How to Measure Knots?

From time to time we get this question, especially when discussing knots in our weatherboards. It can be very confusing and there is still some debate around what is and isn't acceptable, especially when grading timber on site. But thankfully we do have clarity around this thanks to a couple of determinations.

I thought I would address the question around the relationship between NZS3631 (Grading) and NZS3602 (Durability). When discussing weatherboard, we need to put these two standards together.

The best place to start with to understand timber knot standards for cladding is in 3602. Section 111.2.3 and C111.2.3.

111.2.3 Grading requirements additional to those set out in NZS 3631 are as follows:

(a) All holes, resin and bark pockets shall be excluded;

(b) Knot size shall not exceed 50 mm, or 25 mm width for spike knots.

C111.2.3 Dressing grade plus the additional requirements set out above is the lowest grade regarded as suitable in these locations. Where appearance is of major concern, the specification of superior grades should be considered.

Ok, that seems straight forward. Looking at dressing grade standards, cladding must still meet the standards set out in 3602 111.2.3 - knots shall not exceed 50mm.

Where the challenge is, is understanding exactly how to measure a knot. In 3631 (the original timber grading standards) a single knot is measured across the width of the board, contrary to many who believe it's the largest cross section of the knot. We find this in 3631 Figure 13 (below)

Measuring a knot must be done using the method in fig 13 as confirmed by MBIE. Regardless of the profile and way you a using the board etc, MacDirect and majority of timber graders measure knots this way. So when purchasing our mac in our tight knot heart grade you may see some knots that appear larger or elongated, but their measurement is under 50mm when we follow the measurement guidelines.

Spike knots are also mentioned in this same section, but I'll get to that in my next "knot" blog post. We'll also go over bark encasement, resin pockets etc, which are all mentioned in 3602 and 3631.

Thanks

Scott

The Blue line is the correct measure. Red line incorrect. When grading cladding knots