---
title: "Single, double, multiple timber roofs; king post truss"
subject: "Building Technology"
chapter: 4
key: "semester_6/building_technology:timber-roofs-king-post"
source: https://www.bctengineer.com/q/semester_6/building_technology/timber-roofs-king-post
site: BCT Engineering
---

# Single, double, multiple timber roofs; king post truss

Building Technology · Chapter 4: Roofs

- Asked in: 2075 Baisakh, 3+5 marks; 2070 Magh (asked 2 times)
- Video: [Simple Explanation About Roof Truss Design, Parts And Assembly](https://www.youtube.com/watch?v=XWfew8eSv5g) — gregvancom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/timber-roofs-king-post
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4

## Question

Define single/double/multiple timber roofs. Draw neat sketch of king post truss labeling with corresponding elements.

## Short answer

Single roofs use only common rafters (span ≤5 m). Double roofs add purlins (5–9 m). Multiple roofs use trusses like a king post (central post, tie beam, struts) for >9 m, with a king‑post truss spanning 5–8 m.

## Answer

#### Single, double and multiple timber roofs
- **Single roof:** has only common rafters (with or without a tie or collar) and no purlins; used for spans up to about 5 m. Types: lean-to, couple, couple-close and collar beam roofs.
- **Double roof:** has purlins in addition to the common rafters; the purlins reduce the span of the rafters. Used for spans of about 5-9 m.
- **Multiple (triple / framed) roof:** has trusses (principal rafters) carrying purlins, which carry the common rafters; used for spans over about 9 m. Types of trusses: king post, queen post, mansard and combined trusses.

#### King post truss
A **king post truss** is a triangular truss for spans of about 5-8 m, in which a single vertical **king post** is the central tension member, hanging from the ridge and holding up the tie beam.

```text
                      ridge
                       /\
                      /||\
                     / || \     principal rafter
                    /  ||  \
      purlin o     /   ||   \     o purlin
                  / \  ||  / \
           strut /   \ || /   \ strut
                /     \||/     \
       wall    +=======++=======+  tie beam
       plate
               || = king post
```
| Element | Function |
|---|---|
| Tie beam | Horizontal member at the base; resists the outward thrust of the rafters (tension) |
| Principal rafters | Inclined members that carry the purlins/common rafters (compression) |
| King post | Vertical member at the centre; holds up the tie beam from the ridge (tension) |
| Struts | Inclined members from the foot of the king post to the rafters; reduce bending of the rafters (compression) |
| Purlins | Horizontal members over the principal rafters; carry the common rafters |
| Common rafters | Support the battens/boarding and transfer the load to the purlins |
| Cleats | Small blocks that stop the purlins from sliding |
| Wall plate | Distributes the truss load over the wall |
| Straps/stirrups (iron) | Metal straps connecting the king post and tie beam |

The king post truss is suitable for spans of 5-8 m.

## Check yourself

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

1. Multiple roof used for spans?
   A. Under 2 m
   B. Under 5 m
   C. None
   D. Over about 9 m with trusses
   Answer: D. Trusses carry purlins. (Key point: Multiple roof)

2. King post is?
   A. Horizontal
   B. Central vertical tension member holding tie beam from ridge
   C. Wall plate
   D. Compression strut
   Answer: B. Tension member. (Key point: King post)

3. Struts in king post truss?
   A. Tension members
   B. Reduce bending of rafters in compression
   C. Carry tie
   D. Cover
   Answer: B. Inclined compression. (Key point: Struts)

4. King post span?
   A. 5-8 m
   B. 1-2 m
   C. 15 m
   D. 30 m
   Answer: A. Between single roof and queen post. (Key point: Span)

## Related in Building Technology

- [Queen post truss and its elements](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aqueen-post-truss) — Building Technology, chapter 4, asked 2×
- [King post, queen post, mansard trusses; thatch](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aking-queen-mansard-thatch) — Building Technology, chapter 4
- [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×
- [Roof covering; double and triple roofs](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-double-triple) — 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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- [Truss: force method, double-diagonal truss](https://www.bctengineer.com/question.md?key=semester_5%2Ftheory_of_structures_ii%3Atruss-force-method-50kn-20kn-6m) — Theory of Structures II, chapter 2
- [Single and double handshaking](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ahandshaking-single-double) — Microprocessor, chapter 4
- [Single vs double earth wire selection](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Asingle-double-earth-wire-criteria) — Transmission and Distribution System Design, chapter 3
- [T/F: double earth wire for 400 kV single circuit](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-400kv-single-circuit-double-earth) — Transmission and Distribution System Design, chapter 3