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

# Couple closed roof and mansard roof truss

Building Technology · Chapter 4: Roofs

- Asked in: 2079 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/couple-closed-mansard
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-4

## Question

Elaborate couple closed roof and mansard roof truss with neat sketches.

## Short answer

A couple-close roof uses rafters and a tie beam at feet, eliminating thrust (span ~5 m). A mansard truss has steep lower slopes (60–70°) and flat upper slopes, creating an attic room; it combines queen/king post elements for large spans.

## 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.

#### Mansard roof truss
A **mansard roof truss** has a double slope on each side: the lower slope is steep (about 60-70 degrees) and the upper slope is flat (about 15-30 degrees). The truss is a combination of a queen post and king post type, with a tie beam. It encloses a room in the roof space (attic) and is used for buildings that need extra living space or large spans.

```text
          ________
         /        \     upper slope 15-30 deg
        /          \
       /            \
      |              |   lower slope 60-70 deg
      |______________|
        tie beam
```

**Elements:** tie beam, principal rafters (lower steep and upper flatter), king/queen posts, struts, collar, purlins at the break line, common rafters and battens. The wide span and the attic room under the truss make it useful for large halls and buildings that need extra accommodation.

## Check yourself

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

1. In a couple-close roof, what does the horizontal tie beam do?
   A. Takes the horizontal thrust so only vertical loads reach the walls
   B. Supports purlins between trusses
   C. Prevents rafters from sagging at mid-span
   D. Carries the attic floor load
   Answer: A. The tie joins the feet of each rafter pair and resists the outward thrust, so walls get only vertical load. (Key point: Role of tie beam)

2. Why is a collar or double roof needed when a couple-close roof span exceeds about 5 m?
   A. The tie beam cannot be fixed
   B. Rafters sag at larger spans
   C. Walls cannot take vertical load
   D. The ridge piece becomes too thick
   Answer: B. Rafters sag if the span exceeds about 5 m, so a collar or double roof is used. (Key point: Span limit)

3. What are the slopes of a mansard roof truss?
   A. Lower slope flat (15-30 degrees), upper slope steep (60-70 degrees)
   B. Single slope of 30-45 degrees
   C. Lower slope steep (60-70 degrees), upper slope flat (15-30 degrees)
   D. Both slopes about 60-70 degrees
   Answer: C. A mansard has a steep lower slope and a flatter upper slope on each side. (Key point: Mansard slopes)

4. What is the main benefit of a mansard roof truss?
   A. Needs no tie beam
   B. Removes need for purlins
   C. Is the simplest and cheapest truss
   D. Encloses usable attic room and suits large spans
   Answer: D. The double slope encloses an attic room and allows large spans, though it is more complex. (Key point: Purpose of mansard)

## Related in Building Technology

- [Couple closed roof versus collar roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acouple-vs-collar) — Building Technology, chapter 4, asked 2×
- [Couple close roof sketch](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acouple-close-roof) — Building Technology, chapter 4
- [King post, queen post, mansard trusses; thatch](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aking-queen-mansard-thatch) — Building Technology, chapter 4

## Related in other subjects

- [Roof truss member forces (1-11, 1-10 etc.)](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aroof-truss-members-1-11-1-10-1-2-2-10-10-11) — Applied Mechanics, chapter 7
- [Max wind force on roof truss, 50 yr, hill crest](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awind-max-force-roof-truss-kathmandu) — Design of Steel and Timber Structure, chapter 8
- [Wind pressure on roof truss, 20 m span, 30 deg](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awind-roof-truss-20m-span-30deg) — Design of Steel and Timber Structure, chapter 8
- [Purlin for roof truss, 10 m span](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Apurlin-roof-truss-10m-span) — Design of Steel and Timber Structure, chapter 8