wood header (lintle) for garage door

[razoray]

Here I have ridge 2x12 with 2x12 rafters which are not at center line of my garage. For header at eave end do I figure roof load from rafter length or from center line which in this case is longer. Note each rafter has a 2x6 collar beam attached.

 

[Trussdoc]

A 2 x 12 ridge board serves only as a nailer, giving minimal support to the roof system. So the correct way to figure the load is touse the full rafter length as the tributary area.

 

[boo1]

Trussdoc is correct, but may be from you question you need more help!

Tributary Area:
The tributary area is found by the 1/2 each side (or just the spacing). The span of a rafter is not based on the measurement along its length. Rather, the span is based on the rafter's "horizontal projection". This is the horizontal distance from the inside surface of the supporting wall to the inside surface of the ridge board. So consider a simple gable roof on a 24-foot wide ranch framed with 2x4 exterior walls and a 1 1/2 ridge: the span would be 11'3 3/4".

Establish live load, dead load and deflection limits; use the appropriate rafter table to determine acceptable Fb and E values; and then select the appropriate species, size and grade from AF&PA's Design Values for Joists and Rafters publication.

Loading
This is multiplied by the loading(dead load (D)+ live load (L)or different combinations)the code specifies various combinations of loads (also known as load cases), that should be evaluated to determine which cases are critical to the structure. The '97 UBC combinations of loads are a significant change from previous editions with an emphasis given to strength design and a move toward ASCE 7 provisions. Load combinations for allowable stress design are specified in '97 UBC 1612.3.1. They are:

D
D + L + (Lr or S)
D + (W or E/1.4) .9D ± E / 1.4
D + .75 [L + (Lr or S) + (W or E/1.4)]

Then use the book; AF&PA's Span Tables for Joists and Rafters (this assigns allowable spans to various combinations of E and Fb); and a copy of Design Values for Joists and Rafters (this has Fb and E values for various species, sizes and grades of dimension lumber).

Compression Perpendicular to the Grain:
The ends of these members must be able to "react" or resist these loads without crushing. AF&PA lists the required compression perpendicular to grain values for joists and rafters for various spans, on-center spacing and loading conditions in its Span Tables for Joists and Rafters. AF&PA's Design Values for Joists and Rafters lists compression perpendicular to grain design values for a variety of species. Just be sure the species design value exceeds the required compression perpendicular to grain value for your structural condition.

 

[boo1]

see:

http://www.awc.org/technical/spantables/tutorial.htm