How to Build a Garden Bench
from Composite Decking (2025
UK DIY Guide)

Step 1: Plan Your Dimensions

First, decide on the size of your bench. UK standards are a good guideline:

• Height

  Typically 40cm – 50cm from the ground to the top of the seat.

• Depth

  Around 40cm – 50cm deep.

• Length

  This depends on how many people you want to seat. A length of 1.8m is comfortable for three adults

Step 2: Gather Your Tools & Materials

Having everything at hand will save time and help you work faster.

• For the Frame

  We strongly recommend using C24 graded pressure-treated timber (e.g., 4 x 2 inches) for the frame. It’s strong, durable, and the UK industry standard for outdoor structures.

• For the Seat

  You can use leftover composite decking boards. They are stronger than wood, won’t rot or decay, and will perfectly match your decking.

• Tools & Fixings

  You will need a tape measure, a spirit level, a power drill, and a mitre or circular saw. For fixings, always use high-quality galvanised structural screws for an outdoor project.

Step 1: Build the Main Bench Frame

The foundation of your bench is a simple rectangular box frame.

• Using your saw, cut your C24 timber joists to create the two long sides and the two short ends of your bench frame.
• Fix the four pieces together at the corners with your galvanised screws to form the outer box.
• Cut shorter pieces of timber (noggins) and install them inside the frame, spaced approximately 400mm apart. This is a crucial step that prevents the frame from twisting and adds huge rigidity.

Step 2: Attach the Support Legs

• Cut your C24 timber to create the legs for your bench (typically 4 – 6 legs for a 1.8m bench). Remember to subtract the thickness of your decking boards from your final desired height when cutting the legs.
• Place the legs on the inside corners of your frame and fix them securely in place by driving screws through the outer frame and into the leg.

Expert Tip: Use at least two screws per join for a strong connection. Pre-drilling the holes will make it easier to drive the screws in and prevent the timber from splitting.

Step 3: Clad the Bench with Decking Boards

• Cut your composite decking boards to the desired length to form the seat.
• Place the boards on top of your frame, ensuring you leave a small 3 – 5mm gap between each board for drainage and expansion.
• Secure the boards to the frame using your chosen fixing method (e.g., specialist composite decking screws).

Step 4: Protect the Feet

Expert Tip for Longevity: To prevent the timber legs from rotting, they must not have direct contact with wet ground. The best practice is to either place your bench on hardstanding like paving slabs or to thoroughly coat the bottom of each leg with a high-quality end-grain wood preserver before use.