Home Roof Construction - Roof Framing Details

INTRODUCTION

• This e-article is about construction of a home roof, notably details of roof framing as home design standards

• The principal focus of this writing is hand-framing, or stick frame construction, of a home roof and only a few aspects relate to roof truss design and application to a home roof, a subject to be addressed hereunder subsequently

• This presentation is a natural follow-on the another hereunder – "Custom Home Design Program Series – Roof Plan Design Pictures & Text" at http://searchwarp.com/swa314068.htm  

Comment:  Home roof construction by hand-framing as opposed to truss-framing, has been and continues to be a decreasingly likely option in custom home design and construction, on two bases –

1. Master carpenters are fewer and fewer in number – a complaint across the fruited plain heard and observed by this custom home designer for years
2. Amidst the days of more and more instant gratification, truss-framed roof structures rule – sometimes over and done in hours and not days.

Though all-in cost differentials narrow, framing a home roof with trusses still appears to be about the time and not the money, unless corners of quality are cut deeply.  

ROOF FRAMING DETAILS

• Members shall be with lumber -  
  • Identified by the grade mark of a lumber grading or inspection bureau or agency approved by the American Lumber Standards Committee (a/k/a ALSC)
  • Group 2 or 3
  • Not less than #2 Grade
  • In decreasing order of preference of specie
    • Douglas Hemlock Fir-Larch
    • Southern Yellow Pine
    • Spruce-Pine-Fir
      • For engineered applications only and then
      • Only as last resort
  • Naturally continuous for a given member
  • Fully dimensioned
  • With members on application
    • Not less than 2 linear inches in nominal thickness in the field
      
• Between all rafters or top chords, whether hand-framed or trussed, respectively, there
  • Shall not be less than 2 linear inches x 4 linear inches sleepers
  • Set flush at top of face to rafter's or top chord's top of face
  • At not greater than 4 linear feet on center
    • At each decking seam 

Comment: This standard is intended to counteract roofing and roof sheathing sways, swales, and similar.   

Comment:  Speaking about cutting quality deeply, it is common for this custom home designer to see roofs framed 24 linear inches on center with OSB at ½ linear inch of so and sag, sag, sag.  This last guideline – about sistering blocks at roof deck joints – will help to offset sag, sag, sag. 

• Rafters shall
  • Be not less than 2 linear inches x 6 linear inches
  • On not greater than 16 linear inch centers
  • Except in unusual circumstances of short runs and light, loads safely borne otherwise

• Purlins and struts 
  • Shall not be less in cross-section dimension than the rafters which they support
  • Shall be continuous
    • Not jointed and
    • Not spliced
  • Shall transfer load only to appropriate bearing point, e.g., engineered beam, load-bearing partition posted below the bearing point, etc. 

Rafter Beam, or Rafter Tie, or Ridge Beam (not to be confused with an engineered ridge beam), and Collar Tie Latitudes, Section in Elevation

• Shall be set at not greater than 4 linear feet on center
• Struts
• Shall be preferably set on the vertical
• Shall be set at not less than 45° interior angle to the vertical
• Shall be set at not less than 60˚ interior angle to the vertical
• Shall be braced in lengths not greater than 5 linear feet
• When a supported roof is burdened, for example, as in anticipated high wind or snow load conditions, or as in a superior, crossing roofline – a canopy or overriding addition – then the main roof struts shall not be less than twice as many on not greater than half the spacing

• Purlins
  • Shall be set on a rafter at midspan or higher only and absolutely
  • Shall transfer load to sufficient bearing structure with a clear and consistent path to the foundation and thence to earth

• Collar ties shall be 
  • Shall be not less than 2 linear inches x 6 linear inches lumber
  • On not greater than 48 linear inches centers
  • Not greater than 1/3 of the rise from the ridge bottom of face to the collar tie bottom of face
  • Not applied in lieu of ceiling joists or rafter beams 

Comment:  Every carpenter knows this.  We wish. 

Skimping comes in the centers – skimpers stretch them out.  Skimping comes in measuring to the collar tie bottom of face – skimpers use the top of face.   

Skimping comes in not securely fastening collar tie butts to rafters.  This last skimp can be a heart-breaker and a frame-breaker.  (Of course, we're talking hand-framed construction, not ridge beam construction.)  Skimping of a sort continues in a condition more broadly misunderstood than we reckoned earlier-on; namely, a confusion that a collar tie can be applied in lieu of sufficient and necessary rafter beams or ceiling joists. 

• A fastening schedule for collar ties (and rafter ties or rafter beams) is suggested herewith –

Collar Tie or Rafter Tie to Rafter and Strongback to Rafter (Face-nailed). 3 nails: 3.50"x 0.162" (16d common). 4 nails: 3"x 0.120"; 3.25"x 0.120"; 3.25"x 0.128" (12d box); 3"x 0.131"; 3.25"x 0.131".  

• Rafter beams, or rafter ties, or tie beams
  • As distinct member or as ceiling joists
  • May be elevated over their rest on top plate top of face for a raised ceiling or for increased structural stability
    • If applied in lieu of a ceiling joist, the rafter beam shall be sized as a ceiling joist
    • By not greater than 1/3 rise as measured from top plate top of face to joist top of face 
      
      
• Ridges, valleys, and hips shall be full vertical, cut depth of fastened framing members 
  
  
• To vent closed valley and hip rafters, flush plumb cut bottom of face to jacks and ridges bottom of face
  
  
• A rafter's seat shall not override the top plate interior without a full-width and full-depth deadman block below 
  
  
• A hip or valley rafter shall seat as close as possible to the heel, and each application shall be independently considered for doubling or even tripling 
  
  
• Ridge support may include members on the vertical, as battens
  
  
• In low pitch roofs
  • Oversized rafters shall not be overcut
    • If cut at all, then
      • They shall be supported by top-bearing joist hangers and
      • Notched not greater than ¼ of rafter width 
        
        
• Roof frames at top chord or rafter tops of face –
  • Shall not be greater than 1/16 linear inch difference between any three on their shared planes
  • Shall be shimmed level for not greater than 1/16 linear inch difference between any three on their shared planes 
    
    
• Rafters
  • Shall be framed directly opposite of one another
    • At hip
    • At valley
    • At ridge
  • Shall be fully seated at end cuts
    
    
• Rafters and roof trusses
  • Shall each be aligned on the vertical to subordinate studs (a/k/a stack framing)
  • Shall each be secured to top plates with
    • Metal hold-down straps from truss or rafter, across both top plates and onto studs
      • Either as one strap or
      • As straps in overlapping combination
  • Toe-nailing hand-framed and truss-framed roof and ceiling structure to top plates shall be prohibited
  • Wall sheathing
    • Shall extend entirety of both top plates and
    • Shall be fastened
      • With not less than 10d (preferably ring-shanked) nails
      • At not greater than 4 linear inches on center to EACH top plate
      • Except at not greater than 3 linear inches on center in lateral-resistant applications
  • Shall be elevated at seats as required to provide not less than 2 linear inch unobstructed clearance between insulation top of face and roof deck bottom of face 
    
    
• Gable ends
  • Shall be braced
    • At not less than 45° interior angle on the vertical
    • At not greater than 60° interior angle on the vertical
    • At not greater than 8 linear feet on center
      • From interior ceiling or partition structure
      • Up to gable-end structure not less than half rise
        • With not less than 1-2 linear inches x 4 linear inches
    • Braced and posted at the lesser of mid-length and 8 feet on center or
      • Doubled or
      • Both
        
        
• Rafters
  • Shall be set on raised plates as needed whereby
    • Ceiling insulation – especially with thickness – shall run to walls
    • Without compression or other compromise to mass overall and
    • With not less than a 2 linear inch air gap assured
      • Overall,
        • Inter-rafter
        • From soffit
        • To ridge
        • As between insulation top of face and
        • Roof deck bottom of face
  • Baffle to block wind from insulation and assure air gap at roof deck
    
    
• No roofing beam or tie shall be amended in any way except for fastening
  • No notches
  • No bores
  • Butts full-face to rafter deck bottom of face
    
    
• Great care shall be taken in ceiling planes amended from single-plane, such that rafter ties and collar ties may need to be applied as or more frequently than guidelined above when ceiling structure parallel to roof structure lines is weakened to tension forces for discontinuous ceiling joists