The popular question is, “Can composite decking be laid over concrete”?

The short answer is yes, you can lay composite decking over concrete, and no, you can’t. 

Let us explain.

First, why would you want to install composite decking over your pre-existing concrete patio?

  • Well, for one, it improved aesthetics. A deck adds style and character and turns an average outdoor space into something special.
  • It also adds value to your home when you go to sell.
  • Safety is another reason. If your concrete slab is damaged by nearby tree roots, it will crack and rise, creating humps that become a tripping hazard. 

Many older homes have concrete patios.  Like many things, they require ongoing maintenance to keep them looking presentable, but sometimes, they are just too old, cracked and worn, and no amount of paint will improve their appearance or safety.  Installing composite decking over the top will provide a clean, safe and modern look with less effort, and if you select a composite deck, also little to no maintenance is involved.

The good news is you don’t have to remove the existing concrete slab in order to replace it with decking.  There’s a cost saving for you right there.

 You can install your deck over the concrete – but it must be installed on a sub-frame.

You cannot just lay composite decking straight onto the concrete surface because it won’t be even, and you need to provide air movement beneath the composite decking boards. 

Let’s assume you have obtained building permits and local council approval, if required, and are ready to go ahead. In addition to installation instructions, we offer helpful hints on building deck frames in this article about deck frames, but some other points to keep in mind include:

1. Ground Clearance Above Concrete

To be able to lay composite decking over concrete, you need to think about levelling your deck with the doorstep.

The minimum ground clearance over concrete for NewTechWood composite decking is 40mm.

Is your proposed deck going to be an acceptable height for your existing door threshold, given the height of your existing concrete patio?  Ideally, when you walk off your doorstep, you want to either step down to your deck or at least be level with it – you do not want to be stepping up to it. 

There is no need to stress as a professional installer will be able to rectify height issues. The use of low-height deck pedestals is beneficial in these circumstances, and their use can save a lot of time and effort.

2. The Sub-Frame

Your sub-frame can be either timber or metal – the choice is yours.  With NewTechWood decking, we provide fixings for both timber and metal fixing. 

We also offer a quick and easy metal (aluminium) sub-frame system,  StructurAL®, which is ideal for the construction of decks over solid surfaces, such as tiled floors or concrete patios.  It provides strength and ease of installation and, when combined with pedestals, enables a minimal increase from the existing floor level to the finished level.  

If you have a height issue with your proposed deck, this system is perfect for constructing a deck with low height requirements.  This aluminium framing system can also be used with the hidden fastener system or with self-drilling deck screws.

Check out the video to see just how easy it is to install.

Note:  Aluminium is a common choice for the framework when installing decking because it has excellent fire-resistant qualities and is long-lasting.  It won’t sag or rot, and you’ll have no trouble with white ants.  It will definitely outlast a timber sub-frame.

3. Levelling the Surface

Whichever material you use for the sub-frame, must be constructed over a level surface. Otherwise, your deck will not be level. 

To create a level surface, you will need to smooth out any uneven areas in the concrete, and you can do this by using a fast-drying levelling compound or ready-mixed concrete.  Don’t worry about how “pretty” your handiwork is because the deck will cover it.  The primary aim is to produce a level surface for the sub-frame to sit on and, subsequently, the deck.

Note:  To avoid water pooling beneath the deck, when you are levelling your concrete base, allow for a slight slope away from your house if one doesn’t already exist. It does not have to be a drastic slope, just enough to enable any water runoff away from your home.

Remember, another easier option to assist the levelling process is to use decking pedestals, because each pedestal can be independently height adjusted.  These are a quicker option, accurate, and much less messy than cement levelling compounds.

4. Sub-Frame – Span requirements

Natural and composite decking have their specific sub-frame span requirements, regardless of whether you are installing over an old concrete patio or any other hard surface.  For our purposes, we will just talk about NewTechWood’s composite deck construction requirements.

Depending on the decking profile board you have chosen, the decking span required for residential projects will range from 350mm for UH02, 450mm for US49, and 400mm for US54.  Deduct 100mm from these measurements for commercial projects.  Note:  The span for NewTechWood’s specific commercial decking board, the US71, is 600mm.

Don’t assume installing over concrete means your deck will automatically and magically be just as steady and solid. To reduce the deck boards flexing underweight and subsequent damage over the long term, these recommended span requirements need to be adhered to.  For commercial projects, installers can add a reinforcing sleeper at intervals which will boost the strength of the sub-frame even further.

5. Sub-Frame – Securing the concrete

Once you’ve taken the appropriate measurements and marked out where your sleepers will be placed on the concrete, you will attach the sleepers to the concrete using dynabolts and strong steel brackets. Check the level as you go, and if you discover any low areas you missed previously, you can use plastic shims to fill the gap and even it out. 

Again, this is a fairly simple process if you use NIVO pedestals, which also do not require securing the concrete.  Talk about time savers!

6. Install the Decking

Once you have a sturdy, level sub-frame in place, you can then lay your composite decking over concrete in accordance with the relevant guidelines for your chosen product.  Using the hidden fastener system provides the most aesthetically pleasing and clean look because the fixings and screws are not visible on the deck surface – they are hidden between the boards, and on the joists.  This system is designed to accommodate the natural expansion and contraction of decking boards, especially composites. 

Whichever framing system you use, if installing a composite deck for the first time, be sure to read the Installation Guidelines before commencing to avoid accidentally voiding the warranty

So you see, you can install over concrete without removing it, and if you want a really quick and easy sub-frame installation to cover your ageing concrete, consider combining a deck framing system, like StructurAl, with deck pedestals, which will provide a lightweight and stable frame with a high load capacity.

Architects, builders and installers, feel free to contact our team of experts, who will be more than happy to explain your options in further detail and answer your questions. 

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