Protection: preservation Understanding how to protect timber

Understanding how to protect timber

Kevin Underwood explores the external factors that can have a negative effect on timber and ways that timber can be protected.

External timber should be protected to offer the best service life. Photo: George Barnsdale

“Timber systems will benefit the most from high levels of protection from precipitation, wind and direct sunshine, for example windows and doors seated under deep eaves or recessed into walls or sheltered by balconies.”

Timber is an abundant, inherently non-toxic, recyclable, biodegradable, sustainable material. However, it does not come without its natural enemies.

Timber is at risk of attack by some fungi and insects that can affect its appearance, strength and long-term performance as a construction material. Employing protective measures allows us to: • increase its service life • improve its characteristics • make it more competitive against other materials.

Classifying wood

Timber is essentially composed of hollow fibres of cellulose, set in a matrix of lignin with discreet chemical extractives and moisture. This ligno-cellulosoic arrangement of components varies, creating a wide range of timber species, each with different appearances and performance characteristics.

Extractives give timber species their colour, smell and their level of natural resistance to attack by fungi and insects. Resistance to fungal or insect attack varies between species.

The inner heartwood contains extractives, but the outer sapwood, which is extractive free, is rich in starches and sugars; this limits the natural resistance of all sapwood to fungi and insects, should the conditions for attack be favourable.

BS EN 350:2016 1 gives guidance on methods for determining and classifying the durability of wood and wood-based materials against biological wooddestroying agents such as wood decay fungi. Annex B of BS EN 335:2013 2 defines five use classes that represent different service situations to which wood and wood-based products can be exposed, and indicates the biological agents relevant to each situation.

A use class is not a performance class and does not give guidance for how long wood and wood-based product will last in service. >>

Table 1: Natural durability of commonly used structural timbers according to BS EN 350. Wood shrinks to varying degrees along the three dimensional axes: very little along the grain and much more across the grain. The shrinkage in the direction of the growth rings (tangentially) is often up to double that at right angles to them (radially). As a result of this, and of irregular, interlocked or spiral grain, various forms of distortion are liable to occur as the piece of timber dries.

It is desirable that all the shrinkage and distortion has occurred before the timber is put to use and this is one reason why timber should be dried before it is used.

The table above, taken from WIS 2/3-71,

3 shows the relationship between use class and natural durability.

Moisture in timber

Wood has a cellular structure and the moisture in green, unseasoned timber is present in two forms: • as free moisture in the cell spaces • as bound moisture in the cell walls.

In drying, the free moisture in the cell spaces is the first to leave the wood, moving to the surfaces and evaporating. With further drying, the bound water leaves the cell walls and the wood starts to shrink and stress and distortion can develop.

The point at which the cells no longer contain free moisture is called the fibre saturation point and, from this stage of drying, if the wood fibres are free from constraint they will shrink to an extent that is roughly proportional to the loss in the bound moisture. In practice, the surface layers of any piece of timber tend to dry to the fibre saturation point before the centre and are brought into a state of tension. If the moisture gradient from the centre to the surface is too steep, splitting and checking may occur.

Fungi

Fungi can discolour timber, cause decay and reduce its strength. They are a form of plant life and require moisture, warmth, air and a suitable medium on which to grow. Unlike green plants fungi do not require light as they lack chlorophyll and are unable to build up organic matter from the carbon dioxide in the atmosphere. Fungi must have organic matter on which to live so they grow directly on living plants or on their dead remains. They cannot live on purely mineral soil.

Fungi decompose wood by secreting acids and enzymes, which, in the presence of moisture, render soluble some of the cellulose and other constituents of the wood. These are then used as nutrients by the fungus. The wood may be changed in composition and texture and lose strength before it is actually absorbed by the fungus.

The characteristic that usually determines whether timber in use remains sound or becomes decayed is its moisture content. Freshly felled green timber may contain its own weight of water and, unless this moisture is removed, decay can quickly become established. If dry timber is allowed to pick up moisture, or if it is used in contact with the soil, it again becomes susceptible to attack.

Fungi require air and respire to form carbon dioxide and water so, once an attack is established, the wood tends to get moister, accelerating the growth of the fungus.

In general, timber is susceptible to attack at any moisture content between fibre saturation point and complete saturation of the wood, which is when the cell cavities are filled with water.

Moisture content greater than 25% is required for active sapstain development, whereas surface mould growth can >>

Impra Wood Protection

Impra Wood Protection is the trading name for RÜTGERS Organics GmbH which is part of the ICIG, a group of companies based in Frankfurt, Germany. Within the ICI Group there are 30 chemical and pharmaceutical companies operating worldwide, turning over circa €2bn and having approximately 6,000 employees.

The Impra Wood Protection manufactures at two locations in Europe at Mannheim in Germany and Barrow in Furness in the United Kingdom. Within the group there are also distribution companies in Germany, Finland and Poland with many more partners worldwide. We believe with our diverse manufacturing capabilities and having plants in both the UK and Germany we have a business model to suit all businesses throughout Europe and the UK. Impra Wood Protection prides itself on a high level of research and development, which helps to keep the products and services at a high level meeting and exceeding the requirements of some of the most difficult applications within the timber industry, with products that have performed exceptionally in the past and will continue to do so well into the future. Impra Wood Protection works closely with its clients and approval authorities throughout the UK to stay ahead of the competition by providing solutions, which will keep its clients at the forefront of their markets, thus giving them many advantages over their competitors. Impra Wood Protection has two distinctive product ranges for both Wood Treatment and Coatings. The impra®lit range is made up of products that are suitable for either vacuum pressure impregnation, dipping and low pressure impregnation, protect against blue stain and mould with the addition of a large range of colourants. impra®lan and profilan® are a range of Wood Coatings for either industrial or professional use and are either solvent or water based depending on the application. These are available in a range of sizes and many different colours and can be applied by either dipping, flow coating or brushing.

Impra Wood Protection Limited

Park Road Industrial Estate / Barrow in Furness Cumbria LA14 4EQ / United Kingdom info@impra.co.uk / +44 84 4800 9665 impra.co.uk

UV and moisture levels can affect the durability of timber. Photo: AkzoNobel

become established at approximately 18%. Both sap-stain and fungi feed on cell contents and stored food reserves in the timber (i.e. starches and sugars), and are therefore generally confined to sapwood. If high relative humidity persists and moisture content increases, sap-stain or mould growth can thrive and cause deterioration of protective coatings in service. The wood-destroying wet-rot and dry-rot fungi can develop when the moisture content of the timber is above 20%.

The first line of defence is to keep timber moisture content below 20%. In timber used externally above ground, this can be achieved by ensuring the design of a product: • does not include water traps • promotes the shedding of rainwater to minimise wetting • promotes air movement.

Timber used internally is unlikely to reach this level of moisture content unless the timber is affected by water from, for example, a leaking pipe or roof. The spores of wood-decaying fungi (such as wet-rot and dry-rot) or staining fungi (such as sap-stain, blue stain and surface mould) are so widespread that preventing contact with these spores is not possible.

A long service life for timber in an environment suitable for fungal development depends on the level of natural durability of the timber species used or the effectiveness of any preservation treatment that has been employed.

Insects

Timber can become unserviceable due to the natural activity of insects, which tunnel into and devour the timber. However, with our relatively cold climate, timber in outdoor environments in the UK is very rarely affected by insect pests; their variety and veracity are not as great as that seen in warmer parts of the world.

Where insect attack is a concern, preservative treatment with a contact insecticide provides a practical preventative measure.

Preservative treatment

Where timber is susceptible to fungal decay or insect attack, it can be treated with a wood preservative. The process of selecting both the type of preservative and its method of application takes into account: • the natural durability and treatability of the timber species • the level of exposure to moisture • the desired service life • the organisms against which the preservative is to provide protection.

Modified wood

It is possible to alter the chemical and structural properties of wood to enhance its natural durability and to reduce the amount of movement that may occur due to variations in moisture content. Changing the structure of wood and/or limiting the availability of nutrients reduces or prevents fungal development and insect attack.

Timber modification processes include: • heat treatment • impregnation of timber with resin • acetylation.

These processes essentially create a new timber product with enhanced durability and dimensional stability. These relatively new technologies provide a wider choice of material for designers and architects to consider – popular for cladding, decking, windows, external doors, landscaping and structural applications.

Protection from coatings

Well-maintained brush or spray-applied paints and stains can extend the service life and use of timber in outdoor conditions. These surface coatings enhance the appearance of the wood and protect it from deterioration as a result of weathering. >>

By limiting the take up of moisture, protective coatings can also limit dimensional changes in the wood and protect against stain fungi and surface moulds.

While providing protection to the timber, coatings themselves are at risk and need to have properties that make them resistant to ultraviolet light and any airborne chemicals present in rainfall or dew forming on the surface. They also need to be able to accommodate any movement in the timber.

Temperature affects the rate of chemical reactions, and this is influenced by the colour of the coating, the angle of exposure and other factors such as the specific heat and mass of the coated timber and ventilation due to air movement.

Direction of exposure

Levels of solar radiation, humidity, temperature and precipitation can vary considerably and will greatly influence the performance of a coated timber system. The impact of these factors is determined by the direction of exposure.

In the UK, south- and west-facing elevations are generally more demanding for coatings, due to their longer hours of exposure to direct and stronger sunlight than east- and northfacing elevations; the risks of mould and algae growth will generally be highest on north-facing walls. The use of darker coatings, which absorb more solar radiation, and become hotter, can exacerbate this effect.

Shelter

In practice, exposure conditions will depend not only on climate but also on the degree of shelter offered by the construction. Timber systems will benefit the most from high levels of protection from precipitation, wind and direct sunshine, for example windows and doors seated under deep eaves or recessed into walls or sheltered by balconies.

The inclination of the surface is also important. Decreasing the angle of exposure from vertical towards horizontal greatly increases the intensity of weathering for sheltered as well as less-sheltered parts.

Sustainability

The preservation of timber allows the specification of softwood species with lower natural resistance to fungi, as well as all sapwood, where these might otherwise not be used or offer only a very short service life outdoors. Making the most of the timber resource and contributing to waste minimisation and sustainability is imperative to retaining timber as a viable and easily obtained building material. In addition, this reduces pressure on the more naturally durable, scarcer and higher value species. n

About the author

Kevin Underwood Technical Director British Woodworking Federation

References

1. BS EN 350:2016 Durability of wood and wood-based products.

Testing and classification of the durability to biological agents of wood and wood-based materials, BSI, 2016

2. BS EN 335:2013 Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products, BSI, 2013

3. WIS 2/3-71 Specifying externally exposed structural timber,

BM TRADA, 2019

Further reading

• WIS 2/3-1 Finishes for external timber, BM TRADA, 2020 • WIS 2/3-16 Preservative treatment for timber – a guide to specification, BM TRADA, 2021 • WIS 2/3-32 Fungi and insect pests in timber, BM TRADA, 2021 • WIS 2/3-60 Specifying timber exposed to weathering,

BM TRADA, 2021 • WIS 2/3-63 Modified wood products, BM TRADA, 2021

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