Requirements of roof; coupled roof

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: material and joints should not support fire spread.
7. Drainage: rainwater must drain freely through gutters and rainwater pipes.
8. Light weight and economy: the roof should be light so 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.

Coupled roof

A couple roof (simple gable) consists of pairs of common rafters joined at the ridge and resting on wall plates, without a tie beam. Because there is no tie the rafters thrust outward, so it is used only for small spans (about 3.6 m) with strong walls.

        ridge
         /\
        /  \ pair of rafters
       /    \
      /      \
   __/        \__
  wall plate   wall plate

A coupled roof can be of two kinds: couple roof (without tie, span up to 3.6 m) and couple-close roof (with tie beam at the feet of the rafters, span up to 4.5-5 m).

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

• Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
• Rafters notched/spiked to wall plate and tie.
• Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
• Used for small houses, sheds and verandahs.