Decking Calculator UK
Planning Steps
- Measure deck length and width precisely, excluding posts or features.
- Select board width matching your supplier (145mm common for UK softwood).
- Choose layout: straight for simple, diagonal/herringbone for patterns needing 15-20% extra length.
- Set joist spacing per manufacturer (400mm typical for stability).
- Add 5-8mm gaps between boards for drainage; 2mm at ends.
- Maintain 50mm minimum airflow gap beneath for ventilation.
Key Principles
Area = length × width. Linear metres = (area × 1000 / board width) × layout factor (1 for straight, 1.41 for 45° diagonal, 1.75 for herringbone) × (1 + waste/100). Joists span width + overhangs at 400-500mm centres. Use UC4 treated timber joists in ground contact. Cover ground with permeable membrane and 50mm gravel for drainage.
FAQs
What gaps between decking boards?
5-8mm between boards, 5mm at posts, 2mm at ends for water flow and air circulation.
Membrane or gravel needed underneath?
Permeable membrane with 50mm gravel layer recommended to block weeds and aid drainage.
Lay decking direct on ground?
No, elevate on joists minimum 50mm for airflow to prevent rot.
Boards without gaps possible?
Avoid; gaps essential for drainage and expansion.
Timber vs Composite
| Aspect | Timber | Composite |
|---|---|---|
| Cost | Lower initial (£2-4/lm) | Higher (£4-7/lm) |
| Durability | 5-10 years untreated | 25+ years low maintenance |
| Maintenance | Annual oil/stain | Minimal cleaning |
| Slip Resistance | Good dry, slippery wet | Consistent textured surface |
| Weight | Lighter | Heavier, stronger span |
Common Pitfalls
Forgetting layout multiplier: Diagonal patterns increase length by 41%; always factor in.
Over-tight joist spacing: 400mm sufficient; closer wastes material without gain.
Ignoring expansion: Softwood swells; gaps prevent buckling.
No sub-base prep: Level ground, add membrane/gravel or risk pooling.