--- title: "Requirements of roof; coupled roof" subject: "Building Technology" chapter: 4 key: "semester_6/building_technology:roof-requirements-coupled" source: https://www.bctengineer.com/q/semester_6/building_technology/roof-requirements-coupled site: BCT Engineering --- # Requirements of roof; coupled roof Building Technology · Chapter 4: Roofs - Asked in: 2071 Magh, 8 marks - Video: [What is a couple roof](https://www.youtube.com/watch?v=TjcIoI7KLzQ) — Architects Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/roof-requirements-coupled - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4 ## Question What are the requirements of roof? Describe coupled roof with necessary sketches. ## Short answer Roof must provide weather protection, structural stability, thermal/sound insulation, durability, fire resistance, drainage, lightness, economy, appearance, and wind safety. A coupled roof uses pairs of rafters joined at the ridge; a couple roof (no tie) spans up to 3.6 m, while a couple-close roof adds a tie beam to eliminate thrust, spanning up to 4.5–5 m. ## Answer #### 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:** material and joints should not support fire spread. 7. **Drainage:** rainwater must drain freely through gutters and rainwater pipes. 8. **Light weight and economy:** the roof should be light so 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. #### Coupled roof A **couple roof** (simple gable) consists of pairs of common rafters joined at the ridge and resting on wall plates, without a tie beam. Because there is no tie the rafters thrust outward, so it is used only for small spans (about 3.6 m) with strong walls. ```text ridge /\ / \ pair of rafters / \ / \ __/ \__ wall plate wall plate ``` A **coupled roof** can be of two kinds: **couple roof** (without tie, span up to 3.6 m) and **couple-close roof** (with tie beam at the feet of the rafters, span up to 4.5-5 m). A **couple-close roof** is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal **tie beam**. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m. ```text ridge /\ / \ rafters / \ / \ __/________\__ wall plate tie beam (wall plate) ``` Features: - Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more. - Rafters notched/spiked to wall plate and tie. - Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used. - Used for small houses, sheds and verandahs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What loads must a good roof carry safely? A. Dead, live, wind and snow loads, and earthquake forces B. Only its own weight C. Only live load D. Only snow load Answer: A. Structural stability needs all these loads transferred to walls or columns. (Key point: Stability) 2. What is required for roof drainage? A. A perfectly level surface B. Proper slope with gutters and rainwater pipes C. No gutters D. Holes in the covering Answer: B. Water must shed quickly and drain freely. (Key point: Drainage) 3. Why is a couple roof limited to about 3.6 m? A. Rafters are too long B. Ridge cannot be fixed C. No tie beam, so rafters thrust outward D. It has purlins Answer: C. The outward thrust on walls limits the span to small ones with strong walls. (Key point: Couple roof) 4. What does the tie beam do in a couple-close roof? A. Supports the ridge board B. Holds the covering C. Reduces rafter span at mid-height D. Takes horizontal thrust at rafter feet, extending span to 4.5-5 m Answer: D. The tie removes the outward thrust on the walls. 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