--- title: "Timber collar beam roof and roof coverings" subject: "Building Technology" chapter: 4 key: "semester_6/building_technology:collar-beam-roof-coverings" source: https://www.bctengineer.com/q/semester_6/building_technology/collar-beam-roof-coverings site: BCT Engineering --- # Timber collar beam roof and roof coverings Building Technology · Chapter 4: Roofs - Asked in: 2069 Bhadra, 8 marks - Video: [Residential Wood Framing Roof Structural Systems | Ridge Beam and Collar Tie](https://www.youtube.com/watch?v=WYb12rtyj78) — Jose Cadena | Structural Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/collar-beam-roof-coverings - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4 ## Question Draw and explain different components of timber collar beam roof with their sizes. Explain different types of roof covering for pitched roof. ## Short answer A collar beam roof uses wall plate (75x100mm), common rafters (50x100mm at 400-600mm c/c), collar beam (40x100mm), ridge board (40x150mm), and battens (25x40mm). Pitched roof coverings include thatch, clay tiles, slates, stone slabs, AC sheets, GI/CGI sheets, aluminium/steel sheets, plastic sheets, and wooden shingles, each with distinct properties like cost, weight, durability, and slope requirements. ## Answer #### Components of a collar beam roof (with usual sizes) A **collar roof** is a couple (or couple-close) roof in which a **collar beam** (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters from sagging and spreading, and allows a slightly larger span (about 5 m) than a couple-close roof. ```text ridge /\ / \ collar/----\ collar beam / \ / \ __/__________\__ wall plate wall plate ``` The collar beam is bolted to the rafters; battens and the covering are fixed over the rafters. | Member | Usual size | |---|---| | Wall plate | 75 x 100 mm | | Common rafter | 50 x 100 mm at 400-600 mm c/c | | Collar beam | 40 x 100 mm (or 50 x 75 mm) | | Ridge board | 40 x 150 mm | | Battens | 25 x 40 mm at 300 mm c/c (for tiles) | #### Types of roof covering for pitched roof | Covering | Main properties and use | |---|---| | Thatch (straw, khar, reed) | Cheapest, good insulation, high fire risk, needs 45-60 degree slope, short life | | Clay tiles (country, Mangalore, Allahabad) | Good insulation, durable, heavy, need close battens; slope 25-45 degrees | | Slates | Durable, fire resistant, neat; heavy and costly; slope above 25 degrees | | Stone slabs (patthar) | Used in hilly Nepal; very heavy, durable, needs strong rafters | | AC (asbestos cement) sheets | Cheap, light, fire resistant; brittle, health hazard (asbestos) | | GI / CGI sheets | Light, easy and quick fixing, durable if coated; noisy, hot, can corrode | | Aluminium / coloured steel sheets | Light, rust-resistant, many colours; costly | | Plastic / fibre / polycarbonate sheets | Light, translucent; low strength, UV ageing | | Wooden shingles | Good insulation, attractive; fire risk, decay | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a collar beam roof, the collar joins pairs of rafters at about what position? A. One-third to one-half of the rise above the wall plate B. At the wall plate level C. At the ridge only D. At two-thirds of the span Answer: A. The collar sits at 1/3 to 1/2 of the rise above the wall plate. (Key point: Collar roof definition) 2. Which is the usual size of the wall plate in the collar roof? A. 25 x 40 mm B. 40 x 150 mm C. 75 x 100 mm D. 50 x 75 mm Answer: C. Wall plate 75 x 100 mm; battens are 25 x 40 mm and ridge board 40 x 150 mm. (Key point: Member sizes) 3. Which statement about thatch roof covering is correct? A. Fire resistant and long-lasting B. Cheapest with good insulation but high fire risk, needs 45-60 degree slope C. Needs a slope of only 10 degrees D. Heavy and needs strong rafters Answer: B. Thatch is cheap and insulating but burns easily, has a short life and needs a steep slope. (Key point: Thatch) 4. What is the main drawback of AC (asbestos cement) sheets? A. Very heavy B. Highly flammable C. Rust quickly D. Brittle and a health hazard from asbestos Answer: D. AC sheets are cheap, light and fire resistant, but brittle and hazardous. (Key point: AC sheets) ## Related in Building Technology - [Roof covering materials for pitched roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-types) — Building Technology, chapter 4, asked 3× - [Collar timber roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acollar-roof) — Building Technology, chapter 4 - [Roof coverings in Terai, pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-pros-cons) — Building Technology, chapter 4 - [Factors for selecting roof covering](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-factors) — 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 - [Define pitched roof and its types](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adefine-pitched-roof-types) — Building Technology, chapter 4, asked 4× ## Related in other subjects - [Wind pressure on pitched roof on hill](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awind-pitched-roof-eaves-10m) — Design of Steel and Timber Structure, chapter 8 - [Timber beam: value of W for stress limit](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Atimber-beam-w-point-loads-udl) — Strength of Materials, chapter 7 - [Sal timber beam, 2.5 m span, 20 kN/m](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-beam-sal-2-5m-20knm) — Design of Steel and Timber Structure, chapter 12 - [Simple timber beam and flitched beam](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Asimple-flitched-beam-sketch) — Design of Steel and Timber Structure, chapter 12