---
title: "Horizontal crack below RCC slab"
subject: "Building Technology"
chapter: 10
key: "semester_6/building_technology:horizontal-crack-below-slab"
source: https://www.bctengineer.com/q/semester_6/building_technology/horizontal-crack-below-slab
site: BCT Engineering
---

# Horizontal crack below RCC slab

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

- Asked in: 2080 Chaitra, 4 marks
- Video: [Cracking in Building: Causes and Solutions](https://www.youtube.com/watch?v=dskKQBk4aME) — Youva Designers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/horizontal-crack-below-slab
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-10

## Question

Explain the causes and preventive measures for the development of horizontal crack in load bearing wall just below the RCC slab.

## Short answer

Horizontal cracks below RCC slabs are caused by thermal expansion of the slab dragging the wall, slab deflection, shrinkage, and lack of a sliding layer. Preventive measures include providing a sliding joint (bitumen/polythene) between slab and wall, thermal insulation, a ring beam, expansion joints, and adequate bearing.

## Answer

A horizontal crack often forms in a load-bearing wall just below the RCC roof slab.

#### Causes
- **Thermal movement:** the RCC slab, heated by the sun, expands more than the wall below, and drags the top of the wall outwards. In cooling, it contracts and the wall cracks horizontally.
- **Different expansion** of concrete and masonry.
- **Slab deflection** causing rotation at the support and lifting of the wall edges.
- **Shrinkage of the slab** in the first few months.
- **Poor bearing** or lack of a sliding layer between slab and wall.
- **Moisture movement** of the wall below.

```text
   ------ RCC slab ------>  expands
   ====================== horizontal crack
   |  brick wall        |
```

#### Preventive measures
- Provide a **sliding joint** (bitumen coat with polythene, or two layers of tarred paper) between the slab and the wall top.
- Provide **thermal insulation** or a reflective finish (lime concrete terracing, white tiles) on the roof.
- Provide a **ring beam** (roof band) to tie the wall.
- Provide **expansion joints** in long slabs.
- Use proper curing, good mortar, adequate bearing (at least 150-200 mm).
- Use a soft **flexible pad** or a bearing plate where the slab ends on the wall.

## Check yourself

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

1. Horizontal cracking just below an RCC slab is mainly due to:
   A. Thermal movement; slab expands more than wall
   B. Foundation settlement
   C. Vegetation
   D. Use of expansion joints
   Answer: A. The heated slab moves relative to the wall. (Key point: Main cause)

2. A sliding joint to prevent this crack consists of:
   A. Rich mortar
   B. Bitumen coat with polythene or tarred paper between slab and wall
   C. Welded steel
   D. Extra bricks
   Answer: B. It lets the slab move over the wall. (Key point: Sliding joint)

3. Which also reduces this crack?
   A. Removing the ring beam
   B. Painting the wall
   C. Thermal insulation or reflective finish on roof
   D. Reducing bearing
   Answer: C. Reducing roof heating reduces differential movement. (Key point: Other measures)

## Related in Building Technology

- [Earthquake protection of load bearing masonry](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aeq-load-bearing-masonry) — Building Technology, chapter 11
- [RCC stair for 6.0 m by 4.2 m lobby](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-public-lobby-6m) — Building Technology, chapter 5
- [Causes and remedies of cracks](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acracks-causes-remedial) — Building Technology, chapter 10, asked 2×
- [Cavity wall construction](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acavity-wall) — Building Technology, chapter 3, asked 3×

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- [Load bearing wall: crushing strength and mortar](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Awall-250cm-79-5kn-axial) — Concrete Technology and Masonry Structure, chapter 8
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