Trim, taut and terrific?
Wooden trim on a building is not really covered by the Standards, writes Ted Riddle. However, common sense and good installation practices will enhance durability.
A colleague at the Building Designers Association recently asked me about the requirements for using durable timber as a gable trim.
My colleague disputed the use of what he thought was Pacific maple by a builder working on a friend’s new house. He was convinced there was a code regulation that required a durable species.
My understanding of building codes and standards is that they are primarily concerned with structural aspects – the risk to life – rather than aesthetics.
The local council or certifying authority looks at the overall appearance in relation to the streetscape, heritage or urban values, but none of this relates to the species allowed in a trim.
My questioner was looking to reference AS5604 to determine suitability. The material in Timber – Natural durability ratings would have given him the species details but not a determination on individual use.
To get that, I referred him to AS1684 Residential timber framed construction. This is a structural code that also offers recommendations for external exposed timber. It provides informative appendices on timber durability – including notes on naturally durable and treated timber – and species and properties.
In the appendix on durability there is a figure, commonly used in other industry publications, showing the possible placement of timber members and how they can be considered in regard to general durability requirements.
Four classifications are considered:
- in-ground contact;
- external above-ground exposed;
- external above-ground protected; and
- internal fully protected and ventilated.
The notes state: “External timbers are regarded as protected if they are covered by a roof projection (or similar) at 30° to the vertical and they are well detailed and maintained.”
I doubt that a gable trim would fit into the classification of ‘protected’ and should therefore be considered ‘above-ground exposed’.
This classification makes reference to structural elements, such as bearers, joists, decking, posts and handrails. However, it would be appropriate for all external trim.
The other appendix, Timber species and properties, has a table that lists all the elements that must be considered when placing timber. It starts with the trade name and botanical name of the species, then shows working properties and a list of common uses.
This appendix references AS5604 for much of its information. However, the species that raised the issue in the first place, Pacific maple, is not listed. That is because it is a commercial name and not officially recognised.
I wrote extensively about ‘maple’ in the spring issue of Building Connection, so for clarification of the name and which timbers it generally covers, please refer to that publication.
Three shorea species listed in the table (many others are not included) are generally considered to be maple:
- Philippine mahogany dark red;
- Philippine mahogany light red; and
- meranti light red.
Philippine mahogany light red and meranti are not recommended for external joinery in the ‘common uses’ columns, being durability class 4.
Philippine mahogany dark red (durability class 3) is shown as commonly used. I doubt that anybody could distinguish these species from each other when in use, so it would be difficult to enforce a requirement if this was the intent.
However, this section of the Standard is informative only and doesn’t even cover items such as gable trim.
The earlier appendix on durability, in which you might have expected some definitive guidance, is also ambiguous.
In ‘external above-ground exposed’ it throws open the use of all durability classes of timber by indicating that detailing, application and maintenance could make low-durability species acceptable in the applications shown.
This clearly demonstrates that there are no real restrictions in this situation. However, common sense should make builders and designers follow the intent set out in the code.
In my experience over the past 40 years or so, some durability class 4 species, such as Douglas fir, have been used extensively for fascias, barge boards and handrails – and even as cantilevered floor joists (something I certainly don’t recommend).
With appropriate attention during installation (as noted in the Standard) they can perform adequately over long periods.
In Australia we have been a bit spoilt with the ready availability of high-durability species – such as blackbutt, spotted gum, red gum and cypress – and some of our practices have reflected a low appreciation of installation detailing.
Europe and North America are heavily reliant on low-durability timbers, mainly softwoods, and have developed installation practices that allow durability class 4 species in most applications.
Coating and regular maintenance are central to these methods, but detailing and installation will achieve a more than adequate service life for low-durability timbers. These are the real keys:
- avoid moisture traps (water must drain from joints and overlaps);
- allow for shrinkage and movement at fixing points or joints; and
- select the right fixings (low-grade coated nails, screws and bolts can deteriorate very quickly).
A document that would be very useful to builders and designers is available from Forests and Wood Products Australia. The Timber Service Life Design Guide is available on the website at www.woodsolutions.com.au/Resources/Timber-Service-Life-Design
You will have to sign up for a Wood Solutions account, which is free and gives you access to a great deal of resource material, regular updates, coming events and a newsletter if you tick the appropriate boxes on registration (no feedback if you leave them blank).
This design guide also comes with the software TimberLife Educational Software, which may also be useful, particularly for those that prefer electronic resources.
The Timber Service Life Design Guide is one of a series, all of which will be helpful depending on the topic and your needs.
There is another in the series Stairs, Balustrades and Handrails Class 1 Buildings – Construction that deals extensively with timber species in external applications and again gives guidance on selection, fixing and installation.
None of this information answers the original question about using low-durability species in external trim applications, but it does establish that no regulation exists.
The advice provided by AS1684 and the Wood Solutions design guides shows there are many acceptable species, starting at above-ground durability class 3 and moving up through the natural durability classes. Finally there’s a widely available range of H3 treated softwood, including finger-jointed and primed material.
Understanding the available species and sensible installation will help to overcome the durability issue.
When you think back to the original predicament with some form of maple being used as external trim, our investigation has shown us that a number of the in-ground class 4 species (because of the above-ground rating system) such as Philippine mahogany, meranti (probably the timber being questioned in the first instance), alpine ash, mountain ash, etc, are acceptable, if not already in common use.
The real answer is not to question the timber species being used, although common sense should prevail, but to make sure it is installed correctly.