The roofs are supposed to be of Baltic fir covered with Countess slates laid on inch boards; the maximum horizontal wind force is taken at 45 lbs. per foot super, acting only on one side of the roof at a time, equivalent to a normal wind pressure of 30 lbs. per square foot for a pitch of 30°, and 40 lbs. per square foot for a pitch of 45°.

The common rafters to be 1 ft. from centre to centre, but in sheltered positions they may be placed 1 ft. apart in the clear.

1 Tredgold's formulae for the scantlings of King and Queen Post Roofs are given in Part II.

Roofs Without Ceilings

Pitch up to 30°.

King

Post.

Trusses

10' centre to centre.

J Tie Beam.

Depth includes 3" for joints.

Principal Rafters.

King Post.

Struts.

Straining Beam.

§ Purlins.

10 ft. bearing.

Common Rafters.

20

3"

X

4 1/2"

3"

X

5"

3"

X

2 3/4"

3"

X

3"

-

5"

X

7 1/2"

2"

X

3 1/2"

22

3"

X

4 3/4"

3"

X

5 1/2"

3"

X

2 3/4"

3"

X

3 1/2"

-

5"

X

7 3/4"

2"

X

3 3/4"

24

3 1/2"

X

4 1/2"

3 1/2

X

5 1/4"

3 1/2"

X

2 3/4"

3 1/2"

X

3 1/2"

-

5"

X

8"

2"

X

4"

26

3 1/2"

X

4 3/4"

3 1/2"

X

5"

3 1/2"

X

2 3/4"

3 1/2

X

4"

-

5"

X

8 1/4"

2"

X

4 1/4"

28

4"

X

4 1/2"

4"

X

5 1/2"

4"

X

2 3/4

4"

X

4"

-

5"

X

8 1/2"

2"

X

4 1/2"

30

4"

X

4 3/4

4"

X

6"

4"

X

2 3/4"

4"

X

4 1/2"

-

5"

X

8 3/4

2"

X

4 3/4

Queen Posts.

Queen Post.

Trusses 10' centre to centre.

32

4 1/2"

X

4 1/2"

4 1/3"

X

4 1/2"

4 1/2"

X

2 3/4"

4 1/2"

x

2 1/2"

4 1/2"

x

5 1/2"

5"

X

7 1/2"

2"

x

3 1/2"

34

4 1/2"

x

4 3/4"

4 1/2"

X

5"

4 1/2"

X

2 3/4"

4 1/2"

X

2 3/4"

4 1/2"

X

6"

5"

X

7 3/4"

2"

X

3 3/4"

36

4 3/4"

X

4 3/4"

4 3"/

X

5"

4 3/4"

X

2 3/4"

4 3/4"

X

3"

4 3/4"

X

6 1/4"

5"

X

8"

2"

X

4"

38

4 3/4"

X

4 3/4"

4 3/4"

X

5 1/4"

4 3/4"

X

2 3/4

4 3/4"

X

3 1/4

4 3/4

X

6 3/4

5"

X

8"

2"

X

4"

40

4 3/4

X

5"

4 3/4

X

5 1/2"

4 3/4"

X

2 3/4

4 3/4"

X

3 1/4

4 3/4

X

7 1/4

5"

X

8 1/4

2"

X

4 1/4

42

5"

X

5"

5"

x

5 1/2"

5"

X

2 3/4

5"

X

3 1/4"

5"

X

7 1/4"

5"

X

8 1/4"

2"

X

4 1/2"

44

5"

X

5 1/4

5"

X

5 3/4"

5"

X

2 3/4

5"

X

3 1/2"

5"

X

8"

5"

X

8 1/2"

2"

X

4 3/4

46

5 1/4"

x

5 1/4"

5 1/4"

X

5 1/4"

5 1/4"

X

2 3/4"

5 1/4"

X

3 1/2"

5 1/4"

X

8 1/4"

5"

X

8 3/4"

2"

X

5"

Roofs With Ceilings

Pitch up to 30°.

Nature of Roof.

Span in feet.

Common Rafters.

* Collar.

Collar Beam.

8tol8.

Add 1/2" to the depths given in Table for Roofs without Ceilings.

Add |" to depths given in

Table above.

* In fixing the collar to the rafter, the , latter should not be cut into. As regards walls capable or not capable of resisting the thrust of roof see remarks, p. 157.

‡ Tie Beam.

Principal Rafters.

King Post.

Struts.

Straining Beam.

§ Purlins.

10 ft. bearing.

Common Rafters.

King Post.

Trusses 10' centre to centre.

20

4"

X

7"

4"

X

4"

4"

X

3"

4"

X

2 1/2"

_

5"

X

7 1/2"

2"

X

3 1/2"

22

4"

X

7 1/2"

4"

X

4 1/2"

4"

X

3"

4"

X

3"

-

5"

X

7 3/4"

2"

X

3 3/4"

24

4 1/4"

X

8"

4 1/4"

X

4 3/4"

4 1/4"

X

3"

4 1/4"

X

3"

-

5"

X

8"

2"

X

4"

26

4 1/2"

X

8 1/2"

4 1/2"

X

5"

4 1/2"

X

3"

4 1/2"

X

3"

-

5"

X

8 1/4"

2"

X

4 1/4"

28

4 1/2"

X

9"

4 1/2"

X

5 1/2"

4 1/2"

X

3"

4 1/2"

X

3 1/2"

-

5"

X

8 1/2"

2"

X

4 1/2"

30

4 3/4"

X

9 1/2"

4 3/4"

X

5 3/4"

4 3/4"

X

3"

4 3/4"

X

3 3/4"

-

5"

X

8 3/4"

2"

X

4 3/4"

Queen Posts.

Queen

Post.

Trusses 10' centre to centre.

32

4 3/4"

X

7 1/4"

4 3/4"

X

5 1/2"

4 3/4"

X

3"

4 3/4"

X

2 1/2"

4 3/4"

X

6 3/4"

5"

X

7 3/4"

2"

X

3 1/2"

34

4 3/4"

X

7 3/4"

4 3/4"

X

5 3/4"

4 3/4"

X

3"

4 3/4"

X

2 3/4"

4 3/4"

X

7 1/2"

5"

X

7 3/4"

2"

X

3 3/4"

36

4 3/4"

X

8 1/4"

4 3/4"

X

6 1/4"

4 3/4"

X

3"

4 3/4"

X

3"

4 3/4"

X

8 1/4"

5"

X

8"

2"

X

4"

33

5"

X

8 1/2"

5"

X

6"

5"

X

3"

5"

X

3"

5"

X

8 1/2"

5"

X

8"

2"

X

4"

40

5"

X

9"

5"

X

6 1/2"

5"

X

3 1/2"

5"

X

3 1/4"

5"

X

9"

5"

X

8 1/4"

2"

X

4 1/4"

42

5 1/2"

X

9"

5 1/2"

X

6 1/4"

5 1/2"

X

3 1/2"

5 1/2"

X

3"

5 1/2"

X

9"

5"

X

8 1/2"

2"

X

4 1/2"

44

5 1/2"

X

9 1/2"

5 1/2"

X

6 3/4"

5 1/2"

X

3 1/2"

5 1/2"

X

3 1/4"

5 1/2"

X

9 1/2"

5"

X

8 1/2"

2"

X

4 3/4"

46

5 1/2"

X

10"

5 1/2"

X

7 1/4"

5 1/2"

X

4"

5 1/2"

X

3 1/2"

5 1/2"

X

10"

5"

X

8 3/4"

2"

X

5"

For Roofs Of 45"Pitch

Add l"to the depth of common rafters, purlins, and struts, and 4" to the depth of the principal rafters, as given in above Tables.

§ If the purlins, instead of being placed immediately over the joints, are placed at intervals along the principal rafter, increase the depth of the latter, given in the tables, as follows: -

King-post roof

without ceiling

2"

with „

1 1/2"

Queen-post roof

without ceiling

1 1/4"

with „

1/2"

The purlins if placed 2 feet apart and with 10 feet bearing may be made 3" x 6".

The scantlings of the principal rafters, struts, and straining beam can be slightly modified by means of the following rough rule: " For every 1/4" deducted from the lesser dimension of the scantling, add 1/2" to the other dimension, and vice versa." For the tie beam, purlins and common rafters, so long as the depth is about double the breadth, 1/4" deducted from the breadth requires 1/4" to be added to the depth.

‡ The joint of the tie beam with the principal rafter should be placed immediately over the supporting wall. If this cannot be conveniently done, the depth of the tie beam should be increased one or two inches.

These Tables, compiled by Major H. C. Seddon, R.E., from calculations by Lieut. H. R. Sankey, R.E., have been followed in the construction of a large number of W. D. buildings.

Several points connected with roofs, such as the formation of Hips and Valleys, the trimming of rafters round chimneys, etc., must be reserved for Part II., as they do not fall within the limits of this course.