Can Composite Decking Be Laid On Concrete?

Composite decking can absolutely be laid on pre-existing concrete. In fact, it’s actually a fairly simple process and has many benefits when compared to laying it over other surfaces.

As a material, composite also has lots of advantages over other decking materials such as its longevity, durability, ease of care, slip resistance and water resistance.

In this guide, we’re going to take an in-depth look at why installing composite decking over concrete is a good idea and get you up to speed on how to do it yourself.


The Benefits Of Installing Composite Decking Over Concrete

There are a number of benefits to laying composite decking on concrete rather than on other surfaces such as sand or gravel.

  • No worries about weeds – when you lay composite decking on a pre-existing concrete patio it means that no plants or weeds will grow up through the boards.
  • Quick and easy installation – you won’t need to install or pour a base which will greatly reduce the installation time.
  • Gives your garden a new look – laying composite decking over old concrete can refresh the look of your garden and give it a new style. A new composite deck in your garden will also be beneficial if you’re planning to sell your house.
  • Less moisture – a concrete patio provides a moisture-free base, unlike gravel and sand.
  • No need for concrete patio replacements – rather than having to replace a concrete patio that’s discoloured or cracked you can simply cover it up with a composite deck.


Things To Bear In Mind When Installing Composite Decking Over Concrete

Although it’s fairly quick and easy to install composite decking over concrete, there are several things you’ll need to be aware of before you begin. If you pay close attention to these before you start laying your decking then you shouldn’t have any issues.


Water run-off problems

When concrete patios are installed they should be on a slight slope away from your home so that moisture draws down the slope rather than towards your home.

If water is pooling in one particular area it means that the surface isn’t level. This means that you’ll need to use a concrete levelling compound to make sure that the surface is level and sloped away from your home before you lay your composite decking.


Sinking concrete

Newly laid concrete takes quite a long time to settle and in some cases, it can even take a year or more. If you install composite decking on concrete that is still settling then there’s a good chance that the decking will sink over time.

You can make a small mark on your house foundation close to the patio and a mark on the patio itself, and then use a helper and level to measure whether your patio is still settling. This process will need to be carried out over several months.


Patio stairs

If you have stairs that are attached to your patio then you will need to increase the height of the treads before you begin the composite decking installation.

This is needed to make sure that the height of each individual step remains consistent once the composite decking boards or composite decking tiles are installed.

An easier way of dealing with this issue is to lay decking over the steps. However, this will not always be possible and you will need to check the manufacturer’s instructions regarding water drainage and airflow.


Adjoining doors

If you have a door that leads onto your patio you need to make sure that installing a composite deck isn’t going to create a tripping hazard or prevent the door from opening. Remember, a new composite deck will increase the height of the patio surface by at least 25mm.

An easy way to test this is to take a piece of wood or a concrete slab that’s 3 inches tall, place it next to the door on your patio, and see if the door catches it when you open it. You should also try placing it in front of you and then walking out of the door to see if you would trip over it.

If you do identify an issue during this test it doesn’t necessarily mean that installing decking won’t be possible. However, it would probably be best to ask for a professional opinion about how to resolve the problem.



How To Install Composite Decking Over Concrete

Firstly, you’ll need to ensure that you allocate enough time to see the project through successfully. If you’re only installing composite decking over a small surface area then it will probably take you a couple of days.

For large areas, it could take several weeks depending on the size of the area. In this guide, we’ll show you how to install composite decking boards as they are more commonly used than tiles.


Tools you’ll need

  • Cordless drill
  • Hammer drill
  • Level
  • Straight edge
  • Circular saw/Miter
  • Chalk line reel


Materials you’ll need

  • Decking boards
  • Flashing tape
  • Plastic shims
  • Self-levelling compound
  • Pressure-treated wood 2x4s (sleepers)
  • Concrete screws


Step 1

The very first thing you need to do is to make sure that the concrete patio surface is level. If you notice any uneven areas then you can level them our range of decking pedestals. Remember that the surface should be slightly sloped away from your house.

Then, you need to start planning the layout of your composite deck. You should place the plastic joists and all other surface elements on the concrete and check the spacing between them. We recommend 400mm spans between our Clarity and Forma composite deck boards.

If you like, you can then mark out the layout using painter’s tape.


Step 2

The next step is to start installing the plastic joists. Start from your patio’s highest point and place the first plastic bearer into its position and work your way down. If you come across any low spots in the concrete simply raise them using plastic shims. If you need to raise the deck higher than a few mm, then please use our decking pedestals, we have options of fixed and adjustable decking pedestals, anything from 10mm to over 1000mm.

Next, use the hammer drill to drill holes into the concrete, spacers and plastic joists and then secure the recycled plastic joists into the concrete with concrete screws. As you do this, use the straight edge to check that the plastic joists are consistent with each other. Please ensure you leave a gap between the ends of the plastic joists of around 10-20mm, don’t butt up tight.


Step 3

Installing decking boards onto plastic joists that are screwed into concrete isn’t really any different than when you’re building a regular deck. You should start from the outside of the patio and work your way inward.

Screw in the accent boards first as it will help guide you when screwing in the other boards.


Step 4

This last step is only necessary if you’re planning to lay composite decking over the top of concrete patio steps.

Start off by installing two ledger boards with one on either side of the stoop. Then, install double joists on both sides of the stoop and place a double-header beam between the joists and in front of the stoop.

Install a 150x50mm ledger board above the stoop with 150x50mm joists that run out to the header. Make sure that the joists aren’t making contact with the stoop. Once this is done, you can then start to install the boards.



Useful Tips For Laying Composite Decking On Concrete

Pre-drilling holes

To make sure you don’t split or damage the the plastic, timber or aluminium joists you should always use a hammer drill to pre-drill holes into them before securing them with screws.

Proper gapping

Make sure to create proper gapping during the deck installation. This will allow for shrinkage, expansion and contraction, and water drainage. Using hidden fasteners is a good way of doing this.

Anchored posts

If you need to use posts to adjust the height of your composite deck then you need to ensure they’re firmly anchored to the concrete. This will help you to create a stable frame. Try using metal connectors to attach the posts to the concrete.



How much space should I leave in between composite boards?

The right amount of space to leave between the boards is between 6mm and 4mm.The hidden t-clip will leave a 5mm space. Leaving the right amount of space is very important as it allows for necessary water drainage.

Composite also expands slightly during warm weather so having the boards properly spaced will allow for contraction and expansion without the fasteners being placed under extra stress.


How long will my composite deck last?

On average, composite deck boards will last for 25 to 30 years. As they’re made using recycled wood fibres and plastic polymers they don’t require much maintenance and are very resistant to mould and rot.

This gives them longevity and means they shouldn’t need to be replaced for a long time.


How much does it cost to have a composite deck installed?

Laying composite decking materials over concrete is fairly straightforward and you can probably do it yourself. However, if you’d rather hire a professional to do it the costs will vary greatly depending on the size of the outdoor space that you want the decking to be laid on.

The best way to work out how much it will cost you is to measure the area and then ask for a quote from a contractor.


Final Thoughts

As long as you follow all the right steps and make the necessary preparations installing composite decking over concrete is fairly straightforward.

It actually has quite a lot of benefits and when compared with sand or gravel, concrete is a great surface for installing a deck.

Although the process is quite simple, if you have any concerns about things like levelling then it’s always a good idea to consult with one of our professionals before you begin laying your deck. This way, you can be sure there will be no issues further down the line.

So, check out our wide range of composite decking materials and grab your drill and screws and start laying that deck!

Gary is the founder and CEO of Ecoscape UK. Gary graduated from Liverpool JM University in 2007 with a 2:1 in Economics. A passion for innovation, design and sustainability, Gary has put to market numerous wpc products/systems, some of which have UK and European registered designs. Outside of work Gary enjoys spending time with his family, the great outdoors and watching Manchester United.


Comment (01)

  1. Shaun Day
    April 23, 2024

    “Great article on the benefits of using composite decking on concrete! I’ve been considering updating my patio and this seems like the perfect solution. I’m curious about the purchasing process though, as I’ve never bought composite decking before. Do you have any recommendations or tips for finding the right product? Thanks for the helpful information!”

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