What Goes Under Composite Decking? A Complete Guide

Composite decking should sit on a stable, level and well-drained supporting frame. Beneath that frame, the preparation depends on whether the site is soil, concrete, paving or another suitable surface. Each layer, from the ground below to the decking boards above, has a specific purpose in creating a durable installation. 

This guide explains how the build-up changes between surfaces, why drainage and ventilation matter, and what to consider before choosing your decking system and installation materials.

Understand the Four Layers Beneath a Composite Deck

Most composite decking installations include four layers: the existing ground or surface, any required sub-base, the supporting frame and the decking boards. Each layer has a different role in creating a stable, well-drained structure.

A deck built on soil needs more preparation than one installed over a sound concrete slab, although both require a suitable frame.

The Base Is Not the Same as the Frame

The base prepares the site, while the frame supports the decking boards. Neither can replace the other, making both essential for a reliable installation.

Assess the Existing Surface Before Choosing the Build-Up

The correct build-up depends on whether the deck sits on soil, concrete or paving. Before choosing materials, check the surface is stable, drains well and suits the planned installation.

Inspect:

• Ground stability

• Drainage and standing water

• Slopes and levels

• Door thresholds

• Air bricks and damp-proof courses

• Finished deck height

Loose paving or poor drainage should be addressed before installing the frame.

Check the Finished Deck Height

Measure the finished deck level, including the frame and boards. Check door clearances, keep air bricks clear and maintain the correct relationship with damp-proof courses before ordering materials.

Prepare Soil Before Building the Decking Frame

Soil should be cleared, levelled and prepared before installing the supporting frame. Composite decking boards and joists should never rest directly on bare ground.

Preparation usually includes:

1. Remove turf and vegetation.

2. Excavate where needed.

3. Check for unstable ground.

4. Create suitable drainage falls.

5. Install and compact a suitable sub-base.

6. Position the frame on appropriate supports.

Removing grass alone does not create a stable foundation. If you're planning a deck on soil, read our guide on how to lay decking on soil for more detailed advice.

Choose and Compact the Correct Sub-Base

The sub-base should suit the site conditions and chosen frame. Proper compaction improves stability and drainage, while the material and depth should always follow the manufacturer's guidance.

Keep the Frame Off Bare Ground

The supporting frame should never sit directly on soil. Suitable supports reduce moisture exposure, improve drainage and make future inspection easier.

Should You Use a Weed Membrane?

A weed membrane can help reduce unwanted growth beneath a ground-level deck, although it does not replace excavation, compaction, drainage planning or structural support.

Installed in the correct position within the build-up, a permeable membrane helps separate soil from aggregate and can make the area beneath the deck easier to maintain.

A membrane still requires suitable preparation beneath it. Frame supports remain separate from the membrane, allowing the decking structure to perform as intended.

Use a Suitable Frame to Support the Decking Boards

Composite decking boards should be fixed to a supporting frame designed for the chosen decking system. The frame supports the boards, provides secure fixing points and helps keep the surface level. It should be planned as a complete system, including joists, supports and board-end reinforcement where required.

Timber, Composite or Metal Frame?

Timber, composite and metal frames each suit different projects. The right choice depends on site conditions, deck height, moisture exposure and compatibility with the selected decking system. Follow the manufacturer's guidance when choosing frame materials and fixings.

Follow the Correct Joist Spacing

Joist spacing should always match the requirements of the selected decking boards. Provide adequate support beneath board ends and joins, and avoid increasing spacing to reduce material costs. Correct framing improves stability and helps the finished deck perform as intended. Once your frame design is complete, follow our guide on how to install composite decking to complete the next stage of your project.

Choose the Right Sub-Base Material

The right sub-base depends on the existing ground, drainage requirements and frame design. Compacted aggregate may suit some soil installations, while a sound concrete slab or stable paving can provide the existing hard surface for a separate support frame.

Remember, a drainage layer is not the same as a structural support. Loose gravel alone will not provide a suitable base for decking joists.

Install Composite Decking Over Concrete or Paving Carefully

Composite decking can be installed over sound concrete or paving using a suitable support frame. Check the surface is stable, well drained and capable of supporting the planned installation before work begins. Decking boards should never be fixed directly onto concrete or paving.

Do Not Trap Water Between the Frame and Surface

Maintain existing drainage falls and avoid blocking drainage channels beneath the deck. Suitable supports help preserve airflow, while keeping debris clear reduces the risk of standing water around the frame.

Plan Drainage Beneath the Deck

Effective drainage allows water to move away from the supporting frame and beneath the decking. Planning drainage before installation reduces the risk of persistent damp conditions and standing water.

Use this checklist before building:

• Check existing ground falls.

• Maintain drainage outlets.

• Avoid creating low spots beneath the frame.

• Keep gaps between boards clear.

• Consider drainage layers where appropriate.

• Allow access beneath the deck for future inspection.

• Prevent downpipes from discharging directly beneath the structure.

• Keep air bricks unobstructed.

Board gaps allow rainwater through the surface, although they cannot compensate for poor drainage beneath the deck.

Keep Water Away from the Building

The finished deck should not obstruct damp-proof courses, air bricks or existing drainage routes. Where installation alters ground levels beside a property, seek professional guidance before work begins.

Maintain Ventilation Beneath Composite Decking

Air circulation beneath composite decking helps the supporting structure dry after rainfall and limits prolonged moisture exposure. Ventilation should remain unobstructed throughout the life of the deck.

Good practice includes:

• Maintaining suitable airflow around the perimeter where required.

• Keeping debris beneath the deck to a minimum.

• Avoiding fully enclosed sides unless ventilation has been incorporated.

• Following product-specific clearance guidance.

• Keeping inspection areas accessible.

Composite boards are designed to withstand outdoor conditions, although the supporting structure still benefits from good airflow.

Prevent Rats and Pests Beneath the Deck Without Blocking Airflow

Deck perimeters can be protected without restricting ventilation. Good housekeeping beneath the structure reduces the likelihood of attracting pests while preserving drainage and inspection access.

Consider the following:

• Remove leaves and organic debris.

• Avoid storing food or waste beneath the deck.

• Install suitable mesh where appropriate.

• Maintain drainage routes.

• Leave ventilation openings clear.

• Resolve existing pest problems before construction.

If you're planning long-term under-deck maintenance, our guide explaining how to prevent rats under decking provides additional advice.

Avoid These Common Base and Frame Mistakes

Most installation problems begin beneath the decking rather than on the visible surface.

How the Foundation Affects the Life of the Deck

A stable, drained and ventilated foundation helps the decking remain properly supported throughout its service life. While no installation can guarantee a fixed lifespan, careful preparation reduces unnecessary movement, uneven surfaces and moisture-related issues.

Regular inspections also make it easier to identify localised problems before they develop into larger repairs.

What Should You Check Before Ordering Materials?

Confirm the installation plan before purchasing decking boards or accessories.

Your checklist should include:

• Site measurements

• Existing surface type

• Ground preparation requirements

• Sub-base materials

• Weed membrane

• Frame material

• Product-specific joist requirements

• Board direction

• Steps and edge details

• Drainage

• Ventilation

• Clips, screws and trims

• Installation access

Planning hidden components alongside the visible boards helps avoid delays and repeat orders. Once you've confirmed your frame design, you can choose the most suitable composite decking for your project or contact us if you need guidance on compatible components.

When Should You Use a Professional Installer?

Professional installation is recommended for raised decks, unstable ground, complex drainage, retaining structures, attached decks, steps, balustrades or projects with uncertain structural requirements.

A low-level deck on a sound patio may suit an experienced DIY installer. More complex designs benefit from professional planning to ensure the supporting structure meets the project's requirements.

Frequently Asked Questions About What Goes Under Composite Decking

Can Composite Decking Go Directly on Soil?

No. Composite decking boards require a suitable support frame built over prepared ground.

Can Composite Decking Go Straight Over Concrete?

No. A compatible support frame remains necessary, even when the concrete is stable and well drained.

Do You Need Gravel Under Composite Decking?

Not always. Gravel or aggregate may form part of some build-ups, although it does not replace structural supports.

Do You Need Weed Membrane Under Decking?

It can help reduce weed growth beneath ground-level decks, provided the site has already been prepared correctly.

Can Composite Decking Sit on Paving Slabs?

Yes, where the paving is stable, well drained and suitable for supporting an appropriate frame.

What Type of Frame Is Best?

The right frame depends on the decking system, site conditions, moisture exposure and project design.

How Much Space Should Be Left Under Composite Decking?

Follow the clearance and ventilation guidance provided for your chosen decking system and supporting frame.

Composite decking performs best when installed on a stable, level and well-drained supporting frame built over a suitably prepared surface. Assess the site carefully, choose a compatible frame, maintain drainage and ventilation, then follow the installation guidance for your selected system.

Planning the hidden structure before ordering the visible boards creates a stronger installation and helps prevent avoidable problems later. If you're unsure which components suit your project, contact us for advice before placing your order.