How to Calculate Total Decking Materials
Building a timber or composite deck requires much more than just the surface boards. A professional deck is built on a solid structural framework, supported by concreted posts and secured with thousands of screws. Our Decking Project Estimator calculates the complete shopping list for the entire build.
1. The Structural Sub-Frame (Joists)
The sub-frame is the foundation of your deck. In the UK, this is typically built using pressure-treated C16 or C24 timber joists, measuring 100x47mm (4x2) or 150x47mm (6x2).
These joists must be spaced at regular intervals, known as "centres". For standard timber decking, 400mm centres is the industry standard to prevent the boards from bouncing and flexing. If you are using composite decking, the manufacturer will often dictate closer centres, such as 300mm.
2. Support Posts and Postcrete
Unless your deck frame is sitting directly on an existing concrete patio, it needs to be elevated and supported by timber posts (typically 100x100mm) concreted into the ground.
Posts should be spaced roughly every 1.5 metres along the bearer joists. Each post hole requires concrete to secure the post in place. Fast-setting Postcrete is the standard choice, requiring roughly 1.5 bags per hole depending on depth.
3. Decking Boards & The Expansion Gap
Our calculator determines the exact number of surface boards required. It automatically factors in a standard 5mm gap between each board. This gap is vital—timber swells when wet and shrinks when dry. If laid touching, they will buckle and warp.
A 10% waste allowance is standard for standard board calculation to cover saw cuts, squaring off end-grain, and working around drain pipes.
4. Fixings & Ground Cover
Never under-estimate the number of screws required. You need to drive two decking screws into every single point where a deck board crosses a joist. Our estimator calculates this matrix to predict total screw count.
Finally, any vegetation under the deck will grow up through the board gaps to reach sunlight. You must lay a heavy-duty weed control membrane across the soil before building the frame.