--- title: "Steel trusses versus timber; triple timber roof" subject: "Building Technology" chapter: 4 key: "semester_6/building_technology:steel-truss-triple-roof" source: https://www.bctengineer.com/q/semester_6/building_technology/steel-truss-triple-roof site: BCT Engineering --- # Steel trusses versus timber; triple timber roof Building Technology · Chapter 4: Roofs - Asked in: 2077 Chaitra, 3+5 marks - Video: [Manufactured Trusses Explained! Why They're Replacing Stick Framed Roofs](https://www.youtube.com/watch?v=fLwm2yr6RfU) — Deep Structural - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/steel-truss-triple-roof - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4 ## Question Why steel trusses have become popular nowadays as compared to timber? Briefly discuss about the types of triple timber roof. ## Short answer Steel trusses are popular due to higher strength for large spans (30-100m), availability, durability against termites/rot, fire safety, prefabrication speed, uniform quality, and slimmer members. Triple timber roof types include king post truss (5-8m), queen post truss (8-12m), mansard truss for attics, and combined king-queen post truss for spans above 12m. ## Answer #### Why steel trusses are popular compared to timber 1. **Higher strength:** larger spans (30-100 m) are possible with lighter members. 2. **Timber is scarce and costly**, while steel is available in standard sections. 3. **Durability:** not attacked by termites, fungus or rot; long life with painting. 4. **Fire safety:** steel does not burn, unlike timber (though it should be protected, as it loses strength at very high temperatures). 5. **Prefabrication:** quick erection with bolts or welding, little wastage, and members can be reused or recycled. 6. **Uniform quality** and predictable strength, suited to seismic design. 7. **Fewer and slimmer members**, which makes them economical for sheds and halls. #### Triple timber roof (types) A **triple (framed) roof** is used for spans above about 9 m, typically for halls and sheds. It consists of a framework of **trusses** placed at 3-4 m spacing: - **Principal rafters** (of the trusses) carry the **purlins** laid across them. - **Common rafters** rest on the purlins and carry the battens or boarding. - The trusses are tied by a **tie beam** and braced with a king post or queen posts, struts and straining beams. Since three sets of members (trusses, purlins and common rafters) are used, it is called a triple roof. Its types, by the truss used, are: - **King post truss** for spans of 5-8 m. - **Queen post truss** for spans of 8-12 m. - **Mansard truss** (double slope) for roofs needing an attic room. - **Combined (king-and-queen post) truss** for spans above 12 m. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a reason steel trusses are preferred over timber? A. Cheaper raw material that is abundant B. Steel burns more easily C. Not attacked by termites, fungus or rot, and allow larger spans D. Steel needs more joints Answer: C. Steel is durable, strong, fire resistant, and available in standard sections while timber is scarce. (Key point: Steel vs timber) 2. What construction advantage do steel trusses have? A. Must be built entirely on site B. Prefabricated, quickly erected by bolting or welding, and reusable C. Cannot be reused D. Require heavy wastage Answer: B. Prefabrication gives fast erection with little wastage. (Key point: Prefabrication) 3. A triple (framed) roof is used for what span range? A. Above about 9 m B. Up to 3.6 m C. About 4.5-5 m D. About 5-9 m Answer: A. Triple roofs with trusses are for spans above about 9 m, e.g. halls and sheds. (Key point: Triple roof span) 4. Which truss is suited to spans of 8-12 m? A. King post truss B. Couple truss C. Lean-to truss D. Queen post truss Answer: D. King post is 5-8 m, queen post 8-12 m, and combined above 12 m. (Key point: Truss spans) ## Related in Building Technology - [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 - [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× - [Timber collar beam roof and roof coverings](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acollar-beam-roof-coverings) — Building Technology, chapter 4 - [Collar timber roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acollar-roof) — Building Technology, chapter 4 - [Flat roof versus pitched roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aflat-vs-pitched) — Building Technology, chapter 4, asked 2× - [Couple closed roof versus collar roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acouple-vs-collar) — Building Technology, chapter 4, asked 2× ## Related in other subjects - [Steel versus timber structures](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Asteel-vs-timber-advantages) — Design of Steel and Timber Structure, chapter 1 - [Notes and summary: classical and popular music](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Anotes-summary-classical-and-popular-music) — Communication English, chapter 1 - [Merits of timber over steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Amerits-timber-over-steel) — Civil Engineering Materials, chapter 7 - [Timber-steel plate composite member: permissible load](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Atimber-steel-composite-permissible-load) — Strength of Materials, chapter 3