Before The Architect - Rafter Roof Planning, Rafter Roof Framing

Roof Framing

BEFORE THE ARCHITECT – FINE HOME DESIGNING BACKGROUND – UNIQUE HOME DESIGNING ARTICLES

RAFTER ROOF PLANNING, RAFTER ROOF FRAMING

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INTRODUCTION

This e-article is about planning framing a rafter roof, notably details of roof framing as dream home design standards

The principal focus of this writing is hand framing, or stick frame building, of a home rafter roof and only a few aspects relate to truss roof, it's design and application to a house roof, a subject to be addressed hereunder subsequently

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

Master carpenters are fewer and fewer in number – a complaint across the fruited plain heard and observed by this custom dream home designer for years
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.

RAFTER ROOF FRAMING DETAILS

Rafter roof 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.

Roof 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.

Shall be set at not greater than 4 linear feet on center

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

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 not less than 2 linear inches x 6 linear inches lumber

Shall be on not greater than 48 linear inches centers

Shall be on not greater than 1/3 of the rise from the ridge bottom of face to the collar tie bottom of face

Shall not be applied in lieu of ceiling joists or rafter beams

Comment: Every carpenter knows this. We wish.

Comment: 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 building, not ridge beam building.) 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 interiors 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 slope 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

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

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

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

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