Every timber weatherboard moves with the seasons. The question a specifier should ask is not whether a cladding moves but how much, and whether the profile and the fixing are detailed to let it. For Sugi Japanese Cedar the answer is a fraction of a millimetre a season, and the detailing is already in the system.
How much a board moves
Timber swells as it takes on moisture and shrinks as it dries, mostly across the grain. The textbook figure for Japanese Cedar is about 5% tangential shrinkage, against about 4% for Western Red Cedar. Those numbers describe green timber dried to oven-dry, a range no cladding board ever travels on a wall.
What matters is the seasonal swing in service. New Zealand exterior timber settles at roughly 12–16% moisture content and moves within a few points of that through the year. Across a typical 12–18% swing, a 140 mm Sugi board changes width by about 1.4 mm. A 90 mm board moves under a millimetre. The 6 mm negative detail on a vertical shiplap absorbs that many times over.
Why the moisture content at dispatch matters
Sugi cladding is kiln dried to 12–16% before it leaves the Tauranga mill. That is already New Zealand’s exterior equilibrium, so the board goes on the wall close to its in-service moisture content and the large shrinkage has already happened in the kiln. Timber supplied wetter than that does its drying on the building, which is where gaps and cupping come from.
The real risk is cupping, not shrinkage
Sugi weatherboards are flat sawn, which puts the tangential face on the board face. Flat-sawn boards don’t just shrink across the width; they cup away from the heart side when the face and the back take up moisture at different rates. Four details manage it.
1. Board width. Narrower boards cup less and move less. The 140 mm profiles (ST51, ST55, ST61) are the usual choice; the 120 mm ST58 is narrower again. Profiles wider than 150 mm need closer fixing centres.
2. Coat all four faces. The single biggest cause of cupping is an uncoated back. Pre-coat every face, end and cut edge before fixing, as the installation guide specifies. Sugi can supply boards factory pre-coated with the Dryden two-coat exterior oil system, so only cut ends need touching on site. Uneven moisture uptake between face and back is what cups a board; the shrinkage coefficient is a distant second.
3. Let the profile do the restraining. The shiplap join on the vertical profiles (ST51, ST55, ST56, ST57, ST58) and the lap on the bevelback (ST61) hold the board edges by design, so the board cannot curl at the edge without the neighbouring board holding it. The negative detail, 3, 6 or 10 mm depending on profile, is the gap that movement runs into.
4. One fixing per board per stud. For boards up to about 150 mm, fix once per stud, pre-drilled and sized to the installation guide. The instinct with a timber that moves is to add fixings. Don’t. Two fixings across a board’s width restrain it, and a restrained board splits instead of moving.
The cavity
All Sugi exterior profiles are installed over a drained and ventilated cavity to E2/AS1, which is also what the BEAL appraisal (C2549) covers. The cavity keeps the back of the board dry and ventilated, which is as much a movement detail as a weathertightness one: it keeps the face and back of each board at a similar moisture content.
The short version
Sugi moves about a quarter of a millimetre more than Western Red Cedar across a 140 mm board over a season. It arrives at the moisture content it will live at. The cladding system’s profiles, negative details and fixing centres are set for that movement, and the one thing the installer controls, coating all four faces, is the one that matters most.
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