Gable, Gambrel, and Shed Roofs

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Unit 47

• Gable, Gambrel, and Shed Roofs

Gable Roofs Gambrel Roofs Shed Roofs Dormers
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• A gable roof slopes in only two directions.

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• Framework for a gable roof includes ridge board,
  common rafters, and gable studs.

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• A birds mouth is formed by the heel plumb cut
  line and seat line.

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• When calculating true rise when there is a roof
  overhang, the total rise and stand are first
  added. The drop at the ridge is then subtracted.

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• The rafter table on a framing square can be used
  to calculate rafter lengths for pitched roofs
  with 2² to 18² unit rises.

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• Common rafter length is calculated by multiplying
  the total run by the length of the common rafter
  per foot of run.

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• The actual length of a common rafter is
  calculated by deducting one-half the ridge
  thickness perpendicular to the plumb line.

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• A framing square is used to lay out the plumb and
  seat cuts.

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• A framing square or a Speed Square can be used
  to lay out a common rafter for a gable roof. The
  roof in this example has an 8² unit rise and a
  10² overhang. The same procedure is used
  regardless of the layout tool being used.

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• The step-off method combines the procedure of
  laying out the rafters with a procedure of
  stepping off the length of the rafter. In this
  example, the roof has an 8² unit rise and a total
  run of 5-9².

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• When constructing a gable roof, common rafters
  are precut and then fastened to the ridge board
  and wall plates in a continuous operation.

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• When ceiling joists are spaced 16² OC and rafters
  are spaced 24² OC, every other rafter will tie
  into the joists. Proper layout ensures that
  rafters and joists will tie into each other
  wherever possible.

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• Ridge board layout is copied from markings on
  wall plates. Butt joints for ridge boards must
  occur at the center of a rafter.

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• When only a few carpenters are present on a job
  site to set the ridge board, the ridge board is
  secured in position with props and braces.

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• When several carpenters are present to raise a
  gable roof, the method shown is used to set the
  ridge board and nail rafters for a gable roof.

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• Many carpenters prefer to cut the overhang after
  rafters are fastened to the ridge board and wall
  plates. A chalk line is snapped along the rafters
  and a sliding T-bevel is used to lay out the tail
  plumb lines.

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• A gable end overhang can be framed with lookouts
  and a fascia rafter. The fascia rafter is nailed
  to the ridge board and fascia board. Blocking
  rests on the end wall and is nailed between the
  fascia rafter and the rafter next to it. An
  overhang is further strengthened when the roof
  sheathing is nailed to it.

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• A gable end overhang can be framed with a fascia
  rafter supported by the ridge board, outrigger,
  and fascia board. Two common rafters are placed
  directly over the gable ends of the building. The
  fascia rafters are installed between the ridge
  board and the fascia boards. The gable studs are
  notched to fit against the rafter above.

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• Gable studs decrease in length from the ridge
  section toward the walls.

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• The lengths of gable studs can be calculated with
  a framing square.

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• A framing square can be used to find the angle of
  the top cut on gable studs. Combine the unit rise
  and 12² and mark the cut along the unit rise side.

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• Gable studs provide gable wall structure and
  support rafters along their bottom edges.

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• A gambrel roof is essentially a gable roof with a
  double slope on each side. Note the walls
  supporting the meeting points of the upper and
  lower rafters. The length of the rafters for the
  lower portion of the roof is based on a 20² unit
  rise and a 4-6² run. The length of the rafters
  for the upper section is based on a 4² unit rise
  and an 8-0² run.

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• Common rafters for the upper and lower portion of
  a gambrel roof are laid out differently. The cuts
  laid out in this illustration are based on the
  unit rises and runs shown in Figure 47-23.

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• When constructing a gambrel roof, the lower
  portion of the roof is constructed first. If the
  attic space is to be used for a living area, the
  subfloor should be installed before the roof is
  constructed.

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• A shed roof has common rafters with overhangs at
  each end.

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• When laying out common rafters for a shed roof,
  the length of common rafters is based on the unit
  rise and total run of the roof.

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• The construction of a gable dormer is similar to
  that of a gable intersecting roof. A gable dormer
  has a level ridge board.

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• The front wall of a shed dormer is usually
  directly over the exterior wall below. The
  rafters extend from the main ridge board. The
  rafters must be pitched enough to shed water and
  snow.