Understanding Cedar as a weatherboard cladding system

Understanding Cedar as a weatherboard cladding system

Which species is used in NZ?

There are several species of Cedar but the one most commonly imported and used in New Zealand for weatherboards is Western Red Cedar. This member of the cypress family grows in the Rocky Mountains from southern Alaska through northern California. The trees may grow 60 metres tall with trunk diameters of 3 metres.

We will now refer to this timber as "Cedar".

Varying quality

As with most natural products, Cedar comes in a range of qualities. These are determined by where it was grown, how the local environment affected its development, along with its age and maturity.  Also, like all timber, the heartwood is stronger and more durable than the sapwood. 

So like all imported products, its quality is dependent upon the choices of the importer and merchant.  Therefore if it is cheap, there is probably a reason.

Cedar's natural durability

Cedar has some natural characteristics that make it suitable as an exterior cladding.  It has a natural resistance to decay and to repel water.  It also has a high thermal mass which means it assists with insulation better than other timbers.

Profiles of weatherboard used

Cedar is run in a range of weatherboard profiles, designed to be installed horizontally or vertically and during the 1980's it was even popular to install it diagonally.  However, horizontal weatherboards, in either a bevelback or rusticated profile are the most popular in New Zealand.  This is because these two profiles have been used in New Zealand for over 100 years and have proven to be reliable in our environment.

Vertical profiles can be more problematical if the boards warp or cup, as is common with unprotected Cedar. Especially if the profile only allows for a small overlap between boards.  Cupping and warping is more likely if the board has been milled too thin, if it is in a harsh solar environment, or it has not been correctly installed.


Generally accepted maintenance conventions

It is disconcerting for a maintenance planner but there are many different opinions in regard to how Cedar weatherboards should be maintained.  Our role is to advise what maintenance should be undertaken but with Cedar, there appears to be no widely accepted and scientifically proven programme that should be put in place.

However, there is general agreement that painting Cedar with a standard three coat acrylic system is the best, although it is also generally agreed that if you intend to paint your weatherboards, there is little point in using Cedar because one of our cheaper home grown timbers will work fine if it is H3.2 treated.

Architects and property owners typically choose Cedar because they want a certain look.  It might be the weathered greyish look that some people love or the use of stains that complement the grain and tie the buildings in with the local environment or other architectural features on the building. 

Most paint manufacturers and professional painters consider that staining improves the durability of Cedar and typically recommend that the weather faces, in particular, are re-stained every two years, while the other faces can be left for another year or two.  And most have anecdotal evidence that this regime does work.

Others are adamant that oils, either as an oil-based stain or a tinted penetrating oil, such as Drydens, should be used.

However, one area where all of these paint professionals tend to agree is that if the weatherboard is going to be stained, it should be given the first coat on both sides and the cut ends before installation, followed by another coat once installed and the third coat after the first year. 

Scientific evidence

On the other hand, there appears to be no scientific evidence that stains or oils actually improve the durability of Cedar* and while the websites of some professionals and suppliers will make claims as to the improved durability their products or methods will produce, all of this appears to be based on anecdotal evidence, rather than scientific.   

But while science says these things can't have an effect, by comparing one wall that has been treated with a stain with another that has had no treatment - or less frequent treatment - there often appears to be an obvious difference. 

Where science and the industry agree

The one area where all parties do agree is in regard to the quality of the timber, design and installation and in particular, they agree that generally, horizontal weatherboards are more reliable than vertical.

However, if they are to be installed vertically, there should be a good overlap of at least 30mm, they should be installed over a cavity and ideally a RAB, and the timber itself should be of good quality and at least 19mm thick.  There should also be adequate fastening to help prevent the boards from twisting or cupping, and they should be installed according to the supplier's recommendations.

What to do?

It is apparent that treating with oil based stains or tinted oils, particularly if they give you the look you want, is not wrong.  Apart from the cost, there won't be any problems as a result and depending on the location, aspect, materials used and the quality of the job, regular staining is most likely going to improve the durability of your Cedar weatherboards.

What painters, paint manufacturers, architects and timber suppliers all generally agree on, is that if you are going to stain it, you should do this every two to four years depending on the aspect of each face.  On that basis, if your Cedar is easy to access, say at ground level, it's a fairly quick and easy job.  However, if you have some Cedar that can only be accessed with scaffolding, it makes your decisions more difficult.

Based on all of this, the first thing a building owner should do is carry out their own research, consulting those professionals they trust and make their decisions based on what they believe is right for them. 

*At least at this time I haven't been able to find any scientific evidence but my research is ongoing and if anyone is aware of any published works on the subject I would appreciate knowing where I can find them - John Bradley 20 September 2020.

If you have any feedback or questions please use the feedback form.

The Plan Heaven team.

Disclaimer. Plan Heaven is not qualified in law and any comments made on this website are only the opinion of Plan Heaven and should not be regarded as legal advice. Our comments are merely providing some thoughts on how the legislation might be interpreted and how we go about attempting to meet its requirements. You should not rely on this information in isolation and do you own homework and at all times if you wish to be sure of your position relating to legal matters you should seek advice from a suitably qualified lawyer.