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Can Composite Decking be laid on concrete?

Rebecca Smith

February 1, 2021

Yes! Here's how to install composite decking on a concrete base correctly

Can Composite Decking be laid on concrete?

In short, yes! Composite Decking can be laid on concrete AS LONG as you have the correct sub-frame to do so, normally joists. Not sure what that means? Then this blog will most certainly help.

It's really important that you lay your composite decking correctly on a suitable sub-frame. Unfortunately incorrect installation of your sub-frame can lead to long-term problem with your decking and in most cases installation on an incorrect sub-frame will invalidate any warranty you have which is the last thing you want.

In this blog we are here to help guide you in the right direction and give you helpful tips on how to install your decking boards and sub-frame correctly on to concrete.

Note: Please be aware that this is only a guide and the instructions outlined may not be suitable for every project. We strongly recommend speaking to a professional and getting personalised advice for your specific project before laying and building any sub-frames, if you are not qualified, or have no previous relevant experience.

As we mentioned earlier, your decking should always be installed on a flat and stable base. Decking boards must NEVER be laid directly on the ground but instead should be fixed to joists of a suitable sub-frame. In most cases we recommend installing your Alpha Composite Decking on a solid concrete base using plastic composite joists. Whilst we recommend our Alpha Composite joists, as an alternative subframe option - plastic, timber or metal joists can also be used.

What are the benefits of installing your decking on to a concrete base?

  • A faster installation process: A pre-existing solid concrete base cuts installation time in half, as there is no need build a new base prior to installation.
  • Cover up your old concrete: If your existing concrete patio is worn, discoloured and becoming an eyesore you do not have to worry about removing it and re-pouring new concrete. You can cover over your patio with your brand new composite decking
  • Forget the weeds: When you install composite decking on a pre-existing concrete patio, you do not have to worry about laying a weed membrane which you would have to when laying over softer ground
  • Concrete is drier: Concrete provides a dry, moisture-free base that will naturally aid water run off and drainage
  • Save money: While you can remove the concrete before putting in your composite decking, it’s often far more cost-effective to build the composite deck over the concrete if your concrete is in good condition

Things to be aware of

Some common issues to be aware of with your concrete patio include:

  • Water run-off or pooling problems: Concrete should be sloped away from the home to draw moisture away. If you notice areas where water seems to linger, you may need to use a concrete levelling compound to create a flat surface that follows the slope away from your home.
  • Uneven surface: Depending on the condition of your concrete structure, you may need to level the surface to create a strong foundation for your composite deck. You can try using a concrete levelling compound to level the surface.
  • Excessive cracking, crumbling, or sinking: If your concrete slab is in total disrepair with cracks, crumbling, or persistent sinking in one or more areas, you may have to completely remove it before building your composite deck.

Step 1 - Prepare the Patio Area for Decking Installation

Alpha Composite Decking laid on concrete

The concrete base should be at least 100mm thick and have a slope with a decline of 1% - 1.5% that runs away from the property to aid drainage and the run-off of water that may collect under the decking. Tip: You may also want to add 6mm-8mm drainage holes the depth of the base every 4-5m2 to aid drainage.

Step 2 - Laying the joists

Alpha Composite Decking laid on concrete

Spacing - A gap of at least 10mm should be left between the ends of joists and any fixed object such as a walls. Where double joists are required, a gap of 3mm should be left between the 2 joists that lay next to each. Double joists are required where 2 board ends will meet (butt-joins) see picture 6 ahead. Where 2 joist ends meet (butt-joins) an expansion gap of 8mm should be left.

We recommend that the maximum space between joist rows should never exceed 300mm.  Note, boards laid diagonally should be support by joists spaced no more than 250mm apart. Each board cut should be support by a minimum of 3 joists.

To begin attaching the joists to the concrete base, start off by pre-drilling holes in the centre groove of your joists. Your first fixing should be approximately 60-70mm from the joist edge. Continue along the joist screwing at 500mm intervals using a drill bit of sufficient length to create a secure fixing. Expansion screws may be needed. Tip: For extra support and longevity we advise using noggins between joists.

Step 3 - Installing your decking boards

Your sub-frame is now ready for your decking boards to be laid. You can download our full installation guide here to learn how to lay your decking boards correctly.

Remember, if you need any further helps, tips or advice please feel free to get in touch where we'll be more than happy to help.

Are you ready for the next step? Use our FREE composite decking cost calculator to work out exactly how much you'll need for you project.