Timber preservation

1 Using preservatives

Most designers using timber are used to specifiying a range of preservatives. Without question all softwood whether external or internal is routinely coated or injected against a perceived host of rots and insect infestations. Most commonly specified preservative are not benign to people or the environment (see below) yet a huge chemical industry rides on the back of an arguably small or non-existent risk. Prior to 1940 virtually no preservative treatment was carried out, yet the vast majority of buildings (including the houses many of us live in) are still standing, free of rot and infestation.

The problem with timber preservatives

• During the production of impregnated wood, hazardous and carcinogenic substances are released to air, surface water and soil.

• Workers and consumers are exposed to carcinogenic substances during the handling and processing of impregnated wood.

• Impregnating wood hampers the sustainable reuse of wood.

• Hazardous and toxic components of impregnated wood leach to the environment during the installed life of the wood, mainly via direct contact with surface water, but also via contact with rain and moist soil.

• Impregnated wood forms a threat to the environment in waste incineration and landfills.

• Household or uncontrolled outdoor burning of impregnated wood forms a big threat to human health and the environment.

• Treated timber is classed as 'Hazardous Waste'


Specifying preservatives


First choice:

Don’t use preservatives unless required by BS 5589 AND BS 5268:Part 5.

Second choice:

If you have to use preservatives, use Boron based compounds. Borate Oxide (SBX) - Boron compounds are well known, non-toxic preservatives, but are water soluble. Treatment can only be carried out on green timber with a moisture content of over 50% (useful for UK sourced timber)

Not so good:

Cu & Zn Naphthanates, Acypectas Zinc. Recently developed alternatives to CCA (Copper Chrome Arsenate): ammoniacal copper quartenary (ACQ), copper azole and copper citrate.


Creosote, Arsenic, Chromium Salts, Dieldrin (banned in the UK), PCP, Lindane, Tributyl Tin Oxide, Dichofluanid, Permethrin, CCA (The EC restricts its use in buildings other than housing).


2 Durability without preservatives


Insect and fungal attack happens when:

• wood is poorly dried
• wood becomes wet and fails to dry out


Use naturally durable wood

For unfinished external joinery use

• European Oak (durable)
• Sweet Chestnut (durable)
• Western Red Cedar (European grown) (moderately durable)
• Larch (Moderately durable)
• Douglas Fir (Moderately durable)
( more information on species )

- Use seasoned wood

- Avoid using sapwood, especially when using moderately durable wood

- European Oak and Sweet Chestnut contain a great deal of tannin. This will initially stain but then wash off. However, use corrosion resistant fixings (eg ss) and protect surfaces below during this period.

- Be aware of expanding and shrinking glazing beads due to moisture content differential. The glazing sealant used should be able to expand into the gap between glazing and beading.

- Unpainted wood will gradually lose it's original colour and fade to a light grey.

For unfinished cladding use

• European Oak (durable)
• Sweet Chestnut (durable)
• Western Red Cedar (European grown) (moderately durable)
• Larch (Moderately durable)
( more information on species )

- Avoid using sapwood, especially when using moderately durable wood

- ‘Green’ wood can be used and dries naturally. Allow sufficient fixings to absorb shrinkage.

- European Oak and Sweet Chestnut contain a great deal of tannin. This will initially stain but then wash off. However, use corrosion resistant fixings (eg ss) and protect surfaces below during this period.

- There is a relationship between cladding longevity and the depth of eaves. 600mm should be the optimum

- Design a splash zone of no less than 150mm above the ground

- Ventilate behind the cladding

- Ensure the sawn ends are treated


Use seasoned wood

- Drying wood below the fiber saturation point renders it impervious to fungal attack

- Where possible, use air-dried wood in preference to the more energy intensive kiln dried wood. An emerging alternative is wood dried in solar-powered kilns


Durability through design

The design of a building and its components can greatly influence both the performance of the timber and its finish. The main design considerations are to ensure drainage of water and to minimise the absorption of water by wood, noting that end grain is particularly vulnerable.

Protect against surface weathering:

- use durable timber (see above)

- apply protective coatings

- provide shelter and shielding


Protect against wetting:

- separate wood from wet materials

- prevent water traps and capillary paths

- protect from direct wetting

- protect from indirect wetting

- design joints for anticipated exposure conditions

- minimise the surface area on which degradation may occur.

- make details so that there are less cavities and hidden pockets for water to collect in.


Protect from indirect moisture build-up:

- maintain a constant heating cycle

- provide adequate ventilation in areas where high temperatures and humidity occur

- avoid interstitial condensation


Heat treatment

Originating in Finland, the heat treatment of timber is still a relatively new concept in the UK. The treatment involves heating the timber in an oxygen depleted at temperatures between 100 – 150C.
Heat treatment changes wood properties permanently. In particular, it improves its resistance to rot and mould by changing timber's internal structure so as to prevent fungi attack.


Recycled plastic lumber

Recycled plastic Lumber (RPL) is a wood-like product made from recovered plastic or recovered plastic mixed with other materials, which can be used as a substitute for concrete, wood, and metals.

The technology for manufacturing recycled plastic lumber has been in place since the early 1980s. Plastic lumber is most commonly manufactured from post-consumer high density polyethylene (HDPE), but linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) are also used. These plastic feed stocks are derived from such raw materials as post-consumer milk jugs, soda bottles, grocery bags, plastic wrap, bubble rap, detergent bottles, and water bottles, and other used plastic commodities.

The recovered plastic is cleaned, shredded, and ground. The material is then melted and mixed with foaming agents, UV stabilizers, and color pigments. Then it is molded or extruded into lumber or forms. The product is then cooled and inspected. It is now transformed into 'Recycled Plastic Lumber'. The lumber is formed to specified dimensions through either a molding or an extrusion process. This plastic lumber can then be transformed by consumers and manufacturers into a wide range of products including decks, docks, landscape timbers, parking stops, picnic tables, benches, rubbish bins and planters.




BS EN 350-1: 1994 Durability of wood and wood based products. Natural durability of solid wood. Guide to the principles of testing and classification of the natural durability of wood.

BS EN 460: 1994 Durability of wood and wood-based products. Natural durability of solid wood. Guide to the durability requirements for wood to be used in hazard classes.

External Timber Cladding. DG3/2000, amended 2003.
High Performance Wood Windows. TBL 65. Second edition 2004.
TRADA Wood Information Sheets:
2/3 – 1 Finishes for external timber.
2/3 -10 Timbers – their properties and uses.
4 – 14 Moisture in timber.




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