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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 4B
Roof Beams (Rafters) For Plank-&-Beam Construction
16' - 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 less
Stress Grade

           

f = 900

2/3 X 10
6 X 10

8 X 10
3/2 X 12

4/2 X 10
3/2 X 12

 

6 X 12
2/4 X 12

 

f = 1,200

3/2 X 10
3 X 12

6 X 10
2/3 X 10

3/2 X 10
4 X 12

4/2 X 10
3/2 X 12

3/2 X 12

6 X 12

f = 1,500

2/2 X 10
4 X 10

2/2 X 12
3/2 X 10

2/2 X 12

6 X 10
2/3 X 10

4 X 12

3/2 X 12

For L.L. of 20 - 30 psf
Stress Grade

           

f = 900

2/4 X 10
3/2 X 12

4/2 X 12

4/2 X 12
2/4 X 12

5/2 X 12

2/3 X 14
2/4 X 12

10 X 12
4/3 X 12

f = 1,200

4 X 12
6 X 10

2/3 X 12
3/2 X 12

4/2 X 10
3/2 X 12

4/2 X 12
2/4 X 12

8 X 12
6 X 12

8 X 12
2/4 X 12

f = 1,500

3/2 X 10
2/2 X 12

6 X 10
4/2 X 10

2/3 X 10
4 X 12
6 X 10

2/3 X 12
3/2 X 12
8 X 10

3/2 X 13

2/3 X 12
6 X 12
4/2 X 12

 

Live Load (L.L.)

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

Hemlock
eastern

no. 1 select structural

1,300

prime structural

1,200

common structural

1,100

utility structural

950

Hemlock
west coast

select structural

1,600

construction

1,500

structural

1,200

Continued......

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