As well as being popular for light timber framing, the larger dimensional stock is sought after for exposed interior posts and beams because of its good stability and freedom from twist. Glue lamination to produce beams, arches and scaffold planks is also common.
Sustainability of supply
New Zealand’s Douglas-fir forests have helped to reduce soil erosion, improve water quality, increase biodiversity and lessen greenhouse gas emissions into the atmosphere.
All New Zealand growers of Douglas fir who belong to the Douglas-fir Association are members of third-party eco-certification schemes, with most members being certified by the Forest Stewardship Council.
New Zealand-grown Douglas-fir timber was first available in the late 1940s. By 1960, some 60,000 m3 was sawn per annum, growing to over 170,000 m3 by 1988. Today, about 113,000 ha of Douglas-fir is grown in New Zealand – mostly in the South Island – supplying an annual production volume of 155,000 m3 of rough-sawn timber, about 3.5 percent of all New Zealand production timber.
During the past decade, the stocked volumes of Douglas fir have increased, providing assured ongoing supplies throughout the South Island and central and lower North Island. This represents 6 percent of the total plantation forest of New Zealand.
Age Class Distribution (.pdf)
Botanical name: Pseudotsuga menziesii
Other common names: Douglas-fir, Oregon
Strength: Douglas-fir is an excellent framing timber.
Durability: Douglas-fir heartwood is rated as Class 3 durability and can be used for above-ground purposes, but the sapwood is non-durable. The timber cannot be pressure treated with copper, chromium and arsenic (CCA), but can be boron treated to Hazard Classes H3.1 and H1.2.
Finishes: Douglas-fir is easy to stain but difficult to paint.
Working properties: Douglas-fir requires care when machining because of its uneven growth-ring texture. The use of slower feedspeeds, sharp cutter knives and a low cutting-knife angle will help to achieve good machining results.
Appearance: Douglas-fir displays prominent growth-ring bands between the earlywood and latewood. The heartwood is a pale-pinkish colour and the sapwood is near white.
Douglas fir is easily identified by the prominence of growth ring bands between the early and latewood, its pale pinkish coloured heartwood and near-white sapwood.
The latewood and earlywood bands are of considerably different densities and of fairly consistent spacing.
A high proportion of the wood of the tree is heartwood, which is easily distinguished from the sapwood by its pinkish brown colour.
The tree is increasing in available supply, particularly from the Southern regions.
Douglas fir heartwood is rated as Class 3 durability, i.e., average life in ground contact of 5-15 years.
In situations away from ground contact but fully exposed to the weather Class 3 durability indicates that an average life of 15-25 years is likely.
In partly protected situations particularly on vertical or semi-vertical surfaces where water is rapidly shed, a significantly longer life could be expected.Douglas fir sapwood, however, is non-durable.
Douglas fir is available Green or Kiln Dried as untreated or boron treated to H1.2 hazard class.
Use of treated Douglas fir
Douglas fir is not suitable for exterior exposed and in-ground applications, and can not be pressure-treated with CCA to H3.2 hazard class. The standard NZS3602:2003 states Douglas fir is not suitable for exterior cladding or exterior window and door jambs.
Although boron treatment to H3.1 hazard class is readily achievable, it is currently not commonly used.
There are currently no H3.1 treatments approved for Douglas fir so it therefore cannot be substituted for H3.1 treated radiata pine where specified accept through an Alternative Solution, where it may be acceptable for use in weatherboards, fascia and jambs.
When used as framing, boron treated hazard class H1.2 Douglas fir framing is able to be used wherever H1.2 treated radiata pine framing is acceptable.
Use of un-treated Douglas fir
Douglas-fir has been use untreated in house framing in New Zealand since the 1940’s and is traditionally used untreated in house framing in North America and elsewhere throughout the world.
Under the current Acceptable Solutions Untreated Douglas fir can be use in exterior framing in single storey buildings which have masonry cladding (refer to Clause 110 in NZS3602).
Also it can be used untreated for all trusses except in skillion roofs.
All internal framing (except where supporting decks and balconies), internal linings and trim, and internal finishing may use Douglas fir untreated.
The large heartwood zone is not prone to insect borer attack, and therefore the timber can be used in house framing and roof trusses to H1.1 Hazard Class without the need for treatment against insect infestation.
The Department of Building and Housing Guidebook for Builders outlines where Douglas fir can be used, and to what treatment level, for different building members (see below under technical documents).
The Douglas-fir Association has also developed a guideline for untreated Douglas-fir to be used as an Alternative Solution in external framing in low risk buildings. (alternative solutions)
Douglas fir as structural round timber
Douglas fir log buildings that are well designed, constructed and maintained have remained in good condition for 25-30 years.
There is no reason these should not achieve a service life of 50 years or beyond.
Where log buildings have failed in a shorter time period, these have been due to the following failure points:
- Poor protection of logs between felling and installation resulting in badly stained, insect infected or partly decayed logs being installed in the building
- Long construction times, sometimes years, with logs left exposed to the weather during much of that time.
- Design features, which left logs fully exposed to the weather such as, narrow or no eaves, high gable ends on two storey buildings with no protection for lower storey walls.
- Untreated logs used in parapet walls, balcony balustrades and associated structures attached to the building but not protected by the roof.
- Untreated logs extended out with strip verandah or balcony decking nailed directly to them.
- Once only or no application of preservative/protective coatings. Inappropriate protective coatings such as clear polyurethane.
Douglas fir can be used both structurally and decoratively.
Around 60% of NZ grown Douglas fir is strong enough for engineering uses, selected using machine stress grading. Please note these comparative measures of strength are “laboratory” values using standardised short lengths of clear timber.
These will not be the same strength properties as structural lengths of timber.
For the properties of structural length timber please see the tables under the structural design section.
For 12% moisture content (MC) defect-free timber, of average density from trees felled at 30 years of age, the average properties measured on small specimens (20×20 mm cross section) are:
Modulus of elasticity 10 GPa Bending strength 78 MPa Density @ 12% moisture content (MC) 70 - 480kg/m3
(varies 380 - 510kg/m3 across country)
Hardness 3.6kN Tangential shrinkage - green to 12% MC 3.5% Radial Shrinkage - green to 12% MC 1.8%
The low longitudinal shrinkage (0.1%) means that once dry, the timber tends to hold its shape.
The uneven growth ring texture (due to the differences in earlywood and latewood densities) does require machining to be more exacting for appearance uses.
Low density early wood tends to be more prone to pick out in machining operations (planing, turning, shaping).
Grain deviation around knots can result in chip-out when planing and raised grain can occur from compression of early wood in flatsawn boards during dressing.
Slower feedspeeds and greater maintenance of cutter sharpness is required. Lower cutter knife angles (around 20°) tend to reduce chipout .
Douglas fir is prone to splitting when nailing, and nails tend to follow the latewood band leading to nails deflecting from the intended nail direction.
Pre drilling before nailing is recommended for visible connections such as exposed beams and rafters.
Douglas-fir Design Stresses (.pdf)
Douglas fir stability (.pdf)
Risk Matrix (.pdf)
Timber Treatment DBH (.pdf)
Douglas-fir can be used for:
- Roof trusses and framing.
- Internal panelling.
- Glue laminated beams.
As well as being popular for light timber framing, the larger dimensional stock is sought after for exposed interior posts and beams because of it relatively good stability and freedom from twist.
Glue lamination to produce beams, arches and scaffold planks is also common.
Gluing and coating considerations
Douglas fir is very easy to stain but difficult to paint, with priming coats being rapidly absorbed.
Douglas fir is suitable as a laminated product, and can be used for glu-laminated beams and rafters.
Douglas fir is a fairly dry timber when first felled, with a heartwood moisture content of 50%.
This allows the timber to be dried quickly, and with less energy than radiata pine.
- Case Study
- Fiordland Lodge
An accommodation lodge and resort, the use of Douglas fir offers a warm and homely atmosphere to guests.
Bordering the Fiordland National park, the clients felt it was important that the building reflected the forest environment.
Douglas fir was chosen at an early stage of the building design, as it allowed the log-cabin design, similar in design to ones in Alaska that had impressed the clients.
It provided the ability to create a similarly impressive log-cabin design in traditional North Amercian techniques with locally grown New Zealand timbers.
The striking natural timber construction complemented by riverstones echoes the outdoor environment.
Sun dried and hand peeled New Zealand Douglas fir logs feature throughout the building with exposed log beams and log trusses supporting sarked timber ceilings.
Owners Ron and Robynne Peacocke state that the distinctive colour and texture of Douglas fir was the main reason for the use throughout the ceiling sarking, over other options.
Interior finishing of the smaller log cabin accommodation is with recycled native New Zealand timber for floors, ceilings, joinery and bench tops.
The Lodge was designed by American log-builder, the late Norman Rhyne of Wanaka.
- For detailed information on vendors supplying Douglas-fir including their location in New Zealand and products supplied refer to the Suppliers Database or go to NZ Farm Forestry Association Marketplace for availability.