How to Safely Build a Pergola Over Your Deck (2026 UK Guide)

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Materials & Tools List
Before You Build
Step-by-Step Guide
Conclusion

Teak wood grain composite decking boards left side view

Introduction

Building a pergola over your composite decking is a great feature to elevate your garden design and, more importantly, provide shade and shelter from the rain. They provide comfort and shelter in the winter months where you can relax.

In our personal view, a pergola turns your deck into a true outdoor extension that can be used all year round.

Materials & Tools List
Before You Build: Critical Planning & Safety Checks
The Build: A Step-by-Step Guide
Conclusion

Materials & Tools List

Materials

Posts: 100mm x 100mm (Pressure-treated C24 timber).
Main Beams: 47mm x 200mm C24 Structural Timber.
Rafters: 47mm x 150mm Secondary Rafters.
Fixings: M10 or M12 Hot-dipped galvanised structural bolts.
Mandatory Safety: High Wind Ties (Truss Clips) for every post-to-beam connection.

Tools

Jigsaw
Hand clamps
Circular saw
Handsaw
Tape measure
Ladders
Drill with 12mm Auger or Spade Bit (for bolt holes)
Wrench/Ratchet (for tightening bolts)

Before You Build: Critical Planning & Safety Checks

This is the most important phase of your project – a pergola adds significant weight and stress to your garden and deck structure.

Do I Need Planning Permission?

In most cases in the UK, you won’t need planning permission if the structure is not over 2.5m in height and doesn’t cover more than 50% of the space of your garden; however, rules are complex. Our expert advice is simple: always check with your local council first to be 100% safe.
Can Your Deck’s Subframe Handle the Load?

A standard decking subframe is NOT designed to support the weight of a full pergola. For this reason, we do not recommend attaching pergola posts directly onto your decking boards without significant reinforcement below. The safest and most robust method for a DIY project is to set the posts in their own concrete footings just outside the deck .

⚠️ Essential UK Safety Fix: Wind Uplift

The UK experiences frequent strong winds and gales. Because a pergola roof acts like a sail, it is susceptible to wind uplift—the force that attempts to lift the structure off the posts.

To ensure your structure is safe, all 47x200mm main joists must be secured to the posts using High Wind Ties (Truss Clips). Standard vertical screwing is often not enough to comply with safety best practices in exposed UK gardens.

Metal pergola on a deck with cream garden furniture

The Build: A Step-by-Step Guide

Step 1: Set Your Posts in Concrete

Mark Your Locations:

Mark out where your four 100mmx100mm timber posts will go, positioned just outside of the corners of your deck.

Deck with steps and under pergola with hot tub

Dig the Holes:

For each post, dig a hole approximately 300mm wide and at least 750mm deep.

Grey decking with picture frame boarder, hot tub and pergola

Prepare for Drainage:

Add a few inches of gravel to the bottom of each hole and place the post on top of an engineering brick (or slate). This prevents the timber from sitting in wet soil, which is the primary cause of rot.

Black pergola with inflatable hot tub and curtians

Set the Posts:

Place a post in each hole. Use a spirit level and temporary timber braces to ensure each post is perfectly vertical (plumb).

Pour the Concrete:

Fill the holes with post-fixing concrete (follow bag’s instructions). As you fill them, use a stick to poke the concrete to remove air bubbles, which can weaken the structure and strength of the solid concrete. For a professional finish, slope the top of the concrete away from the post so rainwater runs off. Allow the concrete to fully cure.

Inflatable hot tub with black pergola and white curtians

Step 2: Build the Roof Frame

Cut Posts to Height:

Once the concrete is set, cut all your posts to their final, level height.

Pergola built over decking with feature back wall and garden furniture

Attach the Main Beams (47mm x 200mm Joists)

The main support beams connect to your posts. For the strongest connection, cut a “notch” into the top of each post for the beam to sit in. Alternatively, use Galvanised Steel T-Shaped Brackets. Critically, ensure you use High Wind Ties to secure these beams to the posts.

Attach the Secondary Rafters (47mm x 150mm)

Install your secondary rafters on top of the main joists. For a professional finish, cut small notches into these rafters so they sit securely over the main beams like jigsaw pieces. Fix them into position with galvanised nails or screws.

Conclusion

Building a pergola is a more complex project than laying a deck, but it creates a stunning, all-year-round feature for your garden. If you are an experienced DIYer, follow these steps carefully. If you are in any doubt, we recommend speaking to a professional for guidance on installation. For a much simpler weekend project to complement your new space, take a look at our guide on what to do with your leftover composite decking.