Frame Gable Roofs 5
Length Of A Common Rafter
The line length of a
can be calculated by using the
Pythagorean theorem or the rafter table on the framing square which is
based on this theorem. Figure 36 shows the
relationship between unit rise, unit run and unit line length. The
'Length Of Com. Per Unit Run (250) is 291.5.
It is the unit line length found on the rafter table.
The span is the total horizontal building width at the time of roof
framing. The run is the total horizontal distance the rafter travels
from the outside plate line to the centre of the ridge. The run is
one-half the span.
The length of a common rafter is measured between
the centre of the ridge board and the outside edge of the top plate. If
the span is 4000 mm then the the run is 2000 mm. The number of units of
run is 2000 divided by 250 equal to( = ) 8. The length of the common
rafter is 8 times 291.5 equal to 2332 mm.
Of A Common Rafter