---
title: "Cavity wall construction"
subject: "Building Technology"
chapter: 3
key: "semester_6/building_technology:cavity-wall"
source: https://www.bctengineer.com/q/semester_6/building_technology/cavity-wall
site: BCT Engineering
---

# Cavity wall construction

Building Technology · Chapter 3: Mortars & Masonry works

- Asked in: 2078 Baisakh, 3 marks; 2073 Magh; 2072 Asoj, 8 marks (asked 3 times)
- Video: [13 Cavity wall Construction](https://www.youtube.com/watch?v=4vMMNU3iSzI) — Ramstam
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/cavity-wall
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-3

## Question

Describe cavity wall with neat sketches (construction, vertical and horizontal sections, elevation, merits and demerits).

## Answer

A **cavity wall** is a wall made of two separate leaves (skins) of masonry, usually brick or block, separated by a continuous air space (cavity) of 50 to 100 mm and tied together by metal ties.

#### Construction
- The **outer leaf** (103 or 115 mm) resists the weather; the **inner leaf** (100 to 230 mm) carries the load.
- **Cavity width**: 50 to 75 mm (normally not more than 100 mm).
- **Wall ties**: Galvanised or stainless steel ties (vertical twist, butterfly or fish-tail type) placed at 900 mm horizontally and 450 mm vertically, staggered, sloping down towards the outer leaf, with a drip.
- **DPC**: Placed at the base, at openings and at the top of the wall, with a flashing (cavity tray), so that water in the cavity is thrown out; weep holes at 1 m centres in the outer leaf at the bottom.
- The **cavity** is kept clean, and it is closed at the eaves, openings and top by a ring beam or a solid closer.
- **Insulation** can be fitted in the cavity.

```text
 Horizontal section:           Vertical section:
 |outer|  cavity  |inner|        roof slab
 |_____|<-50mm->|_____|         |  |   |  |
   tie ------------ tie         |  |   |  |
                                ==DPC====(weep hole)
                              plinth
```

```text
 Elevation (outer leaf, x = wall tie):
 |____|____|____|____|____|____|
   x          x          x        <- ties 900 mm apart
 |__|____|____|____|____|____|_|
        x          x          x   <- next row 450 mm higher, staggered
 |____|____|____|____|____|____|
 ======= DPC ===== o weep hole o =
```

#### Merits
1. Prevents rain penetration and dampness.
2. Good thermal insulation (cool in summer, warm in winter).
3. Good sound insulation.
4. Reduces efflorescence on the inner face, and good facing bricks are needed only for the outer leaf.
5. Lighter than a solid wall of the same overall thickness, with adequate stability.

#### Demerits
1. Costlier than a solid wall, by about 20 %.
2. Needs skilled work and supervision; mortar droppings can bridge the cavity.
3. Takes more time to build.
4. Ties can transfer moisture if badly placed.
5. Needs extra DPC and details at openings.

## Check yourself

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

1. A cavity wall has:
   A. A hollow brick only
   B. A single thick leaf
   C. Timber infill
   D. Two leaves separated by a continuous air space tied by metal ties
   Answer: D. Two skins are separated by a 50 to 100 mm cavity. (Key point: Cavity wall definition)

2. Which leaf carries the load?
   A. The cavity
   B. Inner leaf
   C. Outer leaf only
   D. Neither
   Answer: B. The outer leaf takes weather; inner carries load. (Key point: Load-bearing leaf)

3. Weep holes at the bottom of the outer leaf:
   A. Ventilate rooms
   B. Reduce strength
   C. Hold ties
   D. Let water in the cavity drain out
   Answer: D. With DPC/flashing they throw water out. (Key point: Weep holes)

4. A merit of cavity wall is:
   A. Faster to build
   B. Needs no supervision
   C. Prevents rain penetration and dampness
   D. Cheaper than solid wall
   Answer: C. It is also thermally and acoustically better but costlier. (Key point: Merit)

## Related in Building Technology

- [Thermal performance, transmittance of composite wall](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-composite-wall-062ba) — Building Technology, chapter 1
- [Horizontal crack below RCC slab](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ahorizontal-crack-below-slab) — Building Technology, chapter 10
- [English and Flemish bond corner wall](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aenglish-flemish-corner) — Building Technology, chapter 3
- [Vertical circulation elements; RCC dog-legged stair](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-circulation-068ba) — Building Technology, chapter 5
- [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×

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