Japanese Cedar - Knotty Timber is Spotty timber

Why Knotty Grade Timber Is a Poor Choice for Charred Cladding

Knotty grade timber like Sugi or Japanese Cedar can look attractive when it is fresh, raw, or lightly stained. It has character, movement, and visible natural variation. But for charred timber cladding, especially exterior cladding, knotty grade timber is a substandard choice because knots do not char, weather, or age the same way as the surrounding board.

A knot was a branch in the tree. You can see the clear wood grain moves around the harder knot.

The problem is not just visual. It is material behaviour.

Knots are formed where branches grew from the tree. That branch wood is not the same as the clear timber around it. Industry and wood-science sources commonly describe knots as denser, harder, more resinous, and structurally different from the surrounding wood.

Wagner Meters also notes that knots have higher density than the surrounding wood, while a wood-machining study found knot density in white spruce averaged 2.4 times higher than clear wood, with cutting forces up to eight times higher than clear wood.

That matters because charring is highly affected by density. BRANZ research on timber charring found that timber density has a major influence on charring rate, and that higher-density wood chars more slowly. Timber Unlimited makes the same point in practical terms: high-density hardwoods char slower than lower-density softwoods, and timber charring rates are often based on density.

Here you see the knots are not charring like the clear timber around them.

The Core Problem: Knots and Board Timber Char at Different Rates

When a board is charred, the lower-density board timber chars faster and more deeply. The knot, being denser and harder, resists the heat for longer.

That means the surrounding timber face may reach the desired black char level, while the knots are still under-charred.

The board face is charred yet the knot remains uncharred

To push the knots to the same visual depth of char, the operator would need to over-char the rest of the board. That risks making the board too thin, too deeply burnt, and visually overprocessed.

So in practice, knotty grade timber creates an impossible compromise:

You either char the board timber correctly and leave the knots under-charred, or you char long enough to char the knots and over-burn the rest of the board.

That is why knot-heavy timber is not ideal for a consistent exterior yakisugi / shou sugi ban finish.

10 year old knotty cladding and all the knots have lost char and turned grey.

Why Japanese Cedar Knots Lose Char Faster

Even when knots appear black after processing, they often do not hold the char in the same way as the surrounding timber.

The surrounding board has long, open grain running with the board. It chars more evenly and forms a more continuous carbonised surface. Knots are different. Their grain direction is distorted because they are branch material embedded into the trunk.

The grain swirls around the knot, and the knot itself is harder, denser, more resinous, and less uniform than the rest of the board.

This creates a weak point in the visual ageing of charred cladding. Over a short time, the char on knots will erode, or weather back faster than expected. The result is a dark charred wall with pale grey or brown knots showing through.

On a small sample, that might look like “natural character.” On a full façade, it can become a polka-dot effect.

This charred Japanese Cedar has lost the black char from its knots in less than 12 months

Why This Is Worse on Exterior Cladding

Exterior cladding is exposed to sun, rain, wind, thermal movement, and repeated wet-dry cycles. These conditions exaggerate any difference between the board face and the knots.

A knot-heavy board may look acceptable when freshly charred, but the ageing curve is uneven. The flat board timber and the knots weather at different rates. After only a few months, the knots can become visibly lighter while the rest of the board remains dark.

That creates three problems:

First, the façade loses its intended architectural look. Instead of a clean, dark, textured surface, the wall becomes visually spotty.

Second, the knots distract from the board rhythm. On vertical cladding, repeated light knots can dominate the elevation and pull attention away from the form of the building.

Third, the finish becomes harder to represent honestly in samples. A small sample board may not show the full effect, but across 100–200 m² of cladding the knot frequency becomes obvious.

Another example of polka dot spotty effect in less than 2 years.

Knotty Grade Is Not “Character” in Charred Cladding

In ordinary timber applications, knots are often sold as character. That can be fair for rustic interiors, ceilings, cabins, or stained boards where variation is part of the design intent.

But charred cladding is different. The value of the finish often comes from controlled surface depth, shadow, texture, and consistency. The eye reads the wall as a single architectural surface. Excessive knots interrupt that surface.

For charred cladding, knotty grade is not simply a cheaper aesthetic option. It is a lower-performing substrate for the finish.

The issue is especially important with dark exterior cladding because contrast is unforgiving. A small pale knot on a light timber wall may disappear. The same knot on a black charred wall becomes highly visible.

Clear grade timber shows even texture and colour for years

Better Timber Selection for Charred Cladding

For premium charred cladding, the only choice is a clear grade with few or no knots.

The goal is not necessarily perfect, defect-free timber. Timber should still look natural. But the knot size, frequency, and type need to be strongly controlled.

Fewer knots mean the charred finish can age evenly and the wall can retain its intended architectural character for longer.

A good charred cladding substrate should have:

• Clear grain or very Low knot frequency.

• if some knot present they shouls be smaller, tight knots, no dead (bark encased) knots.

• Consistent board density

• Stable moisture content before processing

Conclusion

Knotty grade timber is a poor choice for charred exterior cladding because knots are materially different from the surrounding board.

They are typically denser, harder, and structurally irregular. Because higher-density timber chars more slowly, knots resist charring compared with the lower-density board timber around them.

This makes it difficult to achieve an even char level across the board. Worse, as the cladding ages, the knots lose their black char and turn grey or pale, creating an unwanted spotted appearance across the façade.

For a rustic wall, that may be acceptable. For high-end architectural charred cladding, it is usually a defect in the making.

Buy clear grade timber foro your charred project, there are many great options today.

• Accoya

• TMT Pine

• Kanda

• Sydney Blue gum

• Fijian Mahogany

The simple rule is this: the more knotty the timber, the the more spotty that house. Knotty = Spotty.

The Blackwood Project works with all timbers available in New Zealand and can provide you impartial advise for your project.