# How to Design a Roof – Part 6

Tables 4A & 4B Instructions:

Tables 4A & 4B is designed to simplify the selection of beams or rafters to be used in flat-roof framing (post-and-beam) or for pitched-beam roofs, as rafters. Figures 14, 15, 16 and 17. add further explanation.

Span:

Considered to be from support to support. If the roof is flat, it is the horizontal distance between supports. If the roof is pitched, it is the actual length of the member between supports.

Spacing:

Either 6’ 0" or 8’ 0" is the dimension between the beams (rafters). It is the span of the planks.

 Table 4BRoof Beams (Rafters) For Plank-&-Beam Construction16' - 18' - 20' Rafter Spans 16 feet 18 feet 20 feet Plank Spacing 6' 0" 8' 0" 6' 0" 8' 0" 6' 0" 8' 0" For L.L. of 20 psf or lessStress Grade f = 900 2/3 X 106 X 10 8 X 103/2 X 12 4/2 X 103/2 X 12 6 X 122/4 X 12 f = 1,200 3/2 X 103 X 12 6 X 102/3 X 10 3/2 X 104 X 12 4/2 X 103/2 X 12 3/2 X 12 6 X 12 f = 1,500 2/2 X 104 X 10 2/2 X 123/2 X 10 2/2 X 12 6 X 102/3 X 10 4 X 12 3/2 X 12 For L.L. of 20 - 30 psfStress Grade f = 900 2/4 X 103/2 X 12 4/2 X 12 4/2 X 122/4 X 12 5/2 X 12 2/3 X 142/4 X 12 10 X 124/3 X 12 f = 1,200 4 X 126 X 10 2/3 X 123/2 X 12 4/2 X 103/2 X 12 4/2 X 122/4 X 12 8 X 126 X 12 8 X 122/4 X 12 f = 1,500 3/2 X 102/2 X 12 6 X 104/2 X 10 2/3 X 104 X 126 X 10 2/3 X 123/2 X 128 X 10 3/2 X 13 2/3 X 126 X 124/2 X 12

Either 20 psf or less, or between 20 and 30 psf should cover most code requirements.

f:

The fiber stress in bending for the particular wood designed for.

Note: 2/3 X 6 indicates a built-up member using two 3 X 6 members.

 f - Fiber Stess For Particular Wood Species Species Grade f Douglas fir no. 1 select 1,900 no. 2 construction 1,500 no. 3 standard 1,200 Hemlockeastern no. 1 select structural 1,300 prime structural 1,200 common structural 1,100 utility structural 950 Hemlockwest coast select structural 1,600 construction 1,500 structural 1,200

Continued......

 Part 1 Part 2 Part 3 Part 4 Part 5 Part 7 Part 8