Choosing the Right Joist for Your Decking Project

The choice of joist material has one of the biggest impacts on the long-term performance, safety and durability of any decking installation. Whether you're building a domestic garden deck or a high-rise balcony, the subframe determines how stable your surface finish will be, and how long it will last.

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Thu, 28 Jul 2026

This guide explains when to use aluminium joists, recycled plastic lumber joists and timber joists, helping you choose the right subframe for your project.

Why Decking Joist Choice Matters

A decking board is only as good as the structure beneath it. The correct joist type ensures:

  • Proper load-bearing performance

  • Long-term structural stability

  • Moisture and rot resistance

  • Compliance with fire regulations (especially for balconies)

  • Reduced maintenance over the lifespan of the deck

  • Compatibility with pedestals, drainage systems, and surface finishes

Choosing the wrong joist can lead to warping, softening, uneven decking or even structural failure. Below is a breakdown of each joist type and when you should use them.

1. When to Use Aluminium Decking Joists

Best for: balconies, terraces, high-load areas, commercial projects, fire-regulated installations and modern outdoor systems.

Aluminium joists are widely considered the most durable and stable option for decking subframes. Their exceptional strength-to-weight ratio, dimensional stability, and non-combustible composition make them ideal for contemporary applications.

Use Aluminium Joists When:

A) You’re building on a balcony or terrace

UK fire regulations (BS 8579 and Building (Amendment) Regulations) generally require non-combustible materials for the subframe. Aluminium achieves Class A fire ratings, making it compliant for use under aluminium decking, porcelain paving, fibre cement decking and other non-combustible surfaces.

B) You need a long-lasting, rigid subframe

Aluminium does not warp, twist or rot. It maintains structural integrity over decades, even under continuous load.

C) The installation has low height build-up

When working with restricted build-ups (e.g. thresholds on apartments), aluminium allows:

  • Low-profile joists

  • High strength with minimal height

  • Consistency across the full span

D) You need high load capacity

For commercial spaces, restaurants, public walkways, high-footfall balconies, aluminium offers the highest load resistance.

E) You want a low-maintenance solution

Aluminium is completely rot-proof, insect-proof, and impervious to moisture.

Typical Aluminium Joist Use Cases

  • Residential balconies

  • Podium terraces

  • Commercial roof terraces

  • Steel-frame balcony installations

  • Beneath aluminium decking or porcelain paving

2. When to Use Recycled Plastic Lumber Joists

Best for: ground-level garden decks, areas prone to moisture, DIY installations, long-term low-maintenance projects and eco-led builds.

EnviroBuild’s recycled plastic joists are made from 100% recycled plastic and offer excellent durability for installations not governed by strict fire regulations.

Use Plastic Lumber Joists When:

A) You want a rot-proof alternative to timber

Plastic joists will not rot, split, swell or degrade, making them ideal for:

  • Damp environments

  • Subframes sitting close to the ground

  • Decks built over soil or grass

  • Coastal climates

B) Environmental sustainability is a priority

These joists are made from 100% post-consumer recycled plastic sourced in the UK, giving a circular-economy option to your subframe.

C) You are installing composite decking in a garden setting

Plastic lumber pairs perfectly with composite decking for domestic spaces and DIY projects.

Typical Plastic Lumber Joist Use Cases

  • Traditional garden decking

  • Walkways through wet or forested areas

  • On-soil domestic installations

  • Ground-level composite decks

  • Areas requiring maintenance-free materials

3. When to Use Timber Joists

Best for: low-budget domestic installations, short-term builds and projects not exposed to constant moisture.

Timber joists remain common in DIY garden decking, but they come with the shortest lifespan and highest maintenance requirements.

Use Timber Joists When:

A) The project is low-risk and not moisture-prone

Timber can be a suitable short-term option in dry, well-ventilated areas.

B) The client has strict budget constraints

Timber is usually the lowest-cost material upfront, though the long-term maintenance and replacement costs are higher.

Avoid Timber Joists When:

  • The deck sits very close to the ground

  • Moisture cannot escape

  • The installation requires fire-rated materials

  • You need consistent long-term stability

How to Select the Right Joist (Quick Guide)

Choose aluminium if:

You’re building on a balcony, terrace or any non-combustible requirement.

Choose recycled plastic if:

You’re building a long-lasting, low-maintenance garden deck.

Choose timber if:

You want the cheapest, simplest short-term solution and moisture isn’t a concern.

The Right Joist Ensures a Stable Deck

Your decking subframe impacts performance, safety, fire compliance and lifespan. Aluminium is the gold standard for modern balcony and terrace systems; recycled plastic is best for domestic ground-level installations; timber suits basic, budget applications.

EnviroBuild offers a full range of aluminium joists, recycled plastic lumber joists, pedestals and complete decking solutions, ensuring you can build safely, sustainably and with confidence.