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How to plan decking

Kickstart your decking project

A deck is essentially an open-air extension of your living space. Free-standing or attached to a house wall, decking creates a space that's ideal for outdoor entertaining, as a play area for the kids or simply somewhere to relax outdoors. It also offers an inspired solution to common garden problems such as slopes, or spots where the grass won't grow.

Decking is flexible, allowing you to extend, or update the features over time, as well as allowing you to work around fixtures such as trees or posts that you cannot, or don’t want to, remove from the area.

And you don't need to be a carpenter to build a deck - a few power tools, a basic tool kit and some woodworking skills are needed, so in most instances you can undertake the work yourself. Just be sure to plan carefully and check all your measurements, both at the planning stage and as you build. A second pair of hands will be useful too.

Interested in adding some decking to your outdoor space? Read on for our expert advice to help you kickstart your decking project.


There are many options when it comes to decking structures. We're going to run through the three most popular ones:

  • a simple, ground-level deck
  • a ground-level deck with balustrade (also known as railings)
  • a raised (or elevated) deck with balustrade and steps

A simple, ground-level deck

  • Ideal for ground that is level
  • The most simple decking structure to construct and lay
  • Can have footings or be laid directly onto the ground – it’s up to you
  • Consider extra touches like decking lights

A ground-level deck with balustrade

  • Creates an enclosed space - great if you want to be able to keep children and pets in one area
  • Adds a decorative, as well as a practical, feature
  • Balustrades (or railings) can be added during the initial building or added at a later date
  • Consider adding a gate for security

Raised deck with balustrade and steps

  • Suits gardens on a slope
  • A larger-scale project that takes longer to build
  • The space underneath the structure can be used for extra storage
  • Add a skirting panel as a decorative and practical finishing touch

There are different ways of configuring your deck boards. Here are some of our favourite designs explained.


Horizontal deck boards

For the traditional decking look, choose to have your boards laid horizontally. For standard right-angled timber decking, measure a maximum of 450mm between the centre of one supporting joist and the centre of the next.


Diagonal deck boards

A more contemporary, yet still classic option, boards laid diagonally need joists spaced 300mm centre-to-centre for the structure to be secure.


Chevron deck boards

For a decorative chevron pattern that is reminiscent of a parquet floor, your joists should measure 300mm centre-to-centre and you must lay a double joist where the boards meet in order to have space to fix them securely.


Picture frame deck boards

Another attractive option for those who want something a bit more decorative, the picture frame pattern has boards creating a border within the main design. You will need to adjust the framing to support the surface pattern in the corners. Decks are designed to take advantage of the angles of the joists for maximum stability, as illustrated.

When it comes to laying boards, reversible deck boards can be laid smooth or ridged side uppermost, or you can combine the various finishes for decorative effect. The easiest way to arrange them is at right angles to the supporting joists in a horizontal design – but you don’t have to. Decide on the pattern before you start, as the choice will affect the spacing and number of joists.

How to calculate how many deck boards you need

Once you’ve decided on the size and design of your decking, you’ll need to work out how many deck boards you’re going to need to complete your project.

To do this, we offer two easy-to-follow solutions - a decking calculator table or a simple equation. You don't need to do both, just choose your preferred method. Both options don't work for all calculations, so be sure to check which one best suits your project.

The table and the equation:

  • are for horizontal decking designs only. Diagonal, chevron and picture frame designs require slightly more deck boards and wastage due to the angled cuts.
  • allow for the standard recommended 10% wastage.
  • include the expansion gaps required.

The table is based on our most popular timber board size – 2.4m long and 144mm wide - and includes the expansion gaps for timber (the calculations will be slightly different for Grassedeck, Walksure or solid composite deck boards). Just cross-reference your size of decked area (for example, 2 by 3m) and it presents the number of deck boards you will need.

If you're not opting for timber decking, or are looking to use timber boards in a different size than that referenced in the table, have a go with our decking equation.

For either option, you'll need to know:

  • The material of your deck boards
  • The size of your decking area
  • The length of your deck board, in metres (L for length)
  • The width of your deck board, in metres (W for width)
  • The required expansion gap running between the length of your deck boards, in metres (EGL for Expansion Gap Length)
  • The required expansion gap at the ends of your deck boards (EGW for Expansion Gap Width)

Expansion gaps explained

Deck boards are laid with a gap between each individual one - both along their length and their ends. These are known as expansion gaps as they allow rainwater to easily run through the decking (rather than pool in an unwanted puddle) and for the deck to expand and contract due to changes in the weather.

Timber deck boards require:

  • a 5mm to 8mm gap along the length of the deck boards (EGL)
  • and a 3mm gap along the ends of the deck boards (EGW)

Solid composite deck boards require:

  • a 6mm gap along the length of the deck boards (EGL)
  • and a 3mm gap along the ends of the deck boards (EGW)

Unlike with timber decking, solid composite deck boards don't allow for any flexibility with the size of the expansion gap running along the length of the deck boards, as the fixings used in laying only allow for one single size.


Decking calculator table

Metres 1 2 3 4 5 6 7 8 9 10
1 3 6 9 12 15 18 21 24 27 31
2 6 12 18 24 31 37 43 49 55 61
3 9 18 27 37 46 55 64 73 82 92
4 12 24 37 49 61 73 85 98 110 122
5 15 31 46 61 76 92 107 122 137 153
6 18 37 55 73 92 110 128 146 165 183
7 21 43 64 85 107 128 150 171 192 214
8 24 49 73 98 122 146 171 195 220 244
9 27 55 82 110 137 165 192 220 247 275
10 31 61 92 122 153 183 214 244 275 305

Decking calculator equation

Ensure all measurements are in the same units.

Step 1: Calculate the surface area of your decking

Multiple the length (L) of your decking area by the width (W).

L x W = Decking Surface Area (DSA) in m2

Step 2: Calculate the coverage of your deck boards

Add the deck board width (W) to the Expansion Gap Length (EGL).

Add the deck board's Length (L) to the Expansion Gap Width (EGW).

Multiple these numbers together.

(W + EGL) x (L + EGW) = Single Board Coverage (SBC) in m2.

Step 3: Calculate the number of boards required

Multiple the Single Board Coverage (SBC, as calculated in step 2) by 1.1.

Divide the Decking Surface Area (DSA, as calculated in step 1) by the Single Board Coverage (SBC).

DSA / SBC x 1.1 = Total number of boards needed including wastage

How many deck boards?

Now that you've completed the planning process. it's time to start laying your decking and we have a number of articles to help you every step of the way. So, depending on your project, choose from the following how to guides to take your decking project to the next stage.