---
title: "Methods of preserving timber"
subject: "Civil Engineering Materials"
chapter: 7
key: "semester_3/civil_engineering_materials:preservation-of-timber"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/preservation-of-timber
site: BCT Engineering
---

# Methods of preserving timber

Civil Engineering Materials · Chapter 7: Timber

- Asked in: 2081 Baisakh (new course), 1.5 marks; 2081 Chaitra (new course), 3 marks; 2076 Chaitra, 2 marks; 2069 Chaitra, 2 marks (asked 4 times)
- Video: [Preservation of Timber|| Types of preservatives || Method of preservation || Building Materials](https://www.youtube.com/watch?v=MY2X2a6YZ_Y) — Pranitha adarsh CE_TUTORIAL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/preservation-of-timber
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-7

## Question

Describe briefly the various methods used in preserving timber.

## Short answer

Preservation protects timber from fungi and insects using chemicals. Methods include surface brushing/spraying, dipping, steeping, hot-and-cold open-tank treatment, and pressure treatment (best, deep penetration but costly). Charring the surface is a cheap option. Common preservatives are creosote, CCA, and organic-solvent types.

## Answer

**Preservation** is the treatment of timber with chemicals to protect it from decay by fungi and from attack by insects (termites, borers) and marine borers, so that its life increases. Timber should be seasoned first.

#### Methods
1. **Brushing or spraying:** a preservative (coal tar, creosote, copper sulphate solution) is applied on the surface with a brush or spray in 2-3 coats. It is cheap, but penetration is limited.
2. **Dipping:** timber is dipped in the preservative (for example, copper sulphate solution) for a few minutes or hours; used for small members.
3. **Steeping (soaking):** timber is soaked in a cold or hot preservative solution in a tank for several days; penetration is better.
4. **Hot and cold open-tank treatment:** timber is heated in hot preservative (about 90 C) for several hours and then moved to cold preservative, which is drawn into the wood as it cools.
5. **Pressure treatment (full-cell/Bethell or empty-cell process):** timber in an airtight cylinder is first evacuated, and the preservative is then forced in at 8-12 kg/cm², giving deep penetration; this is the best method but costly.
6. **Charring:** the surface of timber (such as posts) is burnt to a depth of about 15 mm; cheap but of limited use.
7. **Diffusion and sap displacement (Boucherie) methods** are also used for green timber.

Common preservatives: coal tar and creosote (oil type), copper-chrome-arsenic (CCA, water-soluble), and organic-solvent types (pentachlorophenol).

## Check yourself

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

1. What is the purpose of timber preservation?
   A. Remove moisture from the wood
   B. Make timber lighter in colour
   C. Increase its elasticity
   D. Protect against fungi, insects and marine borers to increase life
   Answer: D. Preservatives protect from decay and insect attack. (Key point: Purpose)

2. Which method gives deepest penetration and is best but costly?
   A. Pressure treatment in an airtight cylinder
   B. Brushing
   C. Dipping
   D. Charring
   Answer: A. Pressure forces preservative deeply into the wood. (Key point: Pressure treatment)

3. What does charring involve?
   A. Heating timber in hot preservative at 90 C
   B. Burning the surface of posts to about 15 mm depth
   C. Evacuating timber in a cylinder
   D. Soaking in creosote for days
   Answer: B. Charring burns the surface; cheap but limited. (Key point: Charring)

4. In the hot and cold open-tank process, why is timber moved to cold preservative?
   A. To harden the surface
   B. To dry the wood
   C. As it cools, preservative is drawn into the wood
   D. To char the surface
   Answer: C. Cooling contracts the air in the cells, sucking in preservative. (Key point: Hot and cold process)

## Related in Civil Engineering Materials

- [Why timber is used; seasoning method](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aseasoning-method-and-timber-use) — Civil Engineering Materials, chapter 7
- [Uses of timber and modified timber](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Auses-of-timber-modified-timber) — Civil Engineering Materials, chapter 7

## Related in other subjects

- [Intermediate Sal timber joist, 2.5 m span](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Atimber-joist-sal-600mm) — Design of Steel and Timber Structure, chapter 12
- [Types of timber columns and column bases](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Atimber-column-types-and-bases) — Design of Timber and Masonry Structures, chapter 3