How to Design a Raised Composite Deck That Lasts for Decades

Visual showing a beautifully designed composite decking area, illustrating the clean, modern finish and garden design quality that Duralive Decking delivers across Merseyside and Cheshire.

Introduction: Why the Right Deck Design Matters

In the North West — especially across Merseyside and Cheshire — raised decking areas are a fantastic way to make the most of uneven or sloped gardens. But one of the biggest misconceptions homeowners have is that all decks are built the same. The truth is, a deck that’s been properly designed and ventilated can last decades longer than one that hasn’t.

At Duralive Decking, we’ve seen the difference first-hand. Whether you choose Millboard, NewTechWood or Trex, the way your sub-frame is designed, how airflow is managed, and how the boards are installed makes all the difference to performance and longevity. Here’s how to make sure your deck is built to last.

The Benefits of Composite Over Timber

Traditional timber decking has long been popular, but anyone who’s owned it knows the constant upkeep it demands. It fades, warps, becomes slippery in the rain, and is prone to rot — especially when built close to or in contact with the ground, where moisture is trapped and airflow is restricted.

Composite decking offers the natural look of timber with none of its weaknesses. Here’s why more homeowners across Merseyside and Cheshire are making the switch:

• Weather resistance: Engineered to withstand the UK’s unpredictable weather — from freezing winters to wet summers. (Read our full guide on Weather Resistance of Composite Decking to learn more about how it performs in the British climate).

  
  

• Low maintenance: No need for painting, staining or sealing — just an occasional clean with warm, soapy water. (See our article on The Truth About Composite Decking Maintenance for a simple step-by-step care routine).

  
  

• Slip resistance: Textured finishes provide superior grip, even in wet conditions.

  
  

• Long lifespan: Premium boards like Millboard, NewTechWood, and Trex come with warranties of up to 25 years.

  
  

In short, composite decking gives you the timeless look of wood without the headaches — perfect for busy families who want to enjoy their garden, not maintain it.

How to Design a Deck That Lasts

A raised composite deck is only as good as its design and sub-frame. Even the highest-quality decking boards won’t perform well if the structure beneath them isn’t built correctly.

1. Airflow is Essential — and It Must Be Continuous

Airflow beneath a deck isn’t just a minor detail — it’s one of the most important factors in ensuring your structure lasts for decades. Without ventilation, trapped moisture causes timber sub-frames to rot prematurely and can even affect composite-wood or steel systems over time.

A common misconception is that the gaps between decking boards are enough for airflow. While they allow some surface ventilation, air moves horizontally beneath the deck, not vertically through the gaps. If that airflow path is blocked, moisture will stay trapped and the structure won’t dry properly.

To ensure long-term performance, airflow must be continuous from front to back, with a clear, unobstructed route:

• Front-to-back ventilation: Air doesn’t bend or weave around objects — it needs a straight, open path to flow from one side to the other.

• Space from walls and structures: When building against a house or wall, we always pull the sub-frame back by around 50 mm and leave the decking boards at least 10 mm away from the wall. This creates a clean ventilation channel and reduces the likelihood of dirt, cobwebs, and debris accumulating behind the boards.

• Adequate clearance: There should be a minimum of 50 mm of continuous airflow beneath a timber sub-frame. More is always better, but 50 mm is the minimum space needed for effective cross-ventilation and drainage. Even when building over solid bases like patios or concrete slabs, we raise the frame up on posts or adjustable pedestals to maintain this void and prevent moisture build-up.

• Natural airflow front and back: The front and rear edges of the deck remain open enough to allow air to travel freely beneath the structure. This unobstructed crossflow is all that’s needed to keep the sub-frame dry and healthy.

  
  

When this airflow principle is designed in from the start, the sub-frame stays dry, the boards remain stable, and the entire structure lasts significantly longer.

Technical illustration showing correct airflow beneath a composite decking structure — including a 50 mm gap between the sub-frame and wall, and a 10 mm gap between the deck boards and wall for proper ventilation.

2. Drainage and Ground Preparation

Good drainage is just as vital as ventilation. Without it, water can sit beneath the deck and saturate the sub-frame, dramatically shortening its lifespan. Standing water leads to excess moisture, mould growth, and eventually timber rot — even if the joists have been treated.

At Duralive Decking, we treat the groundworks beneath every deck with the same care as the deck structure itself. It starts with levelling and grading the area to create a slight fall — typically around 1:100 (a 10 mm drop over every metre) — to ensure surface water always flows away from the house or adjoining structures.

We then install a permeable weed membrane, which serves two purposes: it prevents vegetation from growing up through the deck, and it allows rainwater to pass through freely rather than pooling on the surface. On raised decks, we make sure the membrane and sub-base system work together to create a draining, breathable layer below the frame.

For low-profile decks where space is tight, good drainage design becomes even more important. In these cases, we often install a free-draining sub-base made up of compacted gravel or MOT Type 3 (a more permeable alternative to Type 1). This prevents standing water and promotes quick runoff beneath the sub-frame.

If the deck is being built over an existing hard surface such as a patio or concrete slab, we always check that:

• The surface has an adequate fall away from the property.

• There are weep holes or edges that allow water to escape freely.

• The decking frame is raised off the slab on posts or adjustable pedestals to prevent water from being trapped underneath.

  
  

In particularly wet or shaded gardens, we may also recommend installing a French drain or linear channel along the front edge of the deck to catch and redirect water.

Taking the time to prepare the ground properly ensures your sub-frame remains dry, stable, and structurally sound for decades — and it’s one of the main reasons Duralive Decking installations outperform typical or DIY built decking.

3. Choosing the Right Sub-frame

The sub-frame is the backbone of your decking system, and getting it right is essential for long-term performance. While timber frame members are still common, they must be the correct treatment class for the job. Most timber yards sell Use Class 3 (UC3) treated timber, which is intended for above-ground use where it can stay mostly dry. However, in decking, many components — such as posts, supports, or bearers — are in regular contact with moisture or even the ground itself.

For these areas, we always specify Use Class 4 (UC4) treated timber — or better still, recycled plastic — to ensure durability. UC4 timber is pressure-treated for direct ground contact and offers long-term resistance to rot and fungal decay, while recycled plastic completely removes the risk of moisture damage altogether.

If a treated timber sub-frame is being used, we also recommend applying a moisture protection tape along the top of each frame member before laying the decking boards. This creates a waterproof barrier between the frame and the boards, stopping standing water from soaking into the timber where it’s most vulnerable.

The top surface of the deck frame is one of the first areas to deteriorate on any timber deck, as it’s a flat section that collects water and dries slowly. Using a high-quality deck tape can dramatically increase the lifespan of the structure.

Using a good-quality deck tape from a reputable manufacturer, such as Walther Strong, helps protect the top of the sub-frame from moisture penetration. This type of non-butyl, self-sealing tape is designed specifically for timber decking systems and will self-seal tightly around fixings to prevent water ingress. It maintains long-term adhesion without softening in heat or becoming brittle in cold weather. It’s a small addition that can make a big difference to the overall lifespan of your deck structure.

Where budgets and specifications allow, we also use recycled plastic or aluminium framing systems, which combine strength, precision, and complete resistance to rot or warping.

4. Correct Fixings and Expansion Gaps

Every composite brand has its own fixing method and spacing requirements, and following the manufacturer’s guidelines is essential for both performance and warranty coverage.

• Trex: Boards are typically installed using hidden clip fasteners on grooved-edge boards, which allow the decking to expand and contract naturally with temperature changes. For areas where a solid-edge board is preferred — such as step treads, picture frames, or perimeter details — Trex supplies a ¼-inch router bit that allows a groove to be routed into the edge locally where clips will be positioned. This clever solution means solid boards can still be used neatly at the deck edge while integrating seamlessly into the main body of the deck using the same hidden fastener system.

  
  

• NewTechWood: Uses a side-groove clip system that includes both standard (floating) clips to allow movement and locking clips to secure rows in place. This system maintains consistent spacing and prevents any lateral board drift.

• Millboard: Typically face-fixed using Durafix® screws, which self-sink beneath the flexible Lastane® surface layer, leaving a virtually invisible finish with no need for colour-matched caps or plugs. Millboard boards do not use side grooves or hidden clips, but they do offer the DuoFix® Side Fixing Guide, a precision tool that allows screws to be installed neatly through the sides of the boards rather than the top face, maintaining a clean surface finish. When using the DuoFix® system, gaps between boards must be set at 6 mm to allow the guide to fit and ensure proper board spacing and drainage.

  
  

At Duralive Decking, we always follow each manufacturer’s installation guidance to the letter. This ensures every board can move as intended without lifting, buckling, or damaging the fixings over time.

Choosing the Right Decking Brand

Each premium brand has its strengths, and the best choice depends on your design goals and budget:

• Millboard – The most realistic wood-look board available, moulded from real oak timber and hand-coloured for natural variation. Incredibly durable and ideal for high-end outdoor living.

  
  

• NewTechWood – Known for its UltraShield capping, which completely seals each board to resist stains, fading, and moisture. Great all-round performance and a contemporary finish.

  
  

• Trex – A globally trusted brand made from 95% recycled materials, offering excellent value, long warranties, and a broad range of colours.

We supply and install all three, offering honest, independent advice based on what’s right for your garden — not what’s most profitable for us.

Cost vs Value

Composite decking does cost more upfront than softwood, but it’s a clear investment in long-term value. Timber decking typically lasts 8–10 years before it needs major repair or replacement, while a well-designed composite deck can last 25 years or more with almost no maintenance.

When you factor in reduced maintenance, increased durability, and improved property appeal, the long-term cost of composite decking is actually lower — and the value it adds to your outdoor space is significant.

Final Thoughts: Build It Once, Build It Right

Designing a raised composite deck that lasts for decades isn’t about shortcuts — it’s about craftsmanship, planning, and attention to detail.

At Duralive Decking, we combine precision design, proper ventilation, quality materials, and professional installation to deliver outdoor spaces that stand the test of time.

If you’re thinking about composite decking in Merseyside or Cheshire, get in touch today. We’ll help you design and build a deck that looks beautiful, feels solid underfoot, and performs perfectly for decades to come.

📞 Contact Duralive Decking to arrange your free design consultation.