The following table, from Tredgold's Carpentry, gives the scantlings (or sizes) of timbers for King-post roofs with ceilings. The trusses are supposed to be not more than 10 feet apart, the pitch of the roof about 1/4 or 27°, the covering slates, and the timber Baltic fir.

King post Roofs.   Table of Scantlings of Timber, recommended by Tredgold,1 for different spans, from 20 to 3 0 feet.

King-Post Roofs

Table of Scantlings of Timber, recommended by Tredgold,1 for different spans, from 20 to 3 0 feet.

Span.

Tie Beam, T.

King Post, KP.

Principal

Rafters,

PR.

Braces or

Struts,

S.

Purlins, P.

Common

Rafters,

CR.

20

feet.

9 1/2

by

4

4

by

3

4

by

4

3 1/2

by

2

8

by

4 3/4

3 1/2

by

2

22

,,

9 1/2

,,

5

5

,,

3

5

,,

3

3 3/4

,,

2 1/4

8 1/4

,,

5

3 3/4

,,

2

24

,,

10 1/2

,,

5

5

,,

3 1/2

5

,,

3 1/2

4

,,

2 1/2

8 1/2

,,

5

4

,,

2

26

,,

11 1/2

,,

5

5

,,

4

5

,,

4 1/4

4 1/4

,,

2 1/2

8 3/4

,,

5

4 1/4

,,

2

28

,,

11 1/2

,,

6

6

,,

4

6

,,

3 1/2

4 1/2

,,

2 3/4

8 3/4

,,

5 1/4

4 1/2

,,

2

30

,,

12 1/2

,,

6

6

,,

4 1/2

6

,,

4

4 3/4

,,

3

9

,,

5 1/2

4 3/4

,,

2

Queen-Post Roofs, Such As In Fig. 326

Tables of Scantlings of Timber for different Spans, from 32 to 46 feet.

Span.

Tie Beam, T.

Queen Post, QP.

Principal

Rafters,

PR.

Straining

Beam,

SB.

Struts, S.

Purlins, P.

Common

Rafters,

CR.

32

ft.

10

by

4 1/2

4 1/2

by

4

5

by

4 1/2

6 3/4

by

4 1/2

3 3/4

by

2 1/4

8

by

4 3/4

3 1/2

by

2

34

,,

10

,,

5

5

,,

3 1/2

5

,,

5

6 3/4

,,

5

4

,,

2 1/2

8 1/4

,,

5

4

,,

2

36

,,

10 1/2

,,

5

5

,,

4

5

,,

5 3/4

7

,,

5

4 1/4

,,

2 1/2

8 1/2

,,

5

4

,,

2

38

,,

10

,,

6

6

,,

3 3/4

6

,,

6

7 1/4

,,

6

4 1/2

,,

2 1/2

8 1/2

,,

5

4

,,

2

40

,,

11

,,

6

6

,,

4

6

,,

6

8

,,

6

4 1/2

,,

2 1/2

8 3/4

,,

5

4 1/4

,,

2

42

,,

11 1/2

,,

6

6

,,

4 1/2

6 1/4

,,

6

8 1/4

,,

6

4 1/2

,,

2 3/4

8 3/4

,,

5 1/4

4 1/2

,,

2

44

,,

12

,,

6

6

,,

5

6 1/2

,,

6

8 1/2

,,

6

4 1/2

,,

3

9

,,

5

4 3/4

,,

2

46

,,

12 1/2

,,

6

6

,,

5 1/2

7

,,

6

9

,,

6

4 3/4

,,

3

9

,,

5 1/2

5

,,

2

With regard to these tables it should be mentioned that the dimensions given are a safe guide, erring, if at all, on the side of excess of strength. The scantlings for the tie beams may be considerably reduced when there are no ceiling joists attached to them. In practice the width of the King and Queen Posts, Principal Rafters, and Struts is generally made the same as that of the tie beam.

Lighter Scantlings For Roofs

The following tables give scantlings for roofs which have been adopted in a large number of War Department buildings. They will be found to be much lighter and more economical than those given by Tredgold.