Larch is a natural, durable softwood with moderate-to-high moisture movement. On long lengths (like 6.1 m), a visible bow or “spring” is common and expected. Standard installation practice is to fix the board at one end and progressively pull it to line along the battens/joists. Good fixings at the right spacing restrain the board; once fitted, the timber settles toward its in-service shape as it acclimatises.
1) What causes the bend?
Wood moves as its moisture content changes. Larch typically shows tangential shrinkage ~8 % and radial shrinkage ~4 % from green to oven-dry—figures that sit behind the industry “movement class” you’ll see in cladding specs. In the UK, European/British larch is generally classed Medium movement (some suppliers list British larch as “Large” due to faster home-grown growth and greater variability). Either way, a degree of curvature on long boards is normal, not a defect. East Brosmaterialhub.deEcochoiceVastern
On top of moisture movement, softwoods can contain reaction wood (compression wood) or juvenile wood. These produce growth stresses that are released when timber is sawn and dried—one reason a long board can show spring or bow even when it was sawn straight. Longitudinal movement is usually tiny, unless reaction/juvenile wood is present, in which case distortion is more likely. SpringerOpenOSU Extension Service
2) Why it’s more noticeable on longer lengths
The longer the board, the easier it is to see small deviations from straight. A few millimetres of movement per metre add up over 6.1 m. That’s why reputable cladding/decking guides emphasise designing and installing to restrain natural movement rather than expecting zero movement. GOV.UK
3) The accepted fix: start at one end and pull the board to line
Industry guidance for external timber (cladding/decking) is clear: use robust fixings at the correct frequency to restrain movement and keep boards true. In practice, installers fix one end, then work along the length, bringing the board to line at each batten/joist. For stubborn boards, installers use clamps or board-straightening levers; these are standard trade tools precisely for this scenario. GOV.UKTDCAGlenalmond TimberDeckwise
Practical how-to (cladding)
- Batten spacing: Typically 400–600 mm centres (closer for diagonal or higher-movement situations). Closer spacing gives more restraint and straighter lines. National Timber GroupCOFORD
- Fixings per support: For boards ≥100 mm wide, use two fixings per batten at about the quarter points of the board width; pre-drill near ends to prevent splitting. TDCA
- Fixing sequence: Set your starter course straight and level, fix the first end, then progressively pull to the chalk line and fix at each batten. For minor spring, hand pressure often suffices; for more pronounced spring, use a clamp or board lever. GOV.UKDeckwise
- Fixing material: Use stainless steel (A2/A4) for larch to avoid staining and corrosion. Ecochoice
- Moisture content at install: Aim for ~16 % ± 4 % and allow boards to acclimatise on site (stickered, covered, off the ground) before fixing. National Timber Group
Practical how-to (decking or fencing)
- Joist spacing: Typically 400–600 mm centres depending on board section; always two screws at quarter points at every support. Tools like CAMO/Hardwood-Wrench/Bowrench help straighten bowed boards safely and quickly. Glenalmond Timber+1Deckwise
4) “Let the boards settle” — what that means
After installation, timber gradually moves toward the local equilibrium moisture content. Seasonal humidity changes will still cause small, reversible movements across the grain; the fixings hold the line you set on day one. Good detailing (ventilated cavity for cladding, correct gaps for decking) is what allows wood to move harmlessly without telegraphing issues. TDCANational Timber Group
5) When is a board too bent?
Minor spring that can be pulled to line with normal fixings/tools is considered acceptable and is anticipated in installation guidance. If a board shows severe twist, excessive cup, or cannot be brought to line at standard batten spacing without undue force, set it aside for trimming or a different use. Close batten spacing (toward 400 mm) increases restraint and often resolves borderline cases. National Timber Group
6) Storage and handling to minimise issues
- Keep packs flat, off the ground, well-supported, and stickered evenly.
- Cover, don’t wrap: protect from rain/sun but allow airflow.
- Bring packs close to the installation area a few days ahead so boards acclimatise before fixing. timberfocus.com
Key takeaways for your customer
- A bit of bend in long larch boards is normal. Larch’s movement characteristics and natural growth stresses make this expected—especially at 6.1 m. materialhub.deEcochoice
- The standard fix is part of good practice: Start at one end and spring the board into place along the supports using correct fixings and spacing. This is how timber cladding and decking are designed to be installed. GOV.UK
- Once fixed, boards settle and seasonal movement is handled by the detailing and fixings you’ve provided. National Timber Group
References & further reading
On reaction wood / growth stress as a cause of distortion in softwoods. SpringerOpenOSU Extension Service
Forest Research (UK). Wood properties and uses of larch in Great Britain (2024): background on British/European larch. cdn.forestresearch.gov.uk
Material data for larch shrinkage (tangential ~8.2 %, radial ~4.2 %). materialhub.deEast Bros
Timber Development UK / supplier factsheets: Movement class commonly listed as Medium for European/British larch. Some home-grown specs list Large due to variability. vincenttimber.co.ukEcochoiceVastern
Fixing & installation guidance (cladding/decking): TDCA and Timber Trade Federation handbooks—batten/joist spacing, fixing positions, moisture at install, acclimatisation. TDCANational Timber Group+1Glenalmond Timber
On restraining movement with robust fixings (general timber guidance). GOV.UK
