ROOF

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ROOF
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Roof Construction
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Roof Framing

• After the roof design is selected, the next
  decision is the type of roof construction--
  trusses or stick built.
• Trusses.
• Less labor to install trusses than to stick build
  roof.
• Factory built
• Better quality control
• Reduced construction cost
• Hauled to site and lifted into place
• Computers allow complex designs.
• Stick built
• Higher labor costs for complex roofs
• More variability in quality
• High level of skill required to produce complex
  rafters
• Requires more scaffolding and other supports for
  construction

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Roof--Truss

• Different types of trusses can be used for roofs.
• The type used will be determined by the use of
  the building, size of the building and/or the
  owners preferences.
• Most trusses are custom built for the building.
• Trusses can be wood or metal
• Truss nomenclature

Rafters
Gussets
Webs
Bottom Cord
Posts
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Types of Roof Trusses
Different types of roof trusses are
available. Truss manufacturers custom build
trusses for each building
Mono (single slope) Sheds attached to
buildings Spans 20 to 30 ft
Scissors Higher center clearance Spans 20 to 40 ft
Fink Poplar efficient design Spans 20 to 50 ft
Wowe Heavier ceiling loads than fink truss Spans
20 to 50 ft
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Types of Roof Trusses--cont
Pratt Used with or without ceilings Spans 20 to
60 ft
Belgian Extended fink truss Spans up to 80 ft
Bowstring Difficult to construct Spans 40 to 120
ft
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Roof Pitch
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Roof Framing-cont.Pitch

• Whether using roof trusses or stick building the
  roof, the pitch of the roof must be selected.
• Pitch is the slope of the roof.
• Roof pitch is indicated by a fraction(1/3, 1/4,
  Etc.) or slope triangle.
• When a fraction is used, it is the rise over the
  span.
• When a slope triangle is used, it is rise over
  run.

What factors influence the best roof pitch for a
building?
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Roof Framing-cont.Pitch Problem

• Determine the rise for the rafter in the
  illustration for a 1/3 pitch.

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Roof Framing-cont.Slope Triangle

• A triangle (slope triangle) is also used to
  indicate roof slope.
• A slope triangle indicates the rise/run.

• The slope triangle in the illustration indicates
  that for every 12 inches of run there will be 3
  inches of rise.
• Because a ratio is used, the 3 and 12 can have
  any units as long as they are both the same.
  Three (3) feet twelve (12) feet would have the
  same rafter slope as 3 inches and 12 inches.
• The use of the slope triangle reinforces the
  concept the a rafter is the hypotenuse of a right
  triangle.

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Roof Framing-cont.Slope Triangle Problem

• Determine the rise for the rafter in the
  illustration.

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Stick Build Rafters
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Stick Built RafterTerms

• A common rafter is the hypotenuse of a right
  triangle.
• The plumb cut is made so that the ends of two
  rafters will fit together.
• They are fitted flush when used in a rafter
  truss.
• A ridge board is used when the rafters are stick
  built.

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Stick Built Rafter Terms-cont.

• The tail cut can be left several ways.
• Common types are

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Stick Built Rafter Terms-cont.

• The birds mouth is used to increase the contact
  area between the rafter and the top plate.
• Must not extend more than 1/2 way through the
  dimension of the board.
• Excessive depth can cause the rafter to split.

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Stick Built Rafter Steps

• Four (4) steps in laying out a common rafter.
• Mark the angle at the ridge board end (plumb cut)
  of the rafter.
• Determine the rafter length.
• Mark the location and size of the birds mouth.
• Mark the angle of the rafter at the overhang.

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Stick Built Rafter Step 1-Plumb cut

• The angle at the end of a rafter is determined by
  the pitch.
• The angle can be calculated in degrees and laid
  out with a protractor.
• Because a rafter is the hypotenuse of a right
  triangle, a framing square

or a speed square can be used.
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Stick Built Rafter Steps.Step One - Plumb Cut
cont.

• Mark the plumb cut on the rafter.
• Place the 12 inch mark on the body of the square
  on one edge of the board.
• Rotate the square until the inches of rise is on
  the tongue of the square and on the same edge of
  the board.
• Mark along the edge of the tongue.

In this example the rise per foot of run is 8
inches.
Remember the same edge of the square must be on
the same edge of the board.
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Stick Built Rafter Steps.Square Use
Why are these two illustrations examples of
incorrect square use?
The square marks are on two different edges of
the board.
Opposite sides of the square are used.
Does this illustration show correct use?
Yes because both edges of the square are on the
same edge of the board.
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Stick Built Rafter Steps.Step Two - Determine
the length

• The second step is determining the length of the
  rafter.
• The length is the distance from the peak of the
  roof to the outside edge of the top plate.
• One half the thickness of the ridge board must be
  deducted when it is used.
• The rafter length can be determined by
  calculation or by stepping.
• Determining rafter length by calculation
• Pythagorean Theorem

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Stick Built Rafter Steps.Example of Length by
Calculation

• Determine the total length of a common rafter for
  a building with a span of 12 ft. 6 in. and a 1/3
  pitch.
• The building will use a 2 x 6 ridge board and a 6
  in. overhang.

Answer 8 ft
Rafter rise
Rafter length
Subtracting 1/2 of ridge
Overhang rise
Length of overhang
Total length
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Stepping A Rafter
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Rafter Length by Stepping

• The stepping process uses the rise and run.
• Works best with a span that is an even foot.

Step a rafter for a building with a 128 slope
and a 10 foot span.

• A building has eight (8) inches of rise per foot
  of run and the run is five (5) feet. It will use
  a 12 inch overhang.
• The square is aligned like making a plumb cut.
  In this case 12 and 8 are used.
• A line is drawn for the plumb cut and the 12 inch
  mark on the square is marked on the board.

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Rafter Length by Stepping - Second Step

• The square is stepped along the board for each
  foot of run.

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Rafter Length by Stepping - Third Step
Process is continued until the width of the run
is stepped off.
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Rafter Length by Stepping - Fourth Step
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Rafter Length by Stepping - Fifth Step
At this point the length of the rafter will be
correct for a building with a run of five (5)
feet.
If an overhang is used, the additional length
must be stepped off for the overhang.
If a ridge board is used, the rafter length must
be adjusted.
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Stick Built Rafter Steps.Step Six - Birds Mouth
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Stick Built Rafter Steps.Tail Cut

• If the rafter did not have an overhang, it would
  be cut off plumb with the back of the birds mouth.

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Stick Built Rafter Step Seven - Overhang

• Laying out the overhang requires two (2) steps.
• In the first step the 12 inch distance of the
  overhang is marked.

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Stick Built Rafter Step Seven - Overhang-cont.

• The second step is to mark the end cut.
• In this example it is a plumb cut.
• The same ratio is used.
• If a square cut is desired, a line square with
  the edge would be marked.

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Questions