This section is from the book "Notes On Building Construction", by Henry Fidler. Also available from Amazon: Notes on building construction.

Nature of Roof. | Span in feet. | ‡ Tie beam. | Straining beam. | § Purlins. 10 ft. bearing. | Common Rafters. | |||

Post. Trusses 10' centre to centre. | 32 | 4¾"x7¼" | 4¾"x5½" | 4¾"x 3" | 4¾ x 2½" | 4¾"x6¾" | 5"x7¾" | 2" x 3½" |

34 | 43" x 73' | 4¾"x5¾" | 43" x 3" | 4¾" x 2¾" | 4¾"x7½" | 5"x7¾" | 2" x 3¾" | |

36 | 4¾" x 8¼" | 4¾"x6¼" | 4¾" x 3" | 4¾" x 3" | 4¾"x 8¼" | 5" x 8" | 2" x 4" | |

38 | 5"x8½" | 5" x 6" | 5"x3" | 5" x 3" | 5"x8¼" | 5" x 8" | 2" x 4" | |

40 | 5" x 9" | 5"x6½" | 5" x 3½" | 5"x3¼" | 5" x 9" | 5" x 8¼" | 2"x4¼" | |

42 | 5½"x9" | 5½x6¼" | 5½"x3½" | 5½" x 3" | 5½"x9" | 5"x8½" | 2" x 44" | |

44 | 5½"x9½" | 5½"x6¾" | 5½"x3½" | 5½"x3¼" | 5½" x 9½" | 5" x 8½" | 2" x 43" | |

46 | 5½"xl0" | 5½"x7¼" | 5½"x4" | 5½"x3½" | 5½"xl0" | 5"x8¾" | 2" x 5" |

Add 1" to the depth of common rafters, purlins, and struts, and ½" to the depth of the principal rafters, as given in Tables I. and II.

§ 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 :

without ceiling 1¼" | |

with „ ½" |

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 ¼" deducted from the lesser dimension of the scantling, add ¼" 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, ¼' deducted from the breadth requires ¼" 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.

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