---
title: "Roof covering; double and triple roofs"
subject: "Building Technology"
chapter: 4
key: "semester_6/building_technology:roof-covering-double-triple"
source: https://www.bctengineer.com/q/semester_6/building_technology/roof-covering-double-triple
site: BCT Engineering
---

# Roof covering; double and triple roofs

Building Technology · Chapter 4: Roofs

- Asked in: 2067 Asar (old course), 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/roof-covering-double-triple
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4

## Question

What is roof covering? Differentiate double and triple roofs with constructional details.

## 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.

## Answer

#### 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.

```text
 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
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the roof covering?
   A. The structural frame
   B. The outermost weather-resisting layer laid on battens or boarding
   C. The wall plate
   D. The ceiling below
   Answer: B. Covering includes thatch, tiles, slates and sheets. (Key point: Covering def)

2. Which members make up a double (purlin) roof?
   A. Rafters, purlins and binders
   B. Trusses, purlins and common rafters
   C. Only rafters and a tie
   D. Posts and straining beams only
   Answer: A. Double roofs have no truss; purlins rest on binders or gable walls. (Key point: Double roof members)

3. What span suits a triple roof compared with a double roof?
   A. Below 5 m versus above 9 m
   B. Same span
   C. Above 9 m versus about 5-9 m
   D. About 3 m versus 5 m
   Answer: C. Triple roofs use trusses for larger spans. (Key point: Triple span)

4. In a triple roof, what supports the purlins?
   A. Binders between gable walls
   B. The wall plate only
   C. The ridge board
   D. Trusses (principal rafters with king/queen posts)
   Answer: D. Purlins rest on trusses spaced 3-4 m. (Key point: Triple support)

## Related in Building Technology

- [Roof covering materials for pitched roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-types) — Building Technology, chapter 4, asked 3×
- [Factors for selecting roof covering](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-factors) — Building Technology, chapter 4, asked 2×
- [Roof types, functional requirements, timber roofs](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-types-requirements-timber) — Building Technology, chapter 4, asked 5×
- [Single, double, multiple timber roofs; king post truss](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atimber-roofs-king-post) — Building Technology, chapter 4, asked 2×
- [Steel trusses versus timber; triple timber roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asteel-truss-triple-roof) — Building Technology, chapter 4
- [Why temporary construction; single and double scaffolding](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ascaffolding-single-double) — Building Technology, chapter 8

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