How to Build a Decking Subframe


Did you know that 70% of decking failures are due to an unstable subframe? Picture this: you’ve just invested time and money into creating a beautiful deck, only to watch it warp and decay within a year. Heartbreaking, right?

But here’s the good news: it doesn’t have to be this way. Building a stable subframe is the unsung hero of a long-lasting deck, and you’re about to discover how to do it right. 

Whether you’re considering eco-friendly materials or pondering the spacing between joists, this guide will address your concerns and set you on the path to decking success. So, why is this so crucial? Because a deck is only as strong as its foundation.

Introduction to Decking Subframes

When it comes to building a deck, many focus on the visible parts: the decking boards, the railings, and the overall design beneath the surface lies the unsung hero of any deck: the decking subframe.

Importance of a Stable Subframe

Deck subframe construction is vital to any decking project. A decking subframe is not just a structure that sits beneath a deck; it’s the backbone of your entire decking endeavour. Think of it as the foundation of a house.

Just as a house needs a strong foundation to support its weight and withstand external forces, a deck requires a robust subframe to provide support for the decking boards. Without a stable subframe, even the most beautifully designed deck can become unstable, leading to potential safety hazards.

The subframe’s role is pivotal. It ensures that the deck remains level, can support the weight of furniture and people, and can withstand external elements like wind, rain, and snow. The subframe is typically constructed with beams attached to support posts, hidden from view but playing a vital role in the deck’s overall stability. This is why many homeowners who prioritize the quality and stability of the subframe in their projects seek professional decking installation services in London.

Key to Long-lasting Decking

The longevity of your deck is directly tied to the quality and stability of its subframe. A well-constructed subframe, whether made from timber or alternative materials like plastic, ensures that the deck remains sturdy and functional for years to come.

When building a decking subframe, it’s essential to consider the decking subframe design and decking subframe spacing. Proper spacing is crucial to ensure that the deck can support weight without sagging or collapsing. There are various tools and resources available, from a decking subframe spacing calculator to a decking subframe spacing guide, which can assist in determining the optimal spacing for your deck.

The choice of decking subframe materials plays a significant role in the deck’s lifespan. While timber is a popular choice, plastic subframes have gained traction as a durable and eco-friendly alternative. They offer resistance to rot and pests, ensuring a longer-lasting structure.

Materials for Decking Subframe

When it comes to building a decking subframe, the choice of materials plays a pivotal role in determining the longevity, durability, and overall performance of the deck. With advancements in technology and a growing emphasis on sustainability, composite materials have emerged as a popular choice for decking subframes.

Recommended Materials for Composite Decking

Composite Joists: One of the standout materials in the realm of composite decking is the WPC (wood plastic composite) joist. These joists are crafted from a unique blend that combines the best of both wood and plastic. The result? A water-resistant and rot-free material that stands as a recommended solution for subframes. When you’re looking to build your deck with a focus on stability and longevity, composite joists are a go-to choice.

Triton Composite Decking Subframe: Another noteworthy mention in the composite decking arena is the Triton composite decking subframe. This construction marvel is not just about strength and stability, it’s also about being eco-friendly. With its non-slip surface, and resistance to algae growth, rot, splintering, and warping, it’s virtually maintenance-free. So, whether you’re working on a raised deck or a ground-level one, Triton ensures your deck surface remains impeccable.

Benefits of Eco-friendly Materials

The shift towards eco-friendly materials isn’t just a trend, it’s a necessity. And when it comes to decking, the benefits are manifold.

Plastic-free and Environmentally Conscious: One of the frontrunners in the eco-friendly decking materials race is Millboard composite decking. What sets it apart? It’s plastic-free. In an age where environmental concerns are paramount, choosing materials that are kind to the planet is not just a choice, it’s a responsibility.

Durability that Stands the Test of Time: When Trex Protect delved into examining various eco-friendly deck options, from wood and aluminium to PVC and composite, one thing became clear: composite decking materials are in a league of their own when it comes to durability. Whether it’s the scorching sun or the relentless rain, composite deck boards remain unfazed.

Resistance is the Key: One of the standout features of composite wood is its low water absorption rates. This means that issues like mould and fungus are kept at bay. And when water isn’t a concern, you can be assured that your deck won’t rot, warp, or twist. So, whether you’re using a spirit level or ensuring a 10mm expansion gap, with composite materials, you’re always a step ahead.


Decking subframe construction

Building a Decking Subframe on Hard Surfaces

Constructing a decking subframe on hard surfaces requires meticulous planning and execution. The foundation you lay will determine the longevity and stability of your deck. Let’s delve into the essential steps to ensure a robust decking subframe on hard terrains.

Preparing the Area: Ensuring Ground Stability

The initial step in building a decking subframe is to ensure that the ground is sound, stable, and flat. This is the bedrock of your decking project. Whether you’re working on a garden decking or a terrace, the ground’s stability is paramount. If you’re planning to build a basic ground-level deck or a slightly raised deck, the principles remain the same: a stable ground ensures a stable deck.

Using Adjustable Pedestals for Leveling

On encountering a sloped surface, the challenge amplifies. This is where adjustable pedestals come into play. These tools are not just for levelling, they are integral to the decking subframe construction. Adjustable pedestals can support the decking substructure on solid flat foundations, flat roofs, and even on a poolside for that perfect pool decking. Whether you’re a DIYer or a professional, using a decking subframe spacing calculator or a decking subframe spacing guide can assist in determining the optimal placement of these pedestals.

Importance of Ventilation and Drainage

A deck isn’t just about aesthetics, it’s about functionality. And one of the most overlooked aspects of decking is ventilation and drainage. When building on hard surfaces, especially if you’re using solid composite deck boards, ensuring proper ventilation and drainage becomes crucial. By raising the subframe and decking from the ground using adjustable pedestals, you promote efficient drainage. This prevents water from pooling on the decking, ensuring the longevity of your deck. Proper ventilation prevents the buildup of moisture, protecting your decking from potential rot and decay.

Building a Decking Subframe on Soft Surfaces

Constructing a decking subframe on soft surfaces, such as gardens or lawns, presents its own set of challenges. With the right approach and meticulous planning, you can ensure a robust and durable decking structure. Let’s delve into the essential steps for building a decking subframe on softer terrains.

Preparing the Ground: Removing the Top Layer and Adding Gravel

At Pi Gardening, we recommend that you prepare the ground underneath the first beam before you even think about placing it. Start by removing topsoil and vegetation. This will not only get rid of any organic material that may decay over time, but it will also provide a more level surface. Once this is done, add a layer of gravel or firm soil. It serves as a stable base for the subframe, ensuring that the pavement will remain level and stable even when the ground beneath it may shift or settle.

Importance of Water Drainage and Weed Control

Water and weeds – the two arch-nemeses of any decking project. Once your gravel layer is down, it’s time to lay a weed barrier. This dual-purpose layer ensures that those pesky weeds don’t find their way up through your decking. It aids in drainage, ensuring that water doesn’t pool beneath your deck, which could lead to rot or other structural issues.

Recommended Height and Slope Considerations

When setting up your subframe, height and slope are two crucial considerations. The subframe should be at least 150mm above the ground. This elevation ensures proper ventilation, preventing moisture build-up, and also aids in drainage. Always ensure that your subframe slopes away from any adjacent structures, especially your house. This ensures that any water runoff is directed away, preventing potential water damage or flooding issues.

Joists and Spacing

The backbone of any decking structure lies in its joists and the spacing between them. These elements determine the deck’s stability, strength, and longevity. Let’s delve deeper into the intricacies of joists and their spacing.

Recommended Materials and Sizes for Joists

When it comes to joists, the choice of material is paramount. The most commonly recommended materials include:

  • Wood: A traditional choice, wood offers a natural aesthetic and can be treated to resist rot and insects.
  • Steel: Known for its strength and durability, steel is often used in commercial decking projects or where longer spans are required.
  • Composite: A blend of wood fibres and plastic, composite joists offer the best of both worlds – the look of wood and the durability of plastic.

The size of the joists is equally important and is determined by their span and spacing. For instance:

  • 2-by-6 joists should ideally span 9 1/2 feet.
  • 2-by-8 joists are best suited for spans up to 13 feet.
  • 2-by-10 joists can comfortably span 16 1/2 feet.

Spacing Considerations Based on Decking Board Thickness

The thickness of your decking boards directly influences the spacing of your joists. While the standard spacing for deck joists is 16 inches in the centre, some builders opt for a 12-inch spacing to fortify the deck frame or to accommodate longer joist spans. The precise maximum joist spacing will hinge on your joist size, framing material, and the maximum span you’re aiming for.

Importance of Proper Joist Spacing for Stability

The significance of correct joist spacing cannot be overstated. Joists are the primary support for decking boards. If spaced too widely, the boards might sag or warp as time goes by. Uneven spacing can also compromise the deck’s stability, making it potentially unsafe. Ensuring that joists are evenly spaced and in accordance with the decking board thickness is crucial for a deck’s stability and longevity.


Subframe for decking

Support and Fixing the Decking Subframe

Ensuring the stability and longevity of a decking subframe requires meticulous attention to its support and fixing mechanisms. From using robust support posts to selecting the right fixings, every detail matters. Let’s explore the best practices for supporting and fixing a decking subframe.

Using Support Posts and Bearers for Raised Decks

Raised decks, by their very nature, demand additional support to ensure they remain stable and secure. This is where support posts and bearers come into play.

  • Support Posts: These are the vertical structures that provide the primary support for raised decks. Ideally, they should be positioned in footings, acting as anchors for the frame. This ensures that the deck remains elevated and doesn’t sag or collapse under weight.
  • Bearers: Acting as horizontal supports, bearers are attached to the support posts. They provide an additional layer of stability, ensuring that the weight of the deck is evenly distributed across the support posts.

Importance of Sturdy and Corrosion-Resistant Fixings

The choice of fixings can make or break the stability of your decking subframe. It’s imperative to opt for:

  • Sturdy Fixings: These ensure that the various components of the subframe are held together securely, preventing any wobbling or shifting over time.
  • Corrosion-Resistant Fixings: Given that decks are exposed to the elements, using corrosion-resistant fixings is non-negotiable. This ensures that the fixings don’t rust or deteriorate, maintaining the structural integrity of the subframe.

Tips for Affixing the Frame to a House Wall

Connecting your decking subframe to a house wall requires precision and the right techniques. Here are some tips:

  • Use a Ledger Board: A ledger board acts as a bridge between the subframe and the house wall. It provides a solid base to which the subframe can be attached.
  • Lag Screws or Bolts: When securing the ledger board to the wall, it’s advisable to use 63 mm solid composite deck screws or bolts. These provide a robust connection, ensuring that the deck remains firmly attached to the house.


Deck subframe design

Safety and Maintenance Tips

Decking subframes, while providing structural support, also requires consistent attention to safety and maintenance. Ensuring the longevity and safety of your decking subframe is paramount for the overall durability of your deck. Let’s delve into the essential safety and maintenance tips for decking subframes.

Ensuring the Safety of the Deck and its Users

The safety of those using the deck is of utmost importance. Regular checks and preventive measures can go a long way in ensuring this:

  • Regular Inspections: Periodically inspect the decking subframe for any signs of wear, tear, rot, or damage. This proactive approach can help in identifying potential issues before they escalate.
  • Secure Fixings: Ensure that the subframe is anchored securely and that all fixings are tightened appropriately. Loose fixings can compromise the stability of the deck.
  • Safety Features: Consider integrating additional safety features such as handrails, especially for raised decks. Adequate lighting can also enhance visibility during the evenings, further ensuring safety.

Longevity and Maintenance of Tanalised Timbers

Tanalised timbers, given their preservative treatment, offer resistance against rot and decay. They still require maintenance:

  • Treat Cut Ends: Whenever you cut tanalised timbers, ensure that the cut ends are treated. This prevents the exposed sections from becoming vulnerable to decay.
  • Water Runoff: Incorporate a slight decline in your decking design. This aids in water runoff, preventing water accumulation, which can be detrimental to the timber.

Tips for Preventing Premature Decay

The longevity of your decking subframe is closely tied to its resistance to decay. Here are some preventive measures:

    • Proper Ventilation: Ensure that the subframe is well-ventilated. This prevents moisture accumulation, which can lead to rot.
    • Drainage: Facilitate efficient water drainage away from the subframe. Stagnant water can be a breeding ground for mould and fungi, which can hasten decay.
    • Avoid Harsh Chemicals: When cleaning your deck, steer clear of aggressive chemicals or bleach. These can erode the integrity of the subframe and reduce its lifespan.


Building a decking subframe is akin to laying the groundwork for a house. It’s the unseen hero that ensures stability, safety, and longevity. From selecting the right materials to understanding the intricacies of joist spacing, every detail matters. As you embark on this journey, remember that a well-constructed subframe is the cornerstone of a deck that stands the test of time.