Roof types, functional requirements, timber roofs

Roof

A roof is the uppermost part of a building, supported on walls or columns, which covers the building and protects the occupants and contents from rain, snow, sun, wind and heat.

Types of roof

Roofs are classified as:

1. Flat roofs: nearly horizontal (slope up to 10 degrees, about 1 in 40 to 1 in 60 for drainage); RCC or stone slab, jack-arch and terrace roofs.
2. Pitched (sloping) roofs: slope more than 10 degrees; covered with tiles, slates, sheets or thatch over timber or steel frames. Types: lean-to, gable (couple), hip, gambrel, mansard, hip-and-valley, north-light, butterfly, conical.
3. Curved (shell) roofs: shell, dome, vault, barrel and folded plate roofs for large halls.
4. Combination roofs: part flat and part pitched on the same building (for example, a flat RCC terrace with a sloping tiled or sheet roof over one part).

 Lean-to       Gable (couple)    Hip
    /|             /\            /--\
   / |            /  \          /    \
  /__|           /____\        /______\

 Mansard           North light
   ____             /|  /|  /|
  /    \           / | / | / |
 /      \         /__|/__|/__|
 |      |

Functional requirements of a roof

1. Protection from weather: must keep out rain, snow and sun and shed water quickly (proper slope, no leakage).
2. Structural stability: must safely carry dead load, live load, wind load and snow load (and earthquake forces) and transfer them to the walls or columns.
3. Thermal insulation: should keep the interior cool in summer and warm in winter.
4. Sound insulation: should reduce rain noise and outside noise.
5. Durability and low maintenance: resistant to weathering, decay, termites and corrosion.
6. Fire resistance: materials and joints should not help fire to spread.
7. Drainage: rainwater must drain freely through gutters and rainwater pipes.
8. Light weight and economy: the roof should be light so that the supporting walls and foundation are economical.
9. Good appearance: pleasing shape and finish, suited to the building.
10. Lightning and wind safety: firmly anchored against uplift; lightning conductor where needed.

Single timber 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.

 Couple          Couple close        Collar beam
   /\               /\                 /\
  /  \             /  \               /--\ <-collar
 /    \           /____\             /    \
                   tie beam

Double timber roof

A double (purlin) roof is used when the span is more than about 5 m (up to 8-9 m). In addition to the common rafters, it has purlins running parallel to the ridge, which support the rafters at about mid-length, so the rafters can be of smaller section. The purlins span between the end (gable) walls or cross walls; where the length is large they are propped by struts or collar ties. The rafters rest on wall plates at the bottom and meet at the ridge board at the top, and battens and the covering are fixed over them. Typical members are wall plate, common rafters, ridge board, purlins and (where needed) struts and collar ties.

          ridge
           /\
          /  \  common rafter
  purlin o    o purlin
        /      \
       /        \
  ____/__________\____
  wall plate   wall plate