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12

Chapter

Roof Framing

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Objectives

  • List and describe the various types of roofs.
  • Identify the parts of a roof frame.
  • Define the terms slope and pitch.
  • Use a framing square, speed square, and rafter tables.
  • Lay out common rafters.

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Objectives (Cont.)

  • Describe the layout and erection of a gable roof.
  • Find the length of a hip rafter.
  • Explain the design and erection of trusses.
  • Describe the procedure for sheathing a roof.
  • Estimate roofing materials.

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Roof Frames

  • Provide base to which roofing materials will be attached
  • Must be strong and rigid
  • Can add distinctive and decorative character to structure
  • Both live loads and dead loads must be considered in roof design

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Roof Types

  • Gable roof
  • Hip roof
  • Gambrel roof
  • Flat roof
  • Shed roof
  • Mansard roof

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Framing Systems

  • Two basic systems used in framing roofs
    • Conventional, stick-built rafters
    • Truss rafters
  • Rafters are framing members that run from peak to wall plates
  • Roof trusses are prefabricated assemblies

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Roof Supports

  • Supports depend on type of rafter design
    • Outside walls
    • Ceiling joists (beams that hold ceiling materials)
    • Interior bearing walls (partitions that support structures above)

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Parts of Roof Frame

  • Common rafters
  • Hip rafters
  • Valley rafters
  • Hip jacks
  • Valley jacks
  • Cripple jacks

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Roof Frame in Plan View

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Layout Terms and Principles

  • When two sides of right triangle are known, third side can be calculated: H2 = A2 + B2
  • Rafter is like hypotenuse of right triangle

​

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Slope and Pitch

  • Slope refers to incline of roof
  • Slope is given as “x inches-in-12”
  • Pitch is technically ratio of rise to span (twice run)
  • 4-in-12 roof has pitch of 1/6 (4/24)
  • Pitch is also used to mean same as slope
  • It is simpler to call out rise and run: 5-in-12

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Unit Measurements

  • Framing square is basic layout tool for roof framing
  • Divisions called units are used to make rafter layout
  • Stair gauges can be attached to square to save slope on square

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Framing Plans

  • Roof framing information found in plans
    • Rise and run of roof
    • Required overhang
    • Subfascia and type of fascia
    • Type of ridge
    • Other information for roof construction

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Rafter Sizes

  • Use building codes to select size of rafters

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Code Note

  • IRC includes rafter span tables for
    • Roof live load of 20 psf or ground snow load of 30 psf, 50 psf, or 70 psf
    • Ceiling not attached to or attached to rafters
    • Dead load of 10 psf or 20 psf
    • Rafter spacing of 12″, 16″, 19.2″, or 24″
    • Wood species and grade
    • Nominal rafter size: 2 × 4, 2 × 6, 2 × 8, 2 × 10, �or 2 × 12

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Laying Out Common Rafters

  • Rafters can be laid out using
    • Step-off method
    • Rafter table on framing square
    • Construction calculator
  • First lay out, check, and cut pattern rafter
  • Use pattern to mark other rafters of same size and type

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Step-off Method

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Rafter Table

  • Table is on face side of square
  • Find line length of rafter from center of ridge to outside of plate
  • Add overhang and subtract half thickness of ridge board from length

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Speed Square

  • Used to find rafter lengths and determine angle of cuts
  • Is smaller and easier to carry than framing square
  • Larger version is called rafter square

Macklanburg-Duncan

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Erecting Gable Roofs

  • Lay out rafter spacing
  • Attach ridge to several rafters on one side
  • Raise and support assembly while rafters are nailed to plate
  • Install rafters on opposite side

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Gable End Frame

  • Should be assembled after rafters and ridge have been installed
  • Consists of vertical studs running from top plate of bearing walls to end rafters
  • Should be well braced before installing ridge board and rafters

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Hip and Valley Rafters

  • Hip rafters support intersection of two roof surfaces slanting upward
  • Use common rafters to calculate length of hip rafter, including overhang
  • Valley rafters used where two pitched roofs meet to form inside corner

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Hip Jack Rafters

  • Short rafters that run between wall plate and hip rafter
  • Same as common rafters except for length
  • Consistent change in length is common difference

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Valley Jacks

  • Start layout at building line and move toward ridge
  • Use common rafter pattern
  • Extend plumb cut of bird’s mouth to top edge

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Erecting Jack Rafters

  • Assemble cut rafters into roof frame
  • Erect rafters in opposing pairs
  • Straighten any bowed rafters
  • Fasten fascia board to rafter or truss tails

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Special Problems

  • To support ridge of narrow roof section, valley rafter is run to main ridge
  • Valley cripple jack is rafter framed between two valley rafters
  • Hip-valley cripple jacks run between hips and valleys

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Roof Openings

  • May be required for chimneys, skylights, and other structures
  • First complete entire framework
  • Then, lay out and frame openings

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Roof Anchorage

  • Outward thrust along top plate must be considered in framing design
  • End walls need extra support, especially for hip roofs
  • Stub ceiling joists and metal straps are used to reinforce roof

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Collar Ties

  • Tie together two rafters on opposite sides of roof
  • Provide bracing and stiffening to hold ridge and rafters together
  • Purlins may be used to provide extra support

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Dormers

  • Framed structures projecting above sloping roof surface
    • Shed dormers
    • Gable dormers
  • Normally contain vertical window units

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Framing Flat Roofs

  • Ceiling and roof members may be combined system
  • Most designs require overhang
  • Pueblo style
    • Flat roof supported by vigas
    • May have parapet walls

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Gambrel Roof

  • Like gable roof but has four slopes
  • Often used in two-story construction
  • Gives added living space with minimal outside wall framing

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Mansard Roof

  • Has two slopes with lower slope being steeper
  • Extends around all four sides of building
  • Second floor joists extend beyond first floor wall
  • Extension provides support for lower rafters

N. Mitchell/Shutterstock.com

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Special Framing

  • Low-sloping roofs usually need support at several points

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Roof Trusses

  • Frames that carry roof and ceiling surfaces
  • Rest on exterior walls and span width of structure
    • W truss or Fink truss
    • King post truss
    • Scissors truss

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Roof Truss Construction

  • In trusses, joint slippage must be at minimum
  • Special connectors hold joints firmly

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Bracing Truss Rafters

  • Proper bracing for trusses is essential
    • Temporary bracing during construction
    • Inside permanent bracing
  • Clip keeps truss in alignment and allows vertical movement of lower truss chord

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Roof Sheathing

  • Provides nailing base for roof covering
  • Adds strength and rigidity to roof frame
  • Made from plywood or OSB
  • Skip sheathing should be used for wood shingles, metal sheets, or metal tile

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Structural Panels

  • Ideal for roof sheathing
  • Can be rapidly installed
  • Hold nails well
  • Resist swelling and shrinkage
  • May be slid up ladders or placed on special racks

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Installing Sheathing

  • Start at eaves and work up toward ridge
  • Use temporary fasteners at corners to keep panels square
  • Lay down rows of panels from edge to edge of roof
  • Stagger end joints between rows of panels
  • Trim extra overhang at ends of gable roof

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Panel Clips

  • Used to strengthen roof sheathing panels between rafters
  • Eliminate need for blocking on long truss or rafter spans
  • Two clips should be used where supports are 48″ O.C.

The Panel Clip Co.

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Safety Note

  • Work from scaffold to install fascia and lay first row of roof sheathing
  • Never place tools, materials, or debris where they can slide off roof
  • Follow OSHA rules that require fall protection, guardrails on scaffolding, and hard hats

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Estimating Materials

  • Number of rafters required for plain gable roof
    • Multiply length of building by 3/4 for spacing 16″ O.C.
    • Round up and add one more
    • Double figure to include other side of roof
  • For complex roof frames, work from complete framing plan

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Model and Small-Scale Construction

  • Working with models requires much time
  • Often best to construct only part of building
  • Small buildings, such as storage sheds, can be sold to recover cost of materials
  • Scale of 1 1/2″ = 1′-0″ usually makes it possible to apply regular framing procedures

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