--- title: "Roof types, functional requirements, timber roofs" subject: "Building Technology" chapter: 4 key: "semester_6/building_technology:roof-types-requirements-timber" source: https://www.bctengineer.com/q/semester_6/building_technology/roof-types-requirements-timber site: BCT Engineering --- # Roof types, functional requirements, timber roofs Building Technology · Chapter 4: Roofs - Asked in: 2070 Chaitra (old course), 4+4 marks; 2068 Baisakh (old course), 8 marks; 2065 Shrawan (old course), 16 marks; 2064 Jestha (old course), 16 marks; 2062 Poush (old course) (asked 5 times) - Video: [Types of Roof | Building Construction | Civil Engineering](https://www.youtube.com/watch?v=OT9MP104iXY) — SGK Online Education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/roof-types-requirements-timber - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4 ## Question Define roof. What are the types of roof? What are the functional requirements of roof? Describe briefly the single timber roof and its types (and double timber roof) with necessary figures. ## Short answer A roof is the uppermost building cover protecting from weather. Types: flat (slope ≤10°), pitched (>10°), curved shells, and combination. Functional requirements: weather protection, structural stability, thermal/sound insulation, durability, fire resistance, drainage, light weight, appearance, and wind/lightning safety. Single timber roof (span ≤5 m) includes lean-to, couple, couple-close, and collar beam types. Double timber roof (span 5-9 m) adds purlins. ## Answer #### Roof A **roof** is the uppermost part of a building, supported on walls or columns, which covers the building and protects the occupants and contents from rain, snow, sun, wind and heat. #### Types of roof Roofs are classified as: 1. **Flat roofs:** nearly horizontal (slope up to 10 degrees, about 1 in 40 to 1 in 60 for drainage); RCC or stone slab, jack-arch and terrace roofs. 2. **Pitched (sloping) roofs:** slope more than 10 degrees; covered with tiles, slates, sheets or thatch over timber or steel frames. Types: lean-to, gable (couple), hip, gambrel, mansard, hip-and-valley, north-light, butterfly, conical. 3. **Curved (shell) roofs:** shell, dome, vault, barrel and folded plate roofs for large halls. 4. **Combination roofs:** part flat and part pitched on the same building (for example, a flat RCC terrace with a sloping tiled or sheet roof over one part). ```text Lean-to Gable (couple) Hip /| /\ /--\ / | / \ / \ /__| /____\ /______\ Mansard North light ____ /| /| /| / \ / | / | / | / \ /__|/__|/__| | | ``` #### Functional requirements of a roof 1. **Protection from weather:** must keep out rain, snow and sun and shed water quickly (proper slope, no leakage). 2. **Structural stability:** must safely carry dead load, live load, wind load and snow load (and earthquake forces) and transfer them to the walls or columns. 3. **Thermal insulation:** should keep the interior cool in summer and warm in winter. 4. **Sound insulation:** should reduce rain noise and outside noise. 5. **Durability and low maintenance:** resistant to weathering, decay, termites and corrosion. 6. **Fire resistance:** materials and joints should not help fire to spread. 7. **Drainage:** rainwater must drain freely through gutters and rainwater pipes. 8. **Light weight and economy:** the roof should be light so that the supporting walls and foundation are economical. 9. **Good appearance:** pleasing shape and finish, suited to the building. 10. **Lightning and wind safety:** firmly anchored against uplift; lightning conductor where needed. #### Single timber roof 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 Couple Couple close Collar beam /\ /\ /\ / \ / \ /--\ <-collar / \ /____\ / \ tie beam ``` #### Double timber roof A **double (purlin) roof** is used when the span is more than about 5 m (up to 8-9 m). In addition to the common rafters, it has **purlins** running parallel to the ridge, which support the rafters at about mid-length, so the rafters can be of smaller section. The purlins span between the end (gable) walls or cross walls; where the length is large they are propped by struts or collar ties. The rafters rest on wall plates at the bottom and meet at the ridge board at the top, and battens and the covering are fixed over them. Typical members are wall plate, common rafters, ridge board, purlins and (where needed) struts and collar ties. ```text ridge /\ / \ common rafter purlin o o purlin / \ / \ ____/__________\____ wall plate wall plate ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What slope separates flat from pitched roofs? A. 45 degrees B. 30 degrees C. 60 degrees D. 10 degrees Answer: D. Flat up to 10 degrees, pitched above. (Key point: Flat vs pitched) 2. A single roof has? A. Only rafters, no purlins, for spans up to about 5 m B. Purlins and binders C. Domes D. Trusses Answer: A. Single roofs suit small spans. (Key point: Single roof) 3. How does couple-close roof remove outward thrust? A. Ridge piece B. Tie beam joins rafter feet C. Purlin D. Collar beam Answer: B. The tie takes horizontal thrust. (Key point: Couple close) 4. A double roof is used for spans of? A. Over 5 m, up to 8-9 m, with purlins and binders B. Under 2 m C. Over 20 m D. None Answer: A. Purlins on binders reduce the rafter span. (Key point: Double roof) ## Related in Building Technology - [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× - [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 - [Collar timber roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acollar-roof) — Building Technology, chapter 4 - [Functional requirements of ventilation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Avent-requirements) — Building Technology, chapter 1, asked 3× - [Requirements of roof; coupled roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-requirements-coupled) — Building Technology, chapter 4 - [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 ## Related in other subjects - [RoR headworks arrangement and functional requirements](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-headworks-arrangement-requirements) — Hydropower Engineering, chapter 5, asked 3× - [General requirements of functional RoR headworks](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-headworks-requirements-only) — Hydropower Engineering, chapter 5, asked 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