In Building Today June 2015, Mike Fox expressed concern at what he saw as a problem with “shrinking timber
weatherboards”, and at the failure of wood product manufacturers to fully address his concerns.
Weatherboard manufacturers have said that installation procedures must be followed. Of course, timber weatherboards are not the only building product which needs to be handled according to manufacturer’s instructions to get expected performance.
With good manufacturing practices as well as correct handling, installation and maintenance, timber weatherboards have been used successfully for many years, and give highquality, long-term performance.
Rather than get into a “he said she said” situation, let’s look at the facts:
• All timber weatherboards change dimension with changes in seasonal moisture content.
• In summer a pine weatherboard could have as low as 12% moisture content, whereas in winter it could be as high as 18%.
• Different manufacturers use different methods of treatment and painting.
The builder should make himself aware of these differences and ensure the appropriate handling, installation and finishing methods are used.
• While primer, whether one coat or two, stops liquid water (rain) from wetting the timber, it does not prevent the eventual diffusion of water vapour into wood. Given that,
two coats have been proven to be better than one, and will give a longer window of opportunity for painting.
• Weatherboards are normally manufactured in the 12% to 14% moisture content range, say 13%.
• On average, a flat sawn board will change its dimension by 0.3% for every 1% change in moisture content (Properties and Uses of NZ Radiata Pine: Volume One — Wood Properties).
• There are probably boards out there with twice the average shrinkage rate, say 0.6% for every 1% change in moisture content.
• Consider this scenario: a 187mm weatherboard, manufactured at 13% moisture content with double the average shrinkage rate.
In winter dampness, it will swell by about 5.8mm from its manufactured dimension. In summer heat it will shrink 6.9mm from its
winter dimension. However, it would only shrink 1.2mm from its manufactured dimension in summer.
• You can see that if the builder installed the boards and allowed them to absorb moisture before they were fully painted, when summer comes there will be a 6.9mm gap between the
paint line on one board and the bottom of the board above.
The owner will complain about this, and the shouting match between builder, painter and manufacturer will start all over again.
• However if the supplier/builder/painter combined to ensure the boards were painted at their manufactured moisture content, even in this worst case the gap would only be
So the key is to ensure that boards are installed and painted at their manufactured moisture content.
Guidance on correct installation
How to do this? Well, weatherboard manufacturers go to great lengths to give guidance on correct handling and installation of weatherboards so they do not get a chance to swell significantly before they are painted.
These instructions include such things as:
• Ensure your supplier stores boards undercover in a well-ventilated and clear-of ground area.
• Arrange delivery of the weatherboards just before they are to be installed.
• Once on site, store indoors on bearers and off the ground. Note that installation in winter would be more likely to cause problems if these recommendations are not followed.
• If they cannot be stored indoors the boards must be protected from moisture uptake from damp ground.
A ground sheet should be used beneath evenly spaced bearers at least 150mm thick, and a secondary site cover should be used over the factor wrap.
• Before installation, measure the dimensions of the boards and compare to the producer’s specification.
Any amount oversize is extra gap that will show up in summer. Instructions are given in installation manuals and, in some cases, in stickers stuck to every board!
So, what do you do if your plans do not come to fruition, and you are worried that the weatherboards have not been properly stored and may already be significantly oversize — but you have to finish the job by the end of the week.
Well, the age-old solution (and one still carried out by many builders) is to paint the lap of the weatherboard the finishing colour before you install them. Any shrinkage will not be noticeable.
Next month: Jenkin Timber sales and marketing manager Bruce Barclay puts his views forward.