Choosing the right preservative treated timber

Hazard Classification Classes in SA

Choosing the right preservative treated timber

As with most commercially grown timber, South African plantation grown pine and eucalyptus are not durable and are therefore subject to attack from biological wood destroying agents such as fungal decay and insects (woodborers and termites). This is why it is necessary to 'preservative treat' timber with wood preserving chemicals if you require confidence in its performance. The South African wood preservation industry adapted the international classification systems and established the H Class system for different end applications of treated timber, as given in the SANS standards.

The purpose and exposure conditions for which the timber is being purchased defines the treatment required. A piece of timber to be used in the roof need not be treated with the same amount of chemical as a piece of timber being used for a jetty. The treatment level changes with different applications. The chemicals need not penetrate to the same depth, nor need the preservative solution uptake be the same. These two factors are called penetration and retention respectively.

The main chemicals used in this country are CCA (Copper Chrome Arsenate) as a waterborne preservative and Creosote, which is coal tar based. CCA gives the timber a greenish look because of the copper content, whilst Creosote leaves the timber pole dark brown to black. Other chemicals available from only a few selected processing plants, are CuAZ (Copper Azole) and Boron, both waterborne, and ZP (Azole permethrin) as a light organic solvent preservative (LOSP).

To be effective preservative chemicals cannot be brushed on. Seasoned timber is put into a treatment vessel and through various processes (usually vacuum and pressure cycles), the chemicals are deeply impregnated into the timber, thus obtaining the correct penetration depth and chemical retention for the predetermined hazard class. Boron may, under specific conditions require increased moisture levels in the timber, also be applied by means of a dip-diffusion process.

In South Africa we have 5 hazard class levels of treatment provided for in the SANS standards.

 

H2 INTERNAL (Low Hazard)

This is for interior use only and timber treated under this classification should be roof trusses, laminated beams, internally used structural timber, ceiling boards, flooring, panelling, doors, cupboards, skirting, window frames and plywood. Chemicals used here would be mainly waterborne e.g. CCA and Boron as well as LOSP preservatives.

H3 EXTERIOR ABOVE GROUND (Moderate Hazard)

CCA and Creosote are mostly used for this hazard class. H3 covers balustrades, fencing bearers and slats, outdoor decking and beams, garden furniture, laminated beams, weather board, steps, cladding, stairs, log homes, gates, fascia boards and plywood. Spacers and cross arms used with electrical, distribution, telephone and light poles are treated to H3.

H4 GROUND CONTACT (High Hazard)

This level of treatment helps prevent agricultural posts and landscaping structures from rotting and termite attack. Also recommended for treatment in this hazard class are playground structures, fencing, pergolas, carports, flower boxes, decking, bridges and stakes, as well as electrical, distribution, telephone and lighting poles.

H5 FRESHWATER AND HEAVY WET SOILS (HIGH HAZARD)

Timber which falls into this category, is timber exposed to continual wetting or where the timber is planted in wet soil. Timber which will fall into this category could be jetties, drains, walkways, retaining walls and slipways. Agricultural poles exposed to frequent irrigation and fertilisers are also included here as well as poles used as piling and foundation poles for permanent structures, such as timber homes.

H6 MARINE (High Hazard)

Only the use of the CCA chemical with Creosote (dual treatment) is recommended for this application. Only timber treated that is suitable to obtain the required penetration and retention with both these chemicals will offer complete protection against marine borers. Jetties, slipways, retaining walls and walkways will fall under this section.


In South Africa all treatment plants producing treated timber are required to be approved by the National Regulator for Compulsory specifications and must be monitored through an approved third-party product certification scheme. All treated timber is required to show not only the product certification mark of one of the approved certification bodies, but also to which H class and SANS standard the timber has been treated to.

Your treated timber will be marked with one of the symbols listed below:

HAZARD CLASS SYMBOL H2 H3 H4 H5 H6
END USE APPLICATION (typical examples) Internal External Above Ground In Ground Contact In Fresh Water / Wet Soils In Sea Water
Laminated
Beams

Roof Trusses Structural
Timber

Ceiling Boards Flooring
Panelling
Doors
Cupboards
Skirting

Window frames

Plywood
Balustrades

Fencing bearers and slats

Outdoor decking and beams

Garden furniture

Laminated beams

Weather board

Steps
Cladding
Stairs
Log Homes
Gates
Fascia boards
Plywood
Droppers
Landscaping structures

Playground structures

Fencing
Pergolas
Carports
Flower boxes
Decking
Bridges
Stakes
Garden Edging
Transmission Poles
Piling/foundation posts/poles
Agricultural & feedlot posts/poles
Retaining Walls
Slipways
Culverts
Groynes
Flood Gates
Jetties
Drains
Walkways
Piling
Retaining Walls
Slipways
Groynes
Jetties
Walkways