F
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Roof Framing
n
s
p
e
c
t
i
o
n
s
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
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Roof types
e
H
Gable – Most common, built with “common” rafters
o
m
Hip – Provides overhang on all four sides
e
I
Gambrel – Provides more space on second floor
n
s
p
Mansard – Combination of Hip and Gambrel
e
c
t
Shed- Frequently used to attach one structure to another
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

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Definitions
H
o
m
Common rafter – Runs
e
from top plate to
I
n
ridgeboard of a gable
s
p
roof
e
c
t
i
Hip rafter – Runs from
o
n
corner of top plates to
s
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 Geometry Definitions
o
m
Span – Measurement from
e
outside of wall to outside of
I
opposite wall
n
s
Run – One half of span (for
p
e
symmetric roofs)
c
t
Rise – The total vertical
i
o
distance that the roof projects
n
s
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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Overhang – The section
H
of the rafter extending
o
m
past the edge of the wall
e
I
Projection – The
n
s
horizontal distance that
p
e
the overhang covers
c
t
i
o
Rafter tail cuts – Cuts
n
s
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
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o
Rafters vs. Trusses
m
e
Rafters used frequently
I
for remodeling, for
n
s
cathedral ceilings, for
p
e
shed roof additions, for
c
t
full 2nd floor storage, and
i
o
n
spans up to 24’
s
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

F
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Roof Framing
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Sizing Rafters – Rafter
o
m
size (like span tables for
e
floor joists) depends on
I
n
s
spacing, species, load,
p
e
c
and span. Sizing of
t
i
o
rafters typically based
n
s
on snow load in
Northeast. The specific
loads come from the
International building
code
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Determining rafter
m
lengths – There are two
e
I
distances needed for
n
s
p
rafter layout
e
c
Ridge cut to birdsmouth
t
i
o
n
Ridge cut to tail cut
s
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
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To calculate the rafter
H
o
length, the “rafter
m
e
square” contains tables
I
n
that are inscribed in the
s
p
square. The carpenter
e
c
t
can use this information
i
o
n
to avoid the need to
s
work with trigonometric
functions. The square
includes info for
common rafters, hips,
valleys and jacks.
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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o
For example:
m
e
I
n
s
p
e
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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o
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I
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n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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o
Hips and Valley rafters
m
e
can also be calculated
I
n
and laid out using the
s
p
rafter square, with some
e
c
t
important differences;
i
o
n
s
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

F
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Roof Framing
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o
The Hip (or Valley)
m
e
rafter forms a
I
n
diagonal on the roof,
s
p
and the length of that
e
c
t
diagonal is 17” for
i
o
n
each 12” of run of the
s
common rafters.
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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The ridge cut is
m
e
modified to fit into the
I
n
space between the
s
p
common rafters
e
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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The Birds mouth must be
m
dropped, or the top of the
e
I
rafter beveled to account for
n
s
the centerline being lower
p
e
than the edges of the rafter
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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The tail cuts will be
m
e
beveled for solid nailing
I
n
at the outside corner.
s
p
For Hip rafters this is an
e
c
t
outside corner, and for
i
o
n
valley rafters this is an
s
inside corner.
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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o
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I
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n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
C
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H
o
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I
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s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Jack rafters have a
H
o
m
cheek cut where they
e
meet the hip or valley.
I
n
s
Each one is shorter than
p
e
the last by a “common
c
t
i
o
difference”.
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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The concept of
o
m
“common difference”
e
I
will also be applied
n
s
p
when cutting gable end
e
c
studs.
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Dormers – Most
H
o
dormers are either shed
m
e
or gable dormers. They
I
n
are framed with
s
p
common rafters.
e
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Where shed or gable
H
o
dormers meet the main
m
e
roof, the rafters must be
I
n
cut to create either a
s
p
valley or break.
e
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Gable end overhangs –
m
e
Both rafter and truss
I
n
roofs commonly use
s
p
gable end overhangs.
e
c
t
However the overhangs
i
o
n
are framed differently
s
for trusses than for
rafters.
Copyright 2006 The National Association of Certified Home Inspectors

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Roof Framing
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The vast majority of
H
o
new construction uses
m
e
trusses for the roof
I
n
framing. Each truss is
s
p
e
designed for the
c
t
i
individual
o
n
s
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
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There are a number of
o
m
important points in building
e
a truss roof:
I
n
s
p
Proper handling
e
c
Proper lifting and setting
t
i
o
n
Proper temporary bracing
s
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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Proper Handling –
m
e
Trusses are made of
I
n
small dimension lumber
s
p
connected by metal
e
c
t
plates. Side loading,
i
o
n
heat, shock loading can
s
damage metal plates and
greatly weaken truss.
Copyright 2006 The National Association of Certified Home Inspectors

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Roof Framing
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Proper lifting and
o
m
setting – A truss
e
erection plan will show
I
n
s
the location of each
p
e
c
numbered truss.
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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Proper temporary
m
e
bracing – The most
I
n
common cause of truss
s
p
collapse is insufficient
e
c
t
or improper temporary
i
o
n
bracing. Temporary
s
bracing stays in place
until the roof is sheathed
and the permanent
bracing is installed.
Copyright 2006 The National Association of Certified Home Inspectors

F
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Roof Framing
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The result of not
m
e
bracing trusses.
I
n
47 MPH wind speed for
s
p
e
a period of 1 minute.
c
t
i
o
n
s
Copyright 2006 The National Association of Certified Home Inspectors

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Roof Framing
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Permanent Bracing –
H
o
This is usually shown
m
on the truss erection
e
I
diagram. Compression
n
s
p
members will buckle
e
c
easily (and truss will not
t
i
o
develop its design
n
s
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