- 2067 Asar (old course) · 8 marks
Roof covering; double and triple roofs
Short answer
Roof covering is the outermost weather-resisting layer (thatch, tiles, slates, sheets) laid on battens or boarding. A double roof uses common rafters and purlins spanning 5–9 m, while a triple roof adds trusses (king/queen post) with purlins on principal rafters, spanning above 9 m.
What is roof covering? Differentiate double and triple roofs with constructional details.
Roof covering
Roof covering is the outermost weather-resisting layer of a roof (thatch, tiles, slates, sheets), laid on battens or boarding supported by the roof frame.
Difference between double and triple roof
| Point | Double (purlin) roof | Triple (framed) roof |
|---|---|---|
| Members | Common rafters and purlins (with struts or collars where needed) | Trusses, purlins and common rafters |
| Span | About 5-9 m | Above 9 m (up to 20 m or more) |
| Support | Purlins rest on gable walls or cross walls | Purlins rest on trusses (king post, queen post) |
| Truss | None | Yes, spaced 3-4 m |
| Cost | Less | More |
| Use | Houses, small halls | Halls, factories, godowns |
Constructional details
- Double roof: wall plates on both walls; purlins running parallel to the ridge, spanning between the gable or cross walls and supporting the rafters at about mid-length; common rafters resting on the wall plates and purlins and meeting at the ridge board; battens and covering.
- Triple roof: wall plates on the walls; trusses (principal rafters, tie beam, king/queen posts, struts); purlins on the principal rafters, held by cleats; common rafters on the purlins; battens and covering.
Double roof (purlin roof) Triple roof (trussed)
ridge ridge
/\ /\
/ \ common rafters o/||\o purlins on
o o <- purlins o/ || \o principal rafters
/ \ (span between / \ || / \
/ \ gable walls) / \||/ \
__/__________\__ +======++======+ tie beam
wall wall wall king post wall
✓ Answer checked by Claude Opus 5.5 (high effort)
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