Free tool · decking
Decking calculator
Boards, joists, clips and screws for your deck, worked out to the mm. Free, no sign-up.
20 rows of composite boards, 6 mm apart. Tap Frame to see what holds them up.
01 · Size in metres
02 · Boards
03 · Boards run
04 · Height
05 · Joists every
400 suits most boards. Some hollow composite boards ask for 300: check the pack.
What you need
Boards
Frame
Ground
Fixings
Finishing
- Under 30 cm high: a house usually needs no planning permission.
- Falls 15 mm from the house to the front, so rain runs off.
- Every board is one length: no joins.
- Boards 6 mm apart; the last cut to 112 mm to fit.
Rather I build it?
I build decks across London. Send me the size and I'll quote, free.
Rough sizes are fine for a list. Check the board maker's fitting sheet before you buy: it has the last word on gaps, joists and fixings.
How it's worked out
A carpenter's usual rules, not a board maker's. Check its fitting sheet before you buy.
- Boards: composite 146 × 23 in 3.6 m lengths with 6 mm gaps, or softwood 144 × 28 in 2.4, 3.6 or 4.8 m with 5 mm gaps. It picks the length that wastes least.
- Spares: 5% more boards, at least one. When boards join end to end, 10% more for the offcuts.
- The edge board: the last row is cut to fit. If that would leave less than half a board, the first and last are both cut instead, so you never end on a sliver.
- The frame: treated 47 × 100 joists at the spacing you pick, a rim joist across each end, and a row of noggins about every 1.5 m. Boards longer than one length join in one line, over a doubled joist.
- Underneath: adjustable pads under each joist about every 1.2 m. Raised decks get 47 × 150 joists on doubled bearers and 100 × 100 posts about every 1.8 m.
- Fixings: one hidden clip where each composite board crosses a joist, or two screws for softwood. Frame screws: four at each joist end and noggin.
- Finishing: fascia round the three open sides, weed membrane with 10% for overlaps, and joist tape on top of every joist.
- The fall: 5 mm a metre away from the house, so rain runs off.
Questions
How many decking boards do I need?
Divide the width across the boards by one board and its gap (146 + 6 mm for most composite boards) and round up: that's the rows. Each row takes one board when the deck is no longer than a board, more when it is. Add a spare or two. The calculator does it for you and says how wide the edge board is cut.
What joist spacing do I need for decking?
400 mm centres suits most boards: it's what I used on my own deck job in Camden Town. Some hollow composite boards ask for 300, and boards laid on the diagonal need them closer, so check the board maker's fitting sheet.
Do I need planning permission for decking?
For a house in England, usually not, if the deck is no more than 30 cm off the ground, covers no more than half the garden with any extensions and sheds, and isn’t in front of the house. Flats, maisonettes and listed buildings are different. Over 30 cm high, it needs planning permission. The Planning Portal has the details.
Why does a deck fall away from the house?
So rain runs off and away from the wall instead of sitting against it. I allow about 5 mm a metre, so a 3 metre deck is 15 mm lower at the front.
Can you build it for me?
Yes, I build decks across London. Press Get a quote and your size is already filled in, or send me the list on WhatsApp.
A deck I built: a walled courtyard in Camden Town, on a new frame.