---
title: "Why joints are necessary; expansion joint"
subject: "Building Technology"
chapter: 10
key: "semester_6/building_technology:joints-why-expansion"
source: https://www.bctengineer.com/q/semester_6/building_technology/joints-why-expansion
site: BCT Engineering
---

# Why joints are necessary; expansion joint

Building Technology · Chapter 10: Causes and prevention of cracks in buildings

- Asked in: 2070 Chaitra (old course), 4+4 marks; 2068 Baisakh (old course), 2+2+4 marks (asked 2 times)
- Video: [Expansion Joint in Buildings | Expansion joint in construction | Separation joint | Expansion Joint](https://www.youtube.com/watch?v=GMmymQLcS-4) — Civil Engineering Mastery
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/joints-why-expansion
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-10

## Question

Why are joints necessary (important) in building construction? Briefly explain the expansion joint in a building with necessary sketches.

## Short answer

Joints are needed to relieve stress from thermal, shrinkage and differential settlement movements without cracking. An expansion joint is a 20-25 mm gap through the full depth above foundation, filled with compressible filler and surface sealant, placed at 30-45 m intervals in long walls and slabs to permit thermal expansion.

## Answer

#### Why joints are necessary
Buildings change in size because of temperature and moisture, and settle unevenly. Without joints, these movements create stresses that crack the structure. Joints are provided:

- To allow expansion and contraction due to temperature changes.
- To permit shrinkage movement of concrete and masonry.
- To allow differential settlement of different parts.
- To control the location of cracks.
- To limit the size of pours of concrete (construction joints).
- To separate parts with different heights or loads.

#### Expansion joint
**Expansion joint:** a gap of 20-25 mm, provided through the full depth of the structure above the foundation to allow expansion and contraction due to temperature, so that cracks do not form. Used at intervals of 30-45 m in buildings, in long walls, slabs, roofs and parapets.

```text
   Expansion joint in a wall/slab
    slab |  25 mm gap  | slab
    =====|             |=====
    wall |  filler     | wall
         |  (bitumen)  |
         | sealant on top
   foundation continuous below
```
The gap is filled with a compressible filler (bitumen-impregnated fibre board, cork, rubber) and sealed on top with sealant. The two parts are fully separated, often by a double column or wall.

## Check yourself

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

1. Why are joints provided in buildings?
   A. To avoid using reinforcement
   B. To increase load on walls
   C. To make the building heavier
   D. To allow movement from temperature, moisture and settlement and avoid cracking
   Answer: D. Joints give space for movement so stresses do not crack the structure. (Key point: Purpose of joints)

2. An expansion joint is typically:
   A. A 20-25 mm gap through full depth above the foundation at 30-45 m
   B. A 2 mm groove in the floor
   C. A gap through the foundation only
   D. A 200 mm gap every 3 m
   Answer: A. It separates the structure above the foundation to allow thermal movement. (Key point: Expansion joint dimensions)

3. The gap of an expansion joint is filled with:
   A. Rich cement mortar
   B. Compressible filler like bitumen fibre board, sealed on top
   C. Rigid concrete
   D. Nothing, left open
   Answer: B. Compressible filler allows movement and sealant keeps water out. (Key point: Joint filler)

## Related in Building Technology

- [Expansion and construction joints, positions](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ajoints-expansion-construction-positions) — Building Technology, chapter 10
- [Expansion joint in a residential building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aexpansion-joint-350wall) — Building Technology, chapter 10
- [Construction joints in structure](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aconstruction-joint) — Building Technology, chapter 10, asked 2×
- [Retrofitting techniques; why it is important](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-techniques-importance) — Building Technology, chapter 11, asked 3×
- [Types of joints and their uses](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ajoints-types-uses) — Building Technology, chapter 10, asked 2×
- [Types of temporary construction](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Atemp-construction-types-sketch) — Building Technology, chapter 8, asked 2×

## Related in other subjects

- [Pipe joints; expansion joint](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-joints-expansion-joint) — Water Supply Engineering, chapter 8, asked 3×
- [Pipe joints; socket and spigot joint](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-joints-socket-spigot) — Water Supply Engineering, chapter 8, asked 4×
- [Pipe joints; collar joint](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-joints-collar) — Water Supply Engineering, chapter 8
- [Good pipe requirements; expansion joint](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-requirements-expansion-joint) — Water Supply Engineering, chapter 8, asked 2×