Plan decking and edging with reliable expansion calculations. Choose materials, units, and seasonal temperature swings. Avoid buckling, maintain gaps, and protect your planting areas.
| Scenario | Material | Length | Width | ΔT | ΔM | Total ΔL | Gap/End |
|---|---|---|---|---|---|---|---|
| Spring install → summer peak | WPC | 3.6 m | 0.14 m | 25 °C | 8% | ~0.0170 m | ~0.0098 m |
| Cool shade → hot sun | PVC | 12 ft | 5.5 in | 40 °F | 4% | ~0.090 in | ~0.052 in |
| Low movement boards | Fiber-Cement | 3000 mm | 150 mm | 30 °C | 3% | ~1.50 mm | ~0.86 mm |
The calculator estimates dimensional movement from thermal and moisture effects, then combines them for an overall expansion value. It applies coefficients to the board size after converting units consistently.
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Composite boards can change length noticeably as surface temperature rises above ambient. Dark colors, full sun, and low airflow can push board temperature well beyond air temperature. Use a realistic temperature change (ΔT) from installation conditions to peak exposure to avoid underestimating movement. Installing near the middle of the expected range typically reduces extreme gap demands.
Garden edging, planters, and deck borders often experience watering cycles, splashback, and shaded drying conditions. Moisture change (ΔM) represents a percent‑point shift in moisture content, not relative humidity. If boards sit near soil, assume higher ΔM and allow additional clearance around rigid borders and posts.
The thermal coefficient α (mm/m/°C) captures material sensitivity to temperature. PVC‑heavy products commonly show higher thermal response than fiber‑cement boards. The moisture coefficient β converts moisture change into a percent length shift. When a datasheet provides an expansion range, set α and β to match worst‑case conditions for safer detailing.
The calculator recommends a gap per end by splitting total expected movement and applying a safety margin. This helps when boards are fastened against trim, garden borders, or metal edging. For long runs, review the “Boards in run” movement and add expansion breaks, slotted fasteners, or floating details to reduce accumulated stress in practice.
Compare thermal and moisture components to understand which driver dominates your conditions. If moisture is the larger contributor, prioritize drainage, airflow, and spacing at soil interfaces. Export CSV for recordkeeping or supplier review, and PDF for site notes. The on‑page history stores recent scenarios so you can refine assumptions quickly. For procurement, keep the exports with the product batch and installation date for traceability.
It is the recommended clearance at each end of a board so the total movement can be accommodated on both sides. If one end is fixed, increase the free-end gap accordingly.
Use the best estimate of board temperature change from install to peak exposure. In full sun, board temperature can exceed air temperature, so a larger ΔT is often safer.
Think in percent‑points of moisture content change, influenced by watering, rain, and drying time. Near soil or constant shade, choose a higher ΔM than for a well‑ventilated deck.
Convert it into α by dividing by the temperature change used in the datasheet, if provided. When only a range is given, select the upper value to remain conservative.
Movement can accumulate across multiple boards in a straight run. Long runs may need expansion breaks, floating edges, or compliant trim details to prevent buckling and fastener stress.
Use it for planning and scenario checks, then confirm with manufacturer requirements and local conditions. Fastener type, color, sun exposure, and constraints can change the required gap.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.