Timbers and preservatives

Timber is used for many types of footpath work, and it is important that a suitable timber and method of preservative treatment (if necessary) are chosen where durability and safety are significant factors. Wherever possible, use local sources of timber to support woodland management in your area. For footpath work in woodlands, try to use timber from within the wood itself, even if this means lessened durability compared with alternative sources. Any imported hardwoods, which may be used for bridges, gates and signs, should be from sustainable sources.

Timber characteristics

The following table lists some of the different types of timber available in Britain. The properties of the timber of any one species may vary according to the soil and climate in which it was grown.

Working qualities

This refers to ease of drilling and sawing.

Durability

This is the measure of resistance to decay, and refers to untreated timber.

VD Very Durable +25 yrs
D Durable 15-25 yrs
MD Moderately Durable 10-15 yrs
ND Non Durable 5-10 yrs

Heartwood is generally more durable than sapwood. Footbridges should be made of timber of durability MD, D or VD.

Density

This varies with moisture content. Values given are at 12% moisture content.

Use

These are suggested uses in footpath work:

Species	Working qualities	Durability	Density kg/m3	Permeability	Use
SOFTWOODS:
Douglas fir	Good	MD	529	R	Bridge beams, stiles, gates, posts.
Larch	Good	MD	592	VR (sapwood also VR)	Bridge beams, stiles, gates, posts.
Scots pine	Good	MD	689	R (sapwood P)	Bridge beams, stiles, gates, posts.
Western red cedar	Good	MD	368	R	Gates
HARDWOODS:
Ekki	Difficult	VD	1025	ER	Bridge beams, decking.
Iroko	Med/Diff	VD	641	ER	Signs, esp routed letters.
Keruing	Difficult	MD	721	R	Gates, bridge beams and decking.
Oak	Medium	D	625	ER	Gates, stiles, posts.
Sweet chestnut	Good	D	545	VR	Gates, stiles, posts, boardwalks.
For properties of other timbers, see ‘Timbers - their properties and uses’ (TRADA 1991), from which this table was compiled.

Permeability

This measures the extent to which preservative can penetrate a timber. Sapwood is usually more easily penetrated than heartwood. Round timbers are of sapwood on the outside, and with the exception of larch, are easily penetrated.

ER Extremely Resistant (absorbs only small amount)
R Resistant (difficult to penetrate more than 3-6mm)
MR Moderately Resistant (6-18mm penetration in 2-3 hours)
P Permeable (penetrates without difficulty)

Permeability also depends on the moisture content of the timber. Green (freshly cut) timber contains a higher moisture content and is less permeable than seasoned timber.

Structural grade

Structural timbers (as required for bridges) are stress graded according to the proportion of defects in the sawn section. For softwoods the gradings are divided into GS (General Structural Grade), and SS (Special Structural Grade). Where the grading has been done by machine, the gradings are MGS or MSS. The timber is further defined by species eg Grade GS Scots pine. Hardwoods have a single grading HS (Hardwood Structural). Where the selection of a timber species is not important, timber may be specified by its SC (Strength Classification). For further details see British Standard 4978 for softwood, and British Standard 5756 for hardwood.

Softwoods most commonly used for bridges are Douglas fir, larch and Scots pine, either imported or home grown, and meeting strength classification SC3 and SC4 and higher.
Timbers to strength classification SC1 and SC2 are unlikely to be useful for footbridges. Hardwoods used are mostly imported, such as Keruing and Ekki.

Preservative treatments

There are three factors to be considered:

1. The permeability of the timber (see above).
2. The method of application.
3. The type of preservative.

Methods of application

Application of preservative is better done on timber which has already been cut to size.

Brushing and spraying

This is the least effective method, and should only be used on joints and end grain cut after main treatment, when no other method is possible. Apply liberally, using at least four coats.
If this is the only possible method of application, timber should be re-treated every few years to give satisfactory penetration. Creosote penetrates better if heated before application.

Immersion

Usually called ‘dipping’ for periods up to about 10 minutes, and ‘steeping’ if immersed for hours or days. This is only effective for permeable timber, as resistant timber absorbs little even after long periods of steeping. Organic solvents or creosote can be used for dipping. Creosote is normally used for steeping and is a convenient method for the treatment of estate timber which is often permeable sapwood, such as round fence posts.

It is impossible to give optimum immersion periods, but as a general rule aim at a minimum of five minutes for dipping, and 24 hours for steeping. Dipping is the best method where timber, for example newly cut end grain, has to be treated on site.

Double vacuum

This is a commercial process for applying organic solvents. It gives long term protection against decay and insect attack for timber not in contact with the ground.

Pressure

This is a commercial process, and the most effective method of applying preservative. Even resistant timbers will absorb sufficient preservative. All wood merchants should be able
to supply pressure-treated wood, and may also treat wood supplied and cut by the customer.

Types of preservative

Coal tar creosote to BS 144

This is best applied under pressure, but can also be applied by brush or spray. Creosote helps prevent checking (cracking) of timber exposed to the weather. Highly toxic to wood-destroying fungi and insects. Non-corrosive to metals. Variety of shades of brown. Gives some repellance to water which helps retard dimensional movement. Strong smell, making treated timber unpleasant to handle. Unsuitable for handrails or handrail supports. Stains. Cannot be painted over. Inflammability increased for short period after treatment, but once thoroughly dried, is slightly more fire-resistant that untreated timber.

Water-borne salts to BS 4072

These are mostly copper/chrome/arsenic, such as Tanalith or Celcure, and need to be applied under pressure. They have no smell. Forms insoluble compound in treated wood which cannot be leached out. Gives greenish-grey colour to timber. Can be painted over and glued. Not moisture repelling. Timber swells during treatment and should be re-dried to restore to original size before use as joinery. Non-corrosive to metals, non-staining and non-flammable.

Organic solvent

These are organic fungicides/insecticides in an organic solvent, such as Cuprinol or Solignum. Double vacuum is the most effective method of application, followed by immersion and then brush or spray as the least effective method. Preservative resistant to leaching but some lost by evaporation. Non-corrosive and non-staining. Can be painted and glued. Treatment does not cause swelling of the timber and therefore can be used on accurately cut wood. No fire hazard once solvent has evaporated. Not water-repellent, but additives can be included to resist moisture changes in use. Many colours available. Not suitable for timber in contact with the ground. Suitable for signboards, handrails, gates.

Untreated timber

The heartwood of D or VD timber is not usually treated as it is naturally resistant to decay.

Occasionally timber may be used for footpath work immediately after felling; for example as bridge beams, or for building revetments or steps on a woodland path where timber is to hand. Treatment with preservative is not possible because of the high moisture content of the timber. The saving on the cost of supply must be balanced against its reduced resistance to decay, compared with treated timber.