The precise layout pf these cuts is determined by the slope or Pith of the roof
and the width or Span of the building. Terms, which must be understood before calculating
rafter dimensions and roof pith, are: RISE, RUN, and SPAN. The rise of the roof
is the vertical distance measured from the top of the wall plate to the intersection
of the centerline of the rafters. The run of the roof is one-half of the spans.
Span is the horizontal distance form the outside of one wall stud to the outside
of the opposite stud wall. Figure 1-3 graphically illustrates these terms.
Figure 1-4 represents
the ratio between the rise and run of the roof.
The roof slope shown as a fractional pitch denotes the relationship between rise and the span
larger image Figure 1-5
Roof slope may be given on a drawing by showing a slope ratio diagram or fractional
pitch indication. The slope diagram, Fig 1-4 represents the ratio between the rise
and run of the roof. The fractional pitch is calculated using the following formula.
Several roof pitches used in roof construction.
- Roof Pitches
Roof slope may be also
be shown using an angular dimension. However, this method is seldom used because
it is difficult to measure as accurately as either of the other methods. A roof
with a 45 degree slope has a 12:12 or ½ pitch. There are several common
roof pitches used in residential home construction. When designing a roof for your
new home, try to use on of the standard roof pitches.
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