Four types of single roof

A single roof has only rafters (no purlins), with or without a tie. It is used for small spans (up to about 5 m). Its types are:

1. Lean-to roof: rafters rest on a high wall at the top and a low wall/post at the bottom; span about 2.5 m (up to 4 m); used for sheds and verandahs.
2. Couple roof: pairs of rafters meet at the ridge and rest on wall plates; there is no tie, so they push the walls outward; span up to about 3.6 m, used only where the walls are strong.
3. Couple-close roof: same as a couple roof, but the feet of each pair of rafters are joined by a tie beam, which removes the outward thrust; span up to about 4.5-5 m.
4. Collar beam roof: a couple roof in which a collar beam joins each pair of rafters at about one-third to one-half of the height from the wall plate; this reduces sagging of the rafters; span up to about 5 m.

 Lean-to       Couple       Couple close      Collar beam
  |\             /\              /\               /\
  | \ rafter    /  \            /  \             /--\ <- collar
  |  \         /    \          /____\           /    \
  |___\                       tie beam
 high  low wall
 wall  or post

Components shown in the figures

• Rafters: inclined members from the eaves to the ridge (50 x 100 mm at 450-600 mm c/c).
• Wall plate: timber (75 x 100 mm) on top of the wall carrying the rafters.
• Ridge piece / board: at the apex where the rafters meet (40 x 150 mm).
• Tie beam: joins the feet of the rafters in a couple-close roof.
• Collar beam: joins the rafters at 1/3 to 1/2 of the height in a collar roof.
• Battens and covering over the rafters.