--- title: "Four types of single roof" subject: "Building Technology" chapter: 4 key: "semester_6/building_technology:single-roof-four-types" source: https://www.bctengineer.com/q/semester_6/building_technology/single-roof-four-types site: BCT Engineering --- # Four types of single roof Building Technology · Chapter 4: Roofs - Asked in: 2076 Bhadra, 8 marks - Video: [Roof Types & How They Are Constructed - Carpentry 101](https://www.youtube.com/watch?v=tzEuI7yuDqc) — BRUNI ARCHITECTURE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/single-roof-four-types - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4 ## Question Explain four types of single roof with figure and show its components. ## Short answer Four single roof types: lean-to (single slope on high/low walls, up to 4m), couple (rafters meet at ridge, no tie, up to 3.6m), couple-close (tie beam joins rafter feet, up to 5m), collar beam (collar at 1/3-1/2 height reduces sag, up to 5m). Components: rafters, wall plate, ridge board, tie/collar beam, battens, covering. ## Answer A **single roof** has only rafters (no purlins), with or without a tie. It is used for small spans (up to about 5 m). Its types are: 1. **Lean-to roof:** rafters rest on a high wall at the top and a low wall/post at the bottom; span about 2.5 m (up to 4 m); used for sheds and verandahs. 2. **Couple roof:** pairs of rafters meet at the ridge and rest on wall plates; there is no tie, so they push the walls outward; span up to about 3.6 m, used only where the walls are strong. 3. **Couple-close roof:** same as a couple roof, but the feet of each pair of rafters are joined by a tie beam, which removes the outward thrust; span up to about 4.5-5 m. 4. **Collar beam roof:** a couple roof in which a collar beam joins each pair of rafters at about one-third to one-half of the height from the wall plate; this reduces sagging of the rafters; span up to about 5 m. ```text Lean-to Couple Couple close Collar beam |\ /\ /\ /\ | \ rafter / \ / \ /--\ <- collar | \ / \ /____\ / \ |___\ tie beam high low wall wall or post ``` #### Components shown in the figures - **Rafters:** inclined members from the eaves to the ridge (50 x 100 mm at 450-600 mm c/c). - **Wall plate:** timber (75 x 100 mm) on top of the wall carrying the rafters. - **Ridge piece / board:** at the apex where the rafters meet (40 x 150 mm). - **Tie beam:** joins the feet of the rafters in a couple-close roof. - **Collar beam:** joins the rafters at 1/3 to 1/2 of the height in a collar roof. - **Battens and covering** over the rafters. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines a single roof? A. Rafters, purlins and trusses B. Only rafters, with no purlins, used for small spans C. A truss with king post D. A roof with two layers of covering Answer: B. A single roof has only rafters (with or without a tie), up to about 5 m. (Key point: Single roof def) 2. Why is a couple roof used only where walls are strong? A. It is very heavy B. Rafters rest on a ridge only C. No tie, so rafters push the walls outward D. It uses thick purlins Answer: C. Without a tie the rafters thrust outward; span is limited to about 3.6 m. (Key point: Couple roof limit) 3. Which roof has rafters resting on a high wall at the top and a low wall at the bottom? A. Lean-to roof B. Couple-close roof C. Collar roof D. Couple roof Answer: A. A lean-to has a single slope against a higher wall, span about 2.5 m. (Key point: Lean-to) 4. How does a couple-close roof differ from a collar beam roof? A. Couple-close has collar at ridge B. Collar roof has a tie at the wall plate C. They are identical D. Couple-close has a tie at the rafter feet; collar joins rafters at 1/3 to 1/2 height Answer: D. The tie removes outward thrust; the collar reduces sagging. 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