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Project guide

How much decking do I need?

Boards are the easy part. The frame underneath is where most DIY lists go wrong, so this guide follows the Timber Decking & Cladding Association (TDCA) handbook.

Updated Based on manufacturer guidance, sources below

The short answer

A 120mm board with a 5mm gap covers 125mm, so a 2.4m board covers 0.3 m². Boards = deck area / 0.3, plus 10%. A 4 x 3m deck needs 44 boards, 19 lengths of 47 x 150mm joist and beam timber, 4 x 3m posts cut into 12 short posts, and 24 bags of Postcrete.

The formula

  • Boards (2.4m) = length x width / 0.3 x 1.1
  • Joists = length / 0.4 + 1 (400mm centres)
  • Beam lines = width / 2.4 + 1, rounded up; posts every 1.8m along each beam
  • Postcrete = 2 bags per 100 x 100mm post; screws = about 40 per m²

Worked example

A 4 x 3m deck, 0.3m high, with the boards running along the 4m side:

  • Boards: 12 m² / 0.3 = 40, plus 10% = 44 boards
  • Joists: 4 / 0.4 + 1 = 11 at 400mm centres
  • Beams: 3 / 2.4 + 1 = 3 beam lines (outer beams single, middle beam doubled), 8 lengths
  • Posts: 3 lines x 4 = 12 posts, cut from 4 x 3m lengths. Postcrete: 24 bags
  • At this height the joists hang between the beams on 44 joist hangers

Work it out for your job

Change the measurements and the quantities update straight away.

Deck area
12 m²
Joists
11 at 400mm centres
Beam lines
3 (no more than 2.4m apart)
Post positions
12 (every 1.8m)

Your list

  • 44Decking boards 120mm x 2.4m
  • 19Joist and beam timber 47 x 150mm x 3.6m (buy treated)
  • 4Posts 100 x 100mm x 3.0m (cut to 0.9m)
  • 24Postcrete 20kg (2 per post)
  • 3Screws, packs of 200 (~594; use exterior decking screws)
  • 2Decking stain 2.5L
  • 44Joist hangers (low deck)
Price this decking list

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Materials checklist

  • Decking boards120mm boards with 5mm gaps between them.
  • Joists and beams47 x 150mm treated timber. Joists at 400mm centres, beams no more than 2.4m apart.
  • Posts100 x 100mm treated posts every 1.8m along each beam, about 600mm in the ground.
  • Postcrete2 bags per 100mm post in a 300 x 600mm hole.
  • Decking screws2 per board at every joist. Use coated or stainless exterior screws.
  • Joist hangers (low decks)For decks under about 350mm, where the joists sit between the beams.
  • Decking stain or oilAbout 8 m² per 2.5L tin on grooved boards, 2 coats.

You'll also need

  • Weed-control membrane and gravel under the deck
  • M10 coach bolts to fix beams to posts (2 per post)
  • End-grain preserver for cut ends
  • Spirit level, string line, saw, drill/driver and spade

Step by step

  1. Measure and decide the board directionBoards usually run along the longest side. Joists run the other way at 400mm centres.
  2. Plan the beams and postsSpace beam lines no more than 2.4m apart and posts every 1.8m along each beam.
  3. Set the postsDig 300 x 600mm holes and set 100 x 100mm posts in 2 bags of Postcrete each.
  4. Build the frameBolt the beams to the posts, then fix the joists on top, or between the beams on hangers for a low deck.
  5. Lay the boardsLeave a 5 to 8mm gap between boards and fix with 2 screws at every joist.

Tips from the manufacturers

  • TDCA guidance is 47 x 150mm C16 joists at 400mm centres, which span up to about 2.97m. That is why the beams go no more than 2.4m apart.
  • Buy pressure-treated timber for the frame and posts. The TDCA recommends ground-contact treatment (Use Class 4) for the substructure, because it is hard to replace later.
  • Leave 5 to 8mm between boards for drainage and movement (TDCA).
  • Use exterior-grade coated or stainless decking screws. Standard zinc screws rust and stain the boards.

Mistakes to avoid

  • Using 47 x 75mm timber for the frame: it is too small to carry the joists.
  • Putting posts only along the edges, with nothing under the middle of a wide deck.
  • Butting boards tight together, which traps water.
  • Building a deck more than 30cm above the ground without checking planning rules.

Frequently asked questions

How many decking boards do I need for 4 x 3m?

About 44 boards of 2.4m x 120mm, including 10% for cuts and a 5mm gap between boards.

What spacing for decking joists?

400mm centres is the standard for most softwood boards (TDCA). Some composite boards need closer spacing, so check the board maker's instructions.

How many posts for a deck?

Posts go every 1.8m along each beam line, and beam lines are no more than 2.4m apart. A 4 x 3m deck has 3 beam lines of 4 posts, so 12 posts.

Do I need planning permission for decking?

In England, decking is usually permitted development if it is no more than 30cm above the ground and, together with other extensions and outbuildings, covers no more than half the garden. Higher decks usually need permission.

How much Postcrete for a decking post?

Blue Circle allows 2 bags for a 100mm post in a 300 x 600mm hole.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

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