Design

The traditional or 'cut' roof as it has become known is essentially a roof cut and assembled on site from individual timber members. It is most frequently a common rafter and purlin roof, the design of which can be prepared from readily available standard span tables for the individual timber members. Hips and valleys are generally constructed to what has become known as 'good practice' and are less well documented with span tables and specific design aids. The sizing of these members is often left to the architect or engineer and it is not always necessary to provide calculations to prove their adequacy. The design of all new roof structures in England, Wales and Inner London must of course conform with the latest edition of the Building Regulations. In Scotland the Building Standards (Scotland) regulations apply, and in Northern Ireland the Building Regulations (Northern Ireland).

The Building Regulations 2000 set out the statutory standards of construction to be achieved, and whilst the 1991 edition of the regulations contained approved documents, the new ones do not. For guidance on roof design, the Building Regulations refer to a TRADA publication entitled Span Tables for Solid Timber Members in Floors, Ceilings & Roofs (Excluding Trussed Rafter Roofs) For Dwellings. It is their publication reference TRADA technology design aid DA 1/2004. This document covers the design of all members likely to be encountered in a simple roof construction and is an extremely complete guide, giving the background to the designs, listing all current standards relevant to roof member design and to timber grading. A list of the span tables contained in the publication is set out in Table 3.1, and is a reproduction of Table 5 from that document. It is strongly recommended that anyone who is in any way involved with the study, design, alteration, conversion and repair, or the approval of roof structures should have a copy for reference. Full details of TRADA can be found in the bibliography.

Table 3.1 Span tables.

Construction

Timber members

Imposed loading not exceeding kN/m2

Strength classes

C16

C24

Floors

Joists

1.50

Table 6

a i e

7

Ceilings

Joists

0.25

Table 8

Table

9

Binders

0.25

Table 10

Table

11

Pitched roofs with slope of 15° or

Rafters

0.75

Table 12

Table

13

more but less than 22.5°

Purlins

0.75

Table 14

Table

15

Rafters

1.00

Table 16

Table

17

Purlins

1.00

Table 18

Table

19

Pitched roofs with slope of 22.5°

Rafters

0.75

Table 20

Table

21

or more but less than 30°

Purlins

0.75

Table 22

Table

23

Rafters

1.00

Table 24

Table

25

Purlins

1.00

Table 26

Table

27

Pitched roofs with slope of 30° to

Rafters

0.75

Table 28

Ta l e

29

45°

Purlins

0.75

Table 30

Ta l e

31

Rafters

1.00

Table 32

Table

33

Purlins

1.00

Table 34

Table

35

Flat roofs: access for maintenance

Joists

0.75

Table 36

Table

37

only

1.00

Table 38

Table

39

Flat roofs: full access allowed

1.50

Table 40

Table

41

Roof sheeting or cladding for roofs

Purlins

0.75

Table 42

Table

43

with a slope of 10°-35° inclusive

1.00

Table 44

Table

45

Source: TRADA Technology Ltd.

Source: TRADA Technology Ltd.

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