Performance
Durability
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.
Mechanical properties
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 (20x20 mm cross section) are:
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Modulus of elasticity
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10 GPa
|
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Bending strength
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78 MPa
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Density @ 12% moisture content (MC)
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470 - 480kg/m3
(varies 380 - 510kg/m3 across country) |
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Hardness
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3.6kN
|
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Tangential shrinkage - green to 12% MC
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3.5 %
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Radial Shrinkage - green to 12% MC
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1.8 %
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Machining
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.
Technical documents
Douglas-fir Design Stresses - Printable PDF
Douglas Fir Association Alternative Solution - Printable PDF
External wall building solutions using Douglas fir - Printable PDF
Handy reference table: Where can I use Douglas fir? - Printable PDF
Douglas fir stability - Printable PDF
Risk Matrix - Printable PDF
Building benefits of Douglas fir - Printable PDF
Formaldehyde - Printable PDF
Timber Treatment DBH - Printable PDF
New Zealand FRI Bulletin 216: New Zealand Radiata Pine and Douglas Fir – Suitability for Processing (publications order form) - Printable PDF
