Grading Macrocarpa Cypress Weatherboard

We get asked all the time to share how we grade mac building grade timber. In part because there are some significant differences to other timbers that require more care and thought in the grading process. The industry and grading rules are based around radiata pine, which presents some minor issues when grading, but our experience has provided ways to ensure our timber meets code requirements while still maintaining the end user goals of having macrocarpa cypress timber's character and design features.

External use grading:

All Cypress Macrocarpa timber used for external purposes is based around the natural durability of the heart wood. In a natural state, mac is durable to an H3.1 equivalent (class three durability) to treated radiata pine. This means it is ideal as a weatherboard or exposed beams and posts (above ground). However, it must be able to easily be removed and replaced if required.

As our mac is more often than not "knotty", we factor in the external use guidelines set out in NZS 3631 and NZS 3602. These codes work hand in hand, however 3602 supersedes 3631 in grading certain products in relation to knots and defects.

As MacDirect like other suppliers of weatherboard products indicate, "5% off grade" will be supplied allowing for "human error", the builder is required to dock out any non-compliant defects before the cladding is installed.

Grading Weatherboard Knots and Defects:

Here's some things we watch out for.

1. No knots aver 50mm.

2. No Spike knots over 25mm (less than half the width of the board)

3. No resin, bark pockets, holes

4. Heart wood only for cypress/mac to meet "class three" durability.
   

(NZS 3602)

Some other more tricky defects that we watch out for:

Pith?

Pith in radiata pine is considered a down-gradable defect, however in mac it's not usually a problem. In 90% of the cases, it actually is a stunning feature to Mac cladding, but to ensure there is no issues our timber is kiln dried and graded to ensure there is no potential defects like checking in the pith that will arise over time. Pith can be 6mm wide in dressing grade, but it is not found to the same extend in Mac as pine, so is not considered a "downgrading defect" in Mac/Cypress and other timbers like Douglas Fir. We do however consider the potential of this as a defect when grading our weatherboard.

Pith in Cypress Macrocarpa dry (16%) weatherboard.

Checks and Cracks?

Checks up to 2mm are acceptable, however we only accept this after the drying process. Any checks and cracks prior to the kiln drying process is most likely to be a fault and therefore is downgraded.

We know that majority of our customers want our weatherboard for their homes because it not only offers them a natural, chemical free timber alternative, but it also provides a certain character that only MacDirect mac can provide. As the experts in Macrocarpa Cypress timbers and having worked with it now for over 30 years, we provide the support, advice and after sales care needed to ensure that Macrocarpa Cypress timbers will continue to adorn the homes of New Zealanders. This is why we continue to advocate for Mac timbers to ensure homeowners have this choice.

I am really looking forward to reading Murray's blog on historical use on how Mac has been applied to homes in New Zealand for over 100 years. Coming soon.

-Scott Kelso, GM MacDirect