| Use case | Species | Direction | Dimension | MC change | Estimated movement |
|---|---|---|---|---|---|
| Deck board width | Southern Yellow Pine | Tangential | 140 mm | 10% → 16% | ~0.22 mm |
| Planter panel width | Western Red Cedar | Radial | 300 mm | 9% → 18% | ~0.21 mm |
| Gate rail height | White Oak | Tangential | 90 mm | 8% → 15% | ~0.19 mm |
This calculator estimates dimensional change from moisture content change using a linear approximation:
- D = measured dimension (width, thickness, or panel size).
- ΔMC = final MC − initial MC (percentage points).
- C = coefficient per 1% MC (depends on species and direction).
Preset coefficients are derived from typical shrinkage values (tangential or radial) divided by an assumed fiber saturation point of about 28% moisture content.
- Choose a species preset that matches your outdoor lumber.
- Select the movement direction based on grain orientation.
- Enter the across-grain dimension you want to protect.
- Set initial and expected final moisture content values.
- Optional: add a safety margin for harsh seasonal swings.
- Press Calculate and review the recommended movement and gaps.
Why wood moves in outdoor garden builds
Wood is hygroscopic, so it absorbs and releases moisture as weather changes. Outdoor equilibrium moisture content commonly ranges from about 10% in dry seasons to 18% or higher during humid months. That swing can create measurable across‑grain movement, especially in wide boards, panels, and rails used in planters, gates, and decking.
Key inputs that drive the estimate
The calculator focuses on three drivers: the dimension you want to protect, the moisture content change (ΔMC), and a species‑and‑grain coefficient. Tangential movement (flat‑sawn width) is usually the largest, radial movement (quarter‑sawn width) is smaller, and longitudinal movement along the grain is typically negligible for spacing decisions. As a quick check, a 200 mm board at an 8‑point ΔMC can shift by a few tenths of a millimeter.
Species presets and what they imply
Presets use typical tangential and radial shrinkage fractions and convert them into a coefficient per 1% moisture change using an assumed fiber saturation point near 28% MC. Cedar and teak often show lower movement than dense hardwoods. If you have a lab value or a published coefficient, enter it as a custom coefficient for tighter planning.
Turning movement into practical spacing
For decking rows, slat runs, or panel assemblies, total movement can add up across multiple boards. Enter the board count and the number of joints or gaps to spread the change. For example, six boards each moving 0.25 mm can create 1.5 mm of total change, which should be distributed so gaps stay open for drainage. A safety margin helps when exposure is extreme, end grain is unsealed, or installation moisture content is uncertain, reducing the chance of buckling and fastener stress.
Quality checks for reliable results
Measure the correct dimension: across the grain where swelling is highest. Use realistic MC values for your climate and storage method. If you are building a planter or raised bed, prefer durable species, keep boards ventilated, and avoid trapping moisture behind liners. Recheck calculations when changing board orientation, thickness, or fastening pattern.
1) What dimension should I enter for a deck board?
Enter the board width across the grain, because width movement is what affects spacing between boards and edge buckling in humid conditions.
2) What moisture content values should I use outdoors?
Use your best seasonal estimate. Many outdoor projects see roughly 10–18% MC, but shaded, irrigated, or coastal sites can run higher. Use local experience when possible.
3) Should I choose tangential or radial direction?
Choose tangential for flat‑sawn board width. Choose radial for quarter‑sawn width. If you are unsure, tangential is the safer, more conservative choice.
4) Why does the calculator assume a fiber saturation point?
Below fiber saturation, wood shrinks and swells with moisture change. The preset coefficients convert typical shrinkage data into per‑percent movement using an assumed FSP near 28%.
5) What safety margin should I apply?
A 5–15% margin is common for exposed garden builds. Increase it when boards arrive wet, when sun‑to‑shade swings are large, or when gaps must stay functional for drainage.
6) Can I trust the gap-per-joint suggestion?
It is a planning guide. Confirm with fastener type, edge conditions, and local building practices. Use test fits and keep water pathways open so boards can move without binding.
1) What dimension should I enter for a deck board?
Enter the board width across the grain, because width movement is what affects spacing between boards and edge buckling in humid conditions.
2) What moisture content values should I use outdoors?
Use your best seasonal estimate. Many outdoor projects see roughly 10–18% MC, but shaded, irrigated, or coastal sites can run higher. Use local experience when possible.
3) Should I choose tangential or radial direction?
Choose tangential for flat‑sawn board width. Choose radial for quarter‑sawn width. If you are unsure, tangential is the safer, more conservative choice.
4) Why does the calculator assume a fiber saturation point?
Below fiber saturation, wood shrinks and swells with moisture change. The preset coefficients convert typical shrinkage data into per‑percent movement using an assumed FSP near 28%.
5) What safety margin should I apply?
A 5–15% margin is common for exposed garden builds. Increase it when boards arrive wet, when sun‑to‑shade swings are large, or when gaps must stay functional for drainage.
6) Can I trust the gap-per-joint suggestion?
It is a planning guide. Confirm with fastener type, edge conditions, and local building practices. Use test fits and keep water pathways open so boards can move without binding.