Advanced options
Based on standard 3.6m x 144mm boards laid with 5mm gaps. Joist count assumes joists run across the width at 400mm centres; add a subframe perimeter and double joists under board joins.
What The Decking Calculator Works Out
- Deck area: length x width in square metres. Running boards parallel to the house usually looks best and sheds water along the fall.
- Deck boards: the area plus 10% for cuts and waste, divided by the area of one board. The UK standard board is 3.6m x 144mm (about 0.52 m2), so a 4 x 3m deck of 12 m2 needs 26 boards. The 5 to 8mm expansion gaps are absorbed by the waste allowance.
- Joists: one every 400mm along the deck length, plus one at the end, running across the width. Treated joists at 400mm centres suit grooved softwood boards; composite flexes more and wants 300mm.
- Subframe and rules: the frame is levelled with a fall of about 1:100 away from the house, ground-level decks sit on bearers over membrane and gravel, raised decks need posts in postcrete, and decks over 300mm high (or covering more than half the garden) may need planning permission. Leave ventilation underneath; trapped damp kills timber frames.
- Board size and waste (advanced): enter the actual board length and width you are buying (2.4m, 3m, 4.2m and 4.8m lengths and 120mm or 150mm widths are all common) and adjust the waste for diagonal laying (15% to 20%).
- Board material (advanced): composite switches the joist centres to 300mm and drops the oil requirement; hardwood keeps 400mm centres. Softwood is cheapest but needs annual cleaning and re-oiling, hardwood lasts longer, and composite costs most up front but only needs washing.
- Joist timber (advanced): total linear metres of joist including the two long frame members, converted to 4.8m stock lengths. Add noggins and doubled joists under any board-end joins.
- Decking screws (advanced): two screws per board at every joist crossing. Use 63mm stainless or coated decking screws for softwood; composite normally uses hidden clip systems.
- Materials cost (advanced): boards and joist lengths at your prices. Typical UK prices are GBP 8 to GBP 15 per 3.6m softwood board, GBP 25 to GBP 45 for composite, and GBP 15 to GBP 25 per 4.8m treated joist.
- Decking oil or stain (advanced): two coats at about 10 m2 per litre, so a 12 m2 softwood deck needs around 2.5 litres a time, repeated every year or two.
Source and further reading: GOV.UK: planning permission guidance.