copyright 2006 the national association of certified home inspectors roof framing a quick primer the...

Copyright 2006 The National Association of Certified Home InspectorsCopyright 2006 The National Association of Certified Home Inspectors

Roof FramingRoof Framing

A Quick PrimerA Quick Primer

The National Association of CertifiedThe National Association of Certified Home InspectorsHome Inspectors

www.NACHI.orgwww.NACHI.org


Roof FramingRoof Framing Roof typesRoof types

Gable – Most common, built with “common” raftersGable – Most common, built with “common” rafters Hip – Provides overhang on all four sidesHip – Provides overhang on all four sides Gambrel – Provides more space on second floorGambrel – Provides more space on second floor Mansard – Combination of Hip and GambrelMansard – Combination of Hip and Gambrel Shed- Frequently used to attach one structure to anotherShed- Frequently used to attach one structure to another


DefinitionsDefinitions Common rafter – Runs Common rafter – Runs

from top plate to from top plate to ridgeboard of a gable ridgeboard of a gable roofroof

Hip rafter – Runs from Hip rafter – Runs from corner of top plates to corner of top plates to ridgeboard on a hip roofridgeboard on a hip roof

Jack rafter – any rafter Jack rafter – any rafter which does not run the which does not run the full length from plate to full length from plate to ridge ( e.g. – Hip jack, ridge ( e.g. – Hip jack, Valley jack)Valley jack)



Roof Geometry DefinitionsRoof Geometry Definitions Span – Measurement from Span – Measurement from

outside of wall to outside of outside of wall to outside of opposite wallopposite wall

Run – One half of span (for Run – One half of span (for symmetric roofs)symmetric roofs)

Rise – The total vertical Rise – The total vertical distance that the roof projects distance that the roof projects above the top plateabove the top plate

Slope – The rise divided by Slope – The rise divided by the run, always given in terms the run, always given in terms of 12” of run (e.g. 3 on 12 of 12” of run (e.g. 3 on 12 written 3/12)written 3/12)

Pitch – The rise over the span Pitch – The rise over the span



Overhang – The section Overhang – The section of the rafter extending of the rafter extending past the edge of the wallpast the edge of the wall

Projection – The Projection – The horizontal distance that horizontal distance that the overhang coversthe overhang covers

Rafter tail cuts – Cuts Rafter tail cuts – Cuts made to form the made to form the overhangoverhang

Birdsmouth – Cuts made Birdsmouth – Cuts made to sit on the top plateto sit on the top plate

Ridge cut – Cut made to Ridge cut – Cut made to attach to the ridgeboardattach to the ridgeboard



Rafters vs. TrussesRafters vs. Trusses Rafters used frequently Rafters used frequently

for remodeling, for for remodeling, for cathedral ceilings, for cathedral ceilings, for shed roof additions, for shed roof additions, for full 2full 2ndnd floor storage, and floor storage, and spans up to 24’spans up to 24’

Trusses used in most Trusses used in most new construction, for new construction, for spans 24’-60’, and most spans 24’-60’, and most commonly for lower commonly for lower sloped roofs sloped roofs



Sizing Rafters – Rafter Sizing Rafters – Rafter size (like span tables for size (like span tables for floor joists) depends on floor joists) depends on spacing, species, load, spacing, species, load, and span. Sizing of and span. Sizing of rafters typically based rafters typically based on snow load in on snow load in Northeast. The specific Northeast. The specific loads come from the loads come from the International building International building code code



Determining rafter Determining rafter lengths – There are two lengths – There are two distances needed for distances needed for rafter layoutrafter layout Ridge cut to birdsmouthRidge cut to birdsmouth Ridge cut to tail cutRidge cut to tail cut

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



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



For example: For example:



Hips and Valley rafters Hips and Valley rafters can also be calculated can also be calculated and laid out using the and laid out using the rafter square, with some rafter square, with some important differences;important differences; The unit run is 17, not 12The unit run is 17, not 12 The ridge, birdsmouth, The ridge, birdsmouth,

and tail need cheek cuts, and tail need cheek cuts, or some modification or or some modification or the top surface needs to the top surface needs to be beveledbe beveled



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



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



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



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



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



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



Dormers – Most Dormers – Most dormers are either shed dormers are either shed or gable dormers. They or gable dormers. They are framed with are framed with common rafters.common rafters.



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



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



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



There are a number of There are a number of important points in building important points in building a truss roof:a truss roof: Proper handlingProper handling Proper lifting and settingProper lifting and setting Proper temporary bracingProper temporary bracing Proper permanent bracingProper permanent bracing These are explained in notes These are explained in notes

will be found on the paper will be found on the paper that comes with the trussesthat comes with the trusses



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



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



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



The result of not The result of not bracing trusses.bracing trusses.

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



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


copyright 2006 the national association of certified home inspectors roof framing a quick primer the...

Documents

hip roof hip rafter

gable roof hip rafter

rafter square

gable roof common rafter

span roof framing slide

hip roof jack

rafter lengths

ridgeboard roof framing