Rafters

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Chapter
16
Roof Designs
Objectives
• Identify and sketch 12 different types of basic roof
designs.
• Draw a roof that has a typical roof slope (pitch).
• Describe the construction of a typical frame roof.
• Explain the importance of proper attic ventilation
and roof flashing.
• Compare the advantages and disadvantages of
new roofing materials.
• Explain the purpose of a roof plan.
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Types of Roofs
• The roof greatly affects the overall appearance of
a house
• Many standard styles
• Style should complement the basic design
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Gable Roof
• Triangular with a gable at both ends
• Easy to build and sheds water well
• Applicable to most house designs
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Winged Gable Roof
• Attractive design feature
• Gable roof, extended at the peak
• Requires lookout rafters
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Hip Roof
• Sloped roof section or hip at ends
• Slightly more difficult to build than gable roof
• Does not provide for ventilation as well as others
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Dutch Hip Roof
• Hip roof with a small gable at either end
• Steeper gables than hips
• Gables can be used for ventilation
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Flat Roof
• Most economical to build
• Requires “built-up” or membrane roof covering
• Popular in warm climates
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Shed Roof
• More pitch than flat roof
• Used for additions or with other roof styles
• Less than 3:12 pitch requires a built-up roof
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Gambrel Roof
• Also known as “barn roof”
• Two different slopes
• Provides additional headroom
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Mansard Roof
• French design
• Provides additional living space
• Difficult to build
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A-Frame Roof
• Provides roof and walls for structure
• Pitch allows for smaller second floor
• Popular for cottages, homes, churches
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Butterfly, Folded Plate, and Curved Panel
Roofs
• Limited residential construction
• Used in commercial construction
• Can cause drainage problems
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Permission granted to reproduce for educational use only.
Contemporary Roof Types
• Warped roof: Limitless in design
– Common shape is hyperbolic paraboloid
– Made from concrete, plywood, or plastic
– Expensive to build
• Free-form roof: Freedom with design
– Any shape can be formed with fabric and frame
– Strong and weather resistant
– Limited residential use
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Permission granted to reproduce for educational use only.
Green Architecture
• Living Roofs: Covered with soil, capable of
growing plants
• Benefits:
–
–
–
–
–
Used for growing food
Provides excellent insulation
Reduces storm water run-off
Creates habitat for wildlife
May lower air temperatures in large cities
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Traditional Frame
Roof Construction
• Before designing a roof:
– Consider the features of traditional frame roof
construction
– Understand the features impact on the design
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Permission granted to reproduce for educational use only.
Rafters
• Rafter: Structural element of the roof that supports
the sheathing
– Common rafters lay perpendicular to wall plate
– Rafters extend from roof’s ridge to plate or beyond
– Several types
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Rafters
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Rafters
• Rafters are cut to proper lengths by locating ridge
cut, seat cut, plumb cut, and tail cut
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Rafters
• Layout of rafter cuts are determined by:
– Roof slope (roof pitch)
– Clear span
• Additional factors:
– Rise
– Run
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Permission granted to reproduce for educational use only.
Rafters
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Rafters
• Roof slope (roof pitch) is slant of roof
• Shown on drawing as:
– Slope ratio diagram
– Fractional pitch
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Rafters
• Slope diagram
– Indicated by triangle
– Shows ratio between rise and run of roof
– Run is always expressed as 12 units
• Fractional pitch is rise/clear span
– Pitch = Rise/Clear Span
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Rafters
• Rafter sizes depend on:
– Spanned distance
– Rafter spacing
– Supported weight
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Rafters
• Rafters may serve as ceiling joists on low-sloped
roofs
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Cornice
• Cornice: Roof’s overhang at the eave line that
forms connection between roof and side walls
• Three types used in residential construction:
– Open cornice
– Box cornice
– Close cornice
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Open Cornice
• Used with exposed-beam construction,
contemporary, or rustic designs
• Rafter ends:
– Exposed
– Tapered or curved
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Box Cornice
• Space between the ends of projecting rafters and
wall is enclosed with a soffit board
• Three basic types:
– Narrow box
– Wide box with lookouts
– Wide box without lookouts
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Wide Box Cornice With Lookouts
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Close Cornice
• Rafter does not project beyond wall
• Roof is terminated by frieze board and molding
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Rake or Gable End
• Gable end (rake): Extension of gable roof beyond
exterior walls of house
– Proportions should be similar to cornice
– Special framing is used for wide overhang
– Close rake is least expensive to build
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Rake or Gable End
• Framing for gable end with wide overhang
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Roof Trusses
• Roof truss
– Assembly of members
that form rigid framework
that supports roof
• Information needed to
purchase a truss:
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–
–
–
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Roof span
Roof pitch
Truss spacing
Roof load
Permission granted to reproduce for educational use only.
Roof Trusses
• Adequate roof load is 40 pounds per sq. ft.
• Lightweight wood trusses can span distances of
20' to 32'
• Typical residential trusses are 2  4 or
2  6 lumber
• Gussets fasten members of wood truss together
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Permission granted to reproduce for educational use only.
Roof Trusses
• Prefabricated trusses
– Less expensive than conventional framing
– Available in standard widths and pitches
• Common wood trusses:
– W-type truss
– King-post or K-post truss
– Scissors truss
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W-Type Truss
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K-Post Truss
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Scissors Truss
• Scissors truss
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Raised Heel Truss
• When bottom chord of truss extends beyond
exterior wall
• Provides space for additional insulation
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Ventilation
• Attic ventilation is a necessity
– Reduces moisture condensation under sheathing
– Helps cool house interior
• Screened openings or ventilators provide attic
ventilation
– Ventilator openings should be at least 1/300th of
ceiling area
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Ventilation
• Types of ventilators
– Gable ends and underside of overhang
– Ridge and soffit ventilator
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Flashing
• Flashing repels water from areas of potential
leakage
– Used where roof contacts a wall, chimney, roof
valley, or at any roof penetration
• Placement is beneath surface material per
specifications
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Permission granted to reproduce for educational use only.
Flashing
• May be weather-resistant metal
– Aluminum, copper, galvanized sheet metal
• Roof valley flashing can be metal or two layers of
90# roll-type roofing
• Metal drip edge flashing
– Prevents water from entering behind shingles
– Protects fascia and rake boards
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Chimney Flashing
• Composed of shingle flashing and counterflashing
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Gutters and Downspouts
• Gutter
– Collects water from roof and directs it to downspout
that diverts it away from house
– Prevents water from running directly off eaves and
splattering house
– Pitched 1" to 1.5" per 20'
– Available in several styles and materials
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Permission granted to reproduce for educational use only.
Roof Sheathing and Roofing
• Roof sheathing supports roofing material
– Generally plywood or other approved panel product
– Thickness depends on rafter spacing
– 1/2" sheathing-grade plywood is used when rafters
are 16"OC
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Permission granted to reproduce for educational use only.
New Roofing Materials
• Asphalt laminate shingles
• Metal roofing
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Asphalt Laminate Shingles
• Thicker, heavier, and more wind resistant than
traditional asphalt shingles
• Appear three-dimensional
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Roofing Materials
• Metal roofing:
– Gaining wider acceptance for residential
construction
– Resists high winds
– Simulates other roofing materials
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Roof Plan
• Roof plan—Top view plan that shows information
needed to construct roof
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–
–
–
Shape
Roof pitch
Size and location of framing
Size and location of roof vents
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Permission granted to reproduce for educational use only.
Roof Plan
• Roof framing plan shows:
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–
–
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Exterior house walls
Exterior roof lines
Size and location of all roof framing members
Any additional information needed for roof
construction
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Permission granted to reproduce for educational use only.
Employability
• Computer ethics
– Unethical to use computer for personal activities
without permission
– Unethical to access confidential information,
download copyrighted material, or harass others
– Many organizations monitor computer users
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