---
title: "Couple closed roof versus collar roof"
subject: "Building Technology"
chapter: 4
key: "semester_6/building_technology:couple-vs-collar"
source: https://www.bctengineer.com/q/semester_6/building_technology/couple-vs-collar
site: BCT Engineering
---

# Couple closed roof versus collar roof

Building Technology · Chapter 4: Roofs

- Asked in: 2081 Chaitra, 4 marks; 2078 Chaitra, 4 marks (asked 2 times)
- 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/couple-vs-collar
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4

## Question

Compare couple closed roof and collar roof over couple roof with suitable sketch.

## Short answer

A couple-close roof has a tie beam at rafter feet, removing outward thrust and spanning up to 5 m, but limits headroom. A collar roof places a collar beam higher (1/3–1/2 rise), allowing more headroom and slightly longer spans (~5 m), though walls must resist some thrust.

## Answer

#### Couple closed roof
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.

#### Collar roof (collar over couple roof)
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 sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

```text
            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate
```
Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

#### Comparison
| Point | Couple closed roof | Collar roof |
|---|---|---|
| Tie member | Tie beam joins feet of rafters at wall plate level | Collar beam joins rafters at 1/3 to 1/2 of rise |
| Outward thrust on walls | Eliminated by the tie | Reduced but not fully removed |
| Span | Up to about 4.5-5 m | Up to about 5 m (slightly larger) |
| Headroom | Tie reduces headroom | More headroom under collar |
| Rafter stiffness | Rafters may sag at middle | Collar reduces sagging |
| Cost | Slightly less | Slightly more |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Tie in couple close joins?
   A. Purlins
   B. Feet of rafters
   C. Ridge
   D. Mid rafters
   Answer: B. At wall plate level. (Key point: Couple close tie)

2. Collar beam is at?
   A. Wall plate
   B. 1/3 to 1/2 of rise above wall plate
   C. Floor
   D. Ridge
   Answer: B. Reduces sagging. (Key point: Collar position)

3. Thrust on walls?
   A. Eliminated by tie in couple close, reduced in collar
   B. Eliminated in collar
   C. None in either
   D. Increased in both
   Answer: A. Collar only reduces. (Key point: Thrust)

4. Which gives more headroom?
   A. Same
   B. Collar roof
   C. None
   D. Couple close
   Answer: B. Tie reduces headroom. (Key point: Headroom)

## Related in Building Technology

- [Couple closed roof and mansard roof truss](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acouple-closed-mansard) — 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
- [Couple close roof sketch](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acouple-close-roof) — 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×
- [Steel trusses versus timber; triple timber roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asteel-truss-triple-roof) — Building Technology, chapter 4

## Related in other subjects

- [Open loop versus closed loop control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-vs-closed-loop) — Embedded System, chapter 7, asked 6×