Roof Framing
A Quick Primer
The National Association of Certified
Home Inspectors
www.NACHI.org
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
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Roof types
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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
Roof Framing
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Definitions
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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
Roof Framing
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Roof Geometry Definitions
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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
Roof Framing
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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
Roof Framing
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Rafters vs. Trusses
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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
Roof Framing
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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
Roof Framing
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Determining rafter
lengths – There are two
distances needed for
rafter layout
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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
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
Roof Framing
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For example:
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing

Hips and Valley rafters
can also be calculated
and laid out using the
rafter square, with some
important differences;
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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
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
Roof Framing
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The ridge cut is
modified to fit into the
space between the
common rafters
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing

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
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
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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
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
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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”.
Copyright 2006 The National Association of Certified Home Inspectors
Roof Framing
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The 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 Framing
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Dormers – Most
dormers are either shed
or gable dormers. They
are framed with
common rafters.
Copyright 2006 The National Association of Certified Home Inspectors
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
Roof Framing
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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
Roof Framing
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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
Roof Framing
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There are a number of
important points in building
a truss roof:
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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
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
Roof Framing
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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
Roof Framing
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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
Roof Framing
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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
Roof Framing
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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