Roundwood

Availability

Sizes

Poles are available in a wide range of sizes. The diameter is usually called up as “Small End Diameter” (SED) with common sizes being from 125 to 350mm, in 25mm increments.

Lengths are usually in 0.6m increments up to 6.0m long, then in 1.0m increments up to 12.0m or more. Sizes up to 18m in length and 600mm in diameter are available as special orders.

Species

Radiata pine is very suitable for treating as it has a large layer of sapwood.

The heartwood at the centre of most logs is difficult to treat and the penetration of the chemicals is often low (10 – 12 mm).

In general the core of a pile or pole does not become well treated. An alternative to radiata pine was Corsican pine, but it has not been planted for some years due to its slow maturing time, and susceptibility to dothistroma needle blight.

The strength of a radiata pine log is partly related to the rate of growth (the slower the growth the stronger the log).

Forests with closely spaced trees tend to have slower growth rates, with smaller knot sizes and hence yield higher strength poles than more intensively managed forests.

Excellent poles were obtained from early pine forests planted in a close spacing regime, and not pruned nor thinned. In the past, such forests (after growing some 30 to 40 years) produced thin, tall, straight trees (20 m to 30 m) ideal for the pole and post industries, but these forests are now rare.

Faster growing managed forests produce pole material for larger sizes above 200mm SED, with some of the best poles coming from slower growth trees planted in sand dunes.

Unless the designer is confident of the supply of high strength poles or the construction process is in a very controlled situation, it is wise to design for poles of the lower strength group.

Non availability of high strength poles during construction can delay the project and involve substantial redesign and disruption.

Suppliers Database

Applications

Applications include:

• Pole and pile foundations for houses.

• Pole frame or pole platform houses.

• Pole-truss industrial and agricultural buildings.

• Uniform diameter pole structures.

• Retaining Walls.

More information on Applications for Roundwood - Printable PDF

Manufacture

Most poles available in New Zealand are radiata pine.

Some slower growing softwood species such as Corsican pine have been preferred for poles in the past, but their availability is decreasing and there is not a significant difference in strength between softwood species.

NZS 3603 does not distinguish between species but relates characteristic stresses to the basic density in the outer zone of the pole.

Basic density is defined as the oven dry weight of the wood divided by its volume in the green condition.

The outer zone is defined as the outer 20% of the radius of the pole. The data on which design data are based is shown in below where the solid symbols represent average values from tests and the circles and crosses represent individual results.

Bending strength - density relationship in green unpeeled softwood

Source: Courtesy Timber Design Guide, 2007

Performance

The PDF below on Performance includes:

• Pole properties.

• Special aspects to using roundwood.

• Strength.

Roundwood Performance - Printable PDF

Durability

Roundwood is often used in contact with the ground or exposed to the weather.

Because New Zealand grown radiata pine accepts preservatives very easily, properly treated poles can be expected to last 80 years or more in contact with the ground.

However, it is important in construction that notches and cuts in the wood do not expose the less highly treated wood in the core of the pole.

Treatment

Most poles are steam conditioned, then pressure treated with CCA preservatives to ensure durability in outdoor exposure, in accordance with specifications of the Timber Preservation Council.

The most common treatment classes are:

• H3 – Sawn timber for exterior exposure, not in contact with the ground.

• H4 – Sawn or round timber fence posts, agricultural posts, vineyard posts, horizontal sawn timber rails for retaining structures where replacement is relatively simple and easy.

• H5 - House poles and piles, retaining wall poles, transmission poles, horizontal sawn timber rails for retaining walls where replacement is a major exercise. Some lightly fertilized horticultural soil has a fungus that decays the poles at ground level, which can be managed by wrapping the pole with a plastic bag near the ground surface.

• H5b- Slightly higher retention than H5, marketed by some companies specifically for buildings. Only H5 or H5b treated poles are normally used for buildings and retaining walls.

• H6 - Marine exposure, to increase life to 50 years from general 20 year life, the wet area above ground level may be covered with a double layer of densotape and protected with plastic tape (although the practicality of this suggestion should be borne in mind for each situation).

All cuts and notches made after treatment should be brush or spray coated with a copper napthenate preservative, in case untreated wood has been exposed by the cutting.

This includes boards of retaining walls.

Copper napthenate preservative can be sourced from timber and hardware stores, along with other proprietary products (not necessarily containing Copper Napthenate) such as EnsealTM Green (both brush on and aerosol), Metallex Green and Cusol. Refer to the manufacturers’ literature for appropriate uses.

When tops of poles need to be cut on site, they should be cut on an angle or be provided with a metal cover for surface water runoff.

Exposed ends should be sprayed or painted over the cut area with copper napthenate preservative. Bollards with a flat top must be capped with copper or stainless steel plates.

Corrosion

Galvanised steel in contact with wet CCA treated timber is likely to corrode.

This problem is particularly severe with wet treated wood.

The recommended protection is to use a hot-dipped galvanizing process after final manufacturing of the steelwork.

Standard zinc-plated products normally used for internal nailplates and strip bracing are inadequate for connections exposed to the weather and in contact with H5 treated timber.

The worst areas are those that will be exposed to moisture over a long period.

This includes poles close to the ground line and all connections exposed to rain. In these areas, the metal should be protected from the wood with Densopaste or Densotape.

Powder coating has limited success as adequate surface coverage varies on edges and the coating can easily chip before or during installation. Heat-shrink plastic tubing works well on bolts but cannot protect the threads.

Densopaste is excellent, and is more permanent than multi-purpose grease. In severe corrosive situations where salt air or excessive moisture is present, connections may be required to be manufactured from stainless steel, either Type 304 or Type 316.

Exposed connections must be regularly inspected and maintained throughout the life of the structure.

A wide selection of fixings, coatings and advice is generally available from leading builders supply merchants nationwide.