Composite Decking Joists
The Ultimate Guide to Composite Decking Joists: Everything You Need to Know for a Lasting Deck
If you're investing in composite decking, don't overlook one of the most critical components: the subframe. While the decking boards steal the spotlight with their aesthetics and tactile appeal, it’s the joists and substructure beneath that truly determine the longevity, performance, and safety of your outdoor space. This guide explores everything you need to know about composite decking joists — their types, materials, spacing, installation methods, and how they pair with premium decking like Millboard.
What Are Decking Joists?
Decking joists are horizontal support elements fixed atop posts or beams to create the framework on which decking boards are laid. Their primary role is to distribute weight evenly, resist movement, and ensure structural stability. For composite decking, choosing the right joist material and layout is vital to achieving a deck that lasts for decades with minimal maintenance.
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Why the Subframe Matters
The subframe is the silent hero of any decking project. A poor-quality or incorrectly installed subframe can lead to costly problems: sagging boards, unsafe movement, moisture retention, and even structural failure. Given that composite decking is designed to outlast traditional timber, your joists need to match or exceed that longevity to avoid premature deck failure.
Joist Material Options
Timber vs Recycled Plastic vs Aluminium
Treated Timber Joists
Softwood timber is a common, cost-effective option. However, it’s only suitable if treated to Use Class 4, especially in ground contact or moisture-prone environments. Even with treatment, timber remains susceptible to rot, warping, and insect damage over time.
Pros: Inexpensive, widely available, familiar to tradespeople.
Cons: Requires airflow and proper drainage, vulnerable to moisture and decay if incorrectly specified.
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Plas-Pro Recycled Plastic Joists
Plas-Pro joists are made from 100% recycled plastic, offering a rot-proof, moisture-resistant alternative. They are ideal for wet environments such as roof terraces, boardwalks, and pool surrounds.
Pros: Maintenance-free, impervious to rot, usable in damp conditions, environmentally friendly.
Cons: Heavier than timber, lower strength-to-weight ratio, needs closer joist spacing (300–400mm centres).
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DuoSpan Aluminium Joists
The pinnacle of decking subframes. DuoSpan combines rigid aluminium with flexible recycled plastic components, enabling precision installations, strength, and complex designs such as curves, cantilevers, and floating decks.
Pros: Lifetime durability, moisture-immune, no warping or twisting, incredibly straight for quick installation.
Cons: Higher upfront cost, but offers unbeatable long-term value.
Recommended Joist Spacing
Joist spacing should match the application and deck board orientation:
If using Millboard decking boards (e.g., Enhanced Grain or Weathered Oak), always ensure the boards are supported by a minimum of three joists, particularly where cut sections are used.
Installing Joists: Key Considerations
Ventilation: Timber joists require airflow under and around the structure. Plas-Pro and DuoSpan do not.
Fall Gradient: A slight fall of 1:200 away from buildings is advised to aid water drainage.
Gapping: Boards need a 4mm side gap (6mm with DuoFix), and a 1mm end gap. Allow 10mm against walls or abutting structures for expansion.
Fixings: Use only Durafix screws for Millboard products. These fasten through the face with a ‘lost-head’ finish, or use the DuoFix side-fixing system for clean line
Elevation and Level Control: DuoLift Pedestal System
Where height adjustment is needed — such as roof terraces or uneven patios — the DuoLift system simplifies subframe installation. With components stacking from 15mm up to 350mm, it ensures level, precise deck builds without compromising strength.
Sustainability Benefits
Millboard subframes, especially Plas-Pro and DuoSpan, reflect our commitment to sustainable innovation:
- Made from recycled materials.
- Designed to last decades without degradation.
- Require no chemical treatments, paints, or preservatives.
This durability translates to less waste, fewer replacements, and a lower carbon footprint over the lifetime of your outdoor space.
Summary: Which Joist Type Is Right for You?
Build Your Deck to Last
At Millboard, we believe every element of your decking — from the visible surface to the hidden framework — should embody quality, resilience, and design excellence. Whether you're enhancing a home garden, constructing a rooftop terrace, or designing a commercial boardwalk, the right joist system makes all the difference.
Ready to start your project? Find an Approved Installer or contact us for technical support and expert advice.
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