Rafters

January 12, 2018 | Author: Anonymous | Category: Arts & Humanities, Architecture
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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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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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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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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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Permission granted to reproduce for educational use only.

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

© Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only.

Rafters

© Goodheart-Willcox Co., Inc.

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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

© Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only.

Rafters • Rafter sizes depend on: – Spanned distance – Rafter spacing – Supported weight

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Permission granted to reproduce for educational use only.

Rafters • Rafters may serve as ceiling joists on low-sloped roofs

© Goodheart-Willcox Co., Inc.

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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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Roof span Roof pitch Truss spacing Roof load

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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

© Goodheart-Willcox Co., Inc.

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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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Permission granted to reproduce for educational use only.

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

© Goodheart-Willcox Co., Inc.

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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Permission granted to reproduce for educational use only.

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

© Goodheart-Willcox Co., Inc.

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

© Goodheart-Willcox Co., Inc.

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

© Goodheart-Willcox Co., Inc.

Permission granted to reproduce for educational use only.

Roof Plan • Roof plan—Top view plan that shows information needed to construct roof – – – –

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: – – – –

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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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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