roofframing

Information about roofframing

Published on January 11, 2008

Author: Tirone

Source: authorstream.com

Content

Roof Framing:  Roof Framing A Quick Primer The National Association of Certified Home Inspectors www.NACHI.org Roof Framing:  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 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) Roof Framing 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 Roof Framing 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 Roof Framing 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 Roof Framing 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 Roof Framing 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 Roof Framing 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. Roof Framing Roof Framing:  Roof Framing Roof Framing:  Roof Framing 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 Roof Framing 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. Roof Framing Roof Framing:  The ridge cut is modified to fit into the space between the common rafters Roof Framing Roof Framing:  Roof Framing 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. Roof Framing Roof Framing:  Roof Framing Roof Framing:  Roof Framing Roof Framing:  Roof Framing Roof Framing:  The concept of “common difference” will also be applied when cutting gable end studs. Roof Framing Roof Framing:  Roof Framing Roof Framing:  Dormers – Most dormers are either shed or gable dormers. They are framed with common rafters. Roof Framing Roof Framing:  Roof Framing 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. Roof Framing 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. Roof Framing 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 Roof Framing 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. Roof Framing Roof Framing:  Proper lifting and setting – A truss erection plan will show the location of each numbered truss. Roof Framing 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. Roof Framing Roof Framing:  The result of not bracing trusses. 47 MPH wind speed for a period of 1 minute. Roof Framing 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 Roof Framing

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