How to Build a Garden Bench
from Composite Decking (2026
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 the finish you achieved when building 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.

  ⚠️ Expert Advice: Fixings Matter Structural Screws are a MUST. Never use cheap, standard screws with brittle heads; they will snap under the pressure of people sitting down.

  For maximum shear strength and stability, we strongly recommend using M10 Carriage Bolts (Coach Bolts) to secure the legs. These solid steel pins prevent the bench from wobbling over time in a way that screws simply cannot.

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. To secure them, drive structural timber screws through the outer frame and into the leg.

Expert Tip: Use at least two structural screws per join for a strong connection. Alternatively, for the ultimate stability (as mentioned in our tools list), drill through the frame and leg to install M10 Carriage Bolts. This prevents the timber from splitting and stops the legs from wobbling over time.

Step 3: Clad the Bench with Decking Boards

• The Seat: Cut your composite decking boards to length and place them on top of the frame, leaving a 3–5mm gap between each board for drainage and expansion. Secure them using hidden fasteners or composite screws.
• The Sides (Fascia): Don’t forget the sides! To hide the pressure-treated timber frame, measure and cut pieces of Fascia Board (or rip down a decking board) to screw onto the face of the frame. This gives the bench a seamless, professional finish that matches your deck.

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.