Light timber framing
The Light Timber Frame Building Code for New Zealand, NZS 3604, is based on the structural properties of average radiata pine. Compliance with NZS 3604, is the prime requirement for the issue of a building consent on the basis of submitted plans. NZS 3604 is the principal acceptable solution (means of compliance) with the New Zealand Building Code, for light timber construction.
NZS 3604 itself calls up NZS 3602 which is the standard for specifying timber and wood products. This code specifies the species, grade and treatment permitted for the various applications in light timber construction. NZS 3602 lists a number of alternatives to radiata pine.
With ample radiata pine and Douglas fir on the market, why should people be looking for alternatives? The most common reason is simply that the trees exist and are being felled. Rather than burn the timber, it seems worthwhile to use it for a higher value use and structural framing is one of the options.
For instance, New Zealand grown Californian redwood (sequoia sempervirens) has been planted in isolated pockets around the Bay of Plenty and is desirable as a durable cladding material. The remainder of the log, unsuited for weatherboards, is often sawn for framing and the question arises as to whether and under what constraints can this framing, which is of significantly lower strength and stiffness than radiata pine, be used for structural components in building.
Another reason is that the price of radiata pine is often related to the export market. When this market is booming there are opportunities of getting lower cost framing by searching for logs of alternative species.
General properties to be taken into account
The main considerations in looking for structural framing substitutes for radiata pine are:-
- Strength grade for grade
- Nail holding and fastening properties
- Stability and shrinkage
- Drying times for closing in
Stiffness is at the head of the list as stiffness determines the spans for most members in light timber framing. The stiffness and strength of timber are compared in this case on the basis of clear wood properties. I.e. the properties of timber without defects. Defects are controlled by grading rules and the defects which are admitted for certain grades can be expected to have similar percentage strength reduction effects on different species.
Strength and stiffness for timber are related, so a species which is stiff enough will generally also be strong enough for direct substitution for radiata pine, grade for grade.
Lower strength species are generally of low density and this affects joint strength. The fastening recommendations of NZS 3604 presuppose timber of given density and nailholding. Lower density species may require additional fastening provisions in critical areas. On the other hand, very high density species may be difficult to fasten without predrilling.
The resistance of timber to insect attack & fungal decay, its durability, is important as a 50 year life is required for structural purposes by the New Zealand Building Code. NZS 3602 permits the use of planed, kiln dried, untreated radiata pine in dry interior above-ground situations. Otherwise, framing members must be heart, or treated to resist insect attack. Many alternative species such as cypresses have a high percentage of heartwood so may be used untreated, but the sapwood of all species, other than radiata pine as specified above, must be treated for structural use. No sapwood of New Zealand grown species may be regarded as durable so, other than kiln dried planed radiata pine, sapwood must be preservative treated for structural use.
Stability and shrinkage, although last in the list above, are important with respect to quality of finish and ease of assembly of the frame. In severe cases timber may be too distorted to be of any real use for building. Shrinkage can also affect joint strength and stiffness and contribute to squeaking floors. The New Zealand timber grading rules5 place limits on the distortion of timber but if frames are assembled green, it is also important that the timber dries without distorting to the extent that it affects subsequent finishing. In general, cypress species are amongst the most stable of the generally available softwoods and some of the eucalyptus species present the greatest distortion problems. It may save a lot of subsequent disappointment if the stability of the timber is checked by sawing a small sample before embarking on a larger project.
Drying times are important where timber is used green and the frame allowed to dry naturally before closing in. Drying depends on weather conditions as well as timber properties. Some species, such as red beech are very slow to dry naturally and special techniques must be used to kiln dry them