---
title: "Methods of repairing cracks"
subject: "Building Technology"
chapter: 10
key: "semester_6/building_technology:crack-repair-methods"
source: https://www.bctengineer.com/q/semester_6/building_technology/crack-repair-methods
site: BCT Engineering
---

# Methods of repairing cracks

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

- Asked in: 2079 Chaitra, 4 marks; 2079 Jestha, 4 marks; 2070 Bhadra, 8 marks (asked 3 times)
- Video: [How to Repair Wall Cracks On-Site: Step-by-Step Guide. #wallcrack #renovation ](https://www.youtube.com/watch?v=PoHLBSt0XY0) — MIRAXONE BUILDING CONSTRUCTION
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/crack-repair-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-10

## Question

Explain the methods of repairing (remedial methods for) different types of cracks in building.

## Short answer

Repair methods depend on width: hairline (<0.3 mm) fill with cement slurry; small (0.3-1 mm) rake V-groove and fill with 1:3 mortar; medium (1-3 mm) epoxy injection; wide (>3 mm) stitching, jacketing or rebuild. For settlement, underpinning; for RCC, epoxy injection, plate bonding, jacketing.

## Answer

Cracks are repaired by the method that suits their type, width and cause. The cause (such as settlement) should be removed first.

#### Repair by crack width
| Crack width | Remedy |
|---|---|
| Hairline (< 0.3 mm) | Fill with cement slurry or paint-grade filler; fresh plaster |
| Small (0.3-1.0 mm) | Rake out, fill with cement-sand mortar (1:3) or polymer-modified mortar |
| Medium (1-3 mm) | Epoxy injection or cement grout through packers |
| Wide (> 3 mm) | Remove and rebuild, stitching with steel clamps, epoxy mortar, jacketing |
| Foundation settlement | Underpinning, grouting, soil improvement |
| RCC cracks | Epoxy injection, grouting, jacketing, bonding steel plates |
| Roof cracks | Waterproof treatment, expansion joints, sealants |

#### Methods in brief
1. **Surface sealing:** hairline cracks are filled with cement slurry or polymer-modified paste.
2. **Raking and filling:** crack is widened into a V groove (10-20 mm), cleaned, and filled with cement-sand mortar 1:3 or epoxy mortar.
3. **Epoxy injection:** low-viscosity epoxy is injected through packers fixed along the crack; used for structural cracks in RCC.
4. **Grouting:** cement or chemical grout pumped into wide cracks and voids.
5. **Stitching:** U-shaped metal clamps fixed across the crack and embedded in epoxy or mortar.
6. **Jacketing / section enlargement:** extra concrete cover with new reinforcement on columns and beams.
7. **Bonding steel plates or FRP** on the tension face.
8. **Underpinning or soil improvement** for settlement cracks.
9. **Re-plastering with chicken mesh** over joints between different materials.
10. **Expansion joints / sealants** for thermal cracks.

## Check yourself

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

1. Before repairing a crack caused by settlement, what should be done first?
   A. Remove the cause
   B. Plaster over it
   C. Paint it
   D. Inject epoxy at once
   Answer: A. The cause must be removed first, otherwise the crack will reappear. (Key point: First step in repair)

2. In raking and filling, the crack is:
   A. Covered with chicken mesh only
   B. Widened to a V groove of 10-20 mm, cleaned and filled
   C. Left open to dry
   D. Injected through packers
   Answer: B. A V groove is cut, cleaned and filled with 1:3 mortar or epoxy mortar. (Key point: Raking and filling)

3. Epoxy injection through packers is used mainly for:
   A. Hairline plaster cracks
   B. Thermal cracks in pavements
   C. Structural cracks in RCC
   D. Vegetation cracks
   Answer: C. Low-viscosity epoxy bonds structural RCC cracks. (Key point: Epoxy injection)

4. Stitching of a crack uses:
   A. Fibre board filler
   B. Sand-cement slurry only
   C. Bitumen sheets
   D. U-shaped metal clamps embedded in epoxy or mortar across the crack
   Answer: D. Clamps are fixed across the crack to tie the two sides together. (Key point: Stitching)

## Related in Building Technology

- [Retrofitting need; repairing cracks of different width](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aretrofit-repair-crack-width) — Building Technology, chapter 10
- [Causes of cracks in a building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acauses-of-cracks) — Building Technology, chapter 10, asked 3×
- [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×
- [Painting on masonry; causes and remedies of cracks](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Apainting-masonry-cracks) — Building Technology, chapter 9
- [Structural and non-structural cracks](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Acracks-structural-nonstructural) — Building Technology, chapter 10, asked 4×
- [Reasons for cracks in plastering](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aplaster-crack-reasons) — Building Technology, chapter 9