
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingProper 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
Roof FramingProper lifting and setting A truss erection plan will show the location of each numbered truss.
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingProper 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
Roof FramingThe result of not bracing trusses.47 MPH wind speed for a period of 1 minute.
Copyright 2006 The National Association of Certified Home Inspectors
Roof FramingPermanent 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