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 Thermally Modified Hardwood Timber Decking Trial

Special thanks go to Vaughan Kearns of Ruapehu Sawmills for supplying the timber and covering the costs of thermal modification and transport.

Please support Ruapehu Sawmills as sponsor of this trial. Ruapehu Sawmills supply a large range of specialty timbers nationwide.
You'll find them in the Marketplace.

This trial was put in place in December 2013 to test thermally modified Eucalyptus regnans as outdoor decking timber.

Hardwood decking is in high demand, traditionally supplied as tropical rainforest timbers such as kwila, teak and jarrah. Recognition of the non-sustainable and untrustworthy nature with which many tropical forests are managed and harvested provides the opportunity for substitution with plantation timber. Softwood timbers such as radiata pine are no substitute for quality hardwood. Hardwood will remain in demand for outdoor applications primarily because of its unique appearance qualities.

Dark brown colour of thermally modified timber

Unfortunately, fast-growing cold-climate Eucalyptus species grown in New Zealand such as E. regnans, E. delegatensis, E. obliqua, E. fastigata and E. nitens do not meet the durability requirements of the New Zealand building code for use as decking, which has the requirement of 15 years minimum service life exposed to the weather.

Thermally modified eucalyptus holds the promise of durability as outdoor decking along with quality hardwood appearance. The decking wood used in this trial was graded to Farm Forestry Timbers Standard Decking grade.

Why undertake this trial?

The Department of Building and Housing require evidence that a new timber product meets the miniumum requirements for durability. There are several ways to determine durability including graveyard tests, in-service tests and accelerated decay tests. Graveyard tests determine in-ground durability with 20mm or 50mm stakes, which provides an estimate of above-ground durability. Accelerated decay tests take place in a controlled cellar environment which speeds up the decay process and provides an estimate of above-ground durability. Of course the best test for durability of decking is an in-service test as decking. However, this will take up to 15 years for results.

The 4.6m2 section of the deck, freshly laid.
Freshly laid deck after rain, giving an indication of the colour if oiled.

Farm Forestry Timbers has begun the process of determining durability of thermally modified E. regnans with an in-service test because this is the premium method. Although this takes time, evidence of durability is required and faster methods are costly and may not be as reliable. By documenting this trial we hope that this will contribute to the accumulated evidence supporting durability of thermally modified hardwood in New Zealand.


Eucalyptus regnans timber was supplied by Vaughan Kearns of Ruapehu Sawmills as dressed ex 125mm timber. Thickness was 19mm and width 110mm. Edges were arrised and both faces were dressed smooth. The kiln-dried and dressed timber was thermally modified at Tunnicliffes during December 2013 using the Thermowood 230 process. Timber was graded to Farm Forestry Timbers Standard Decking grade (Grade revision 1.1 October 2013) prior to laying.

Installation was undertaken during late December 2013 at a Northland property near Kerikeri. The timber was laid to replace a Eucalyptus saligna deck that was failing after approximately 15 years. The situation is very exposed to weather and clearly highly condusive to decay. The same E. saligna in less severly-exposed decking adjacent to the test site has not decayed at all. Joists on which the thermally modified timber was laid were the original H3.2 treated pine joists. These were new when the E. saligna was laid 15 years previously.
Two sections of deck were laid, one of 4.6 m² directly over 15 year old h3.2 joists and the other 4.2 m² over 15 year old h3.2 joists, cleaned and with grooves ground, then coated with one coat of metalex (4g/L Copper present as copper naphthenate in white spirit). On the 4.2m2 section four new pieces of 90 mm x 19 mm H3.2
Purchased from Carters Kerikeri, January 2014
radiata pine were spaced equally as a control. 

Galvanised decking nails (60mm x 3.15 A.G.) were used and nail holes were pre-drilled. Some stainless steel screws were used for comparative purposes. Maximum spacing between joists was 45 cm.


Because the timber was thermally modified as a dressed product, some water staining
Water staining and indentation where the bundle was tied with straps for kilning
was present on the surface. There was some degree of crook in the boards but it was unknown whether this occured previously to, or during the modification process. Some surfaces were raised
Raised surface with internal checking present
, probably caused by expansion of internal checks during the modification process. Occasional cracks
Cracks were present on the occasional board but it is unknown whether this was caused by the modification process
were present, possibly caused by the modification process. Overall the quality was acceptable, but it is suggested that in future rough sawn timber is modified before dressing. Less degrade might be possible by kiln-drying to a very low moisture content before modifying.

Modified hardwood timber is very light and easy to work. The timber is a rich chocolate brown and could either be oiled to retain this colour, or in this case left to oxidise to the classic grey colour which hardwood timber develops over time (photos to come).

Four pieces of 90x19 mm h3.2 radiata pine were put in the deck as a control (lighter colour).
The radiata is distinctive with the deck-tread facing up.
Severe decay and insect damage to the 15 year old E. saligna. This situation is clearly a high decay site. Wet and warm Northland has perhaps the highest rate of decay in NZ.
Some of the worst decay on the E. saligna, turned over to show the underside.
15 year old H3.2 radiata joists cleaned up after being scrubbed with water and a brush. Some decay was present on these.
Grooves ground into the wood and metalex brushed on before installing the decking timber.
E. saligna decking with deep surface checks that cause decay. It is suspected this deck was originally laid without proper drying.
E. saligna decking with no decay after 15 years.


Disclaimer: While every effort is made to ensure the accuracy of the information provided on this site, Farm Forestry Timbers Society do not accept liability for any consequences arising from reliance on the information published. If readers have any doubts about acting on any articles they should seek confirming, professional advice.

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