Composite Deck

Can Composite Decking Be Installed Over Concrete?

Imagine that you’re heading into the backyard for a spot of gardening. You’ve donned your sunhat, your gloves, and picked up your secateurs and a bucket in which to collect plant trimmings and off-cuts.

Yet as soon as you step outside onto the patio, pain blasts through your big toe and into your foot. You let out a yelp of pain and stumble backwards, only to feel your heel catch on a crack. A wobble runs through your body and you realise with surprise that you’re tipping backwards. You’ve lost your balance!

With a loud ‘ouch!’ you land butt first on the concrete, and now you’re sporting a bruised bottom as well as a swollen toe… both of which are likely throb and give you grief for up to a week.

Later, when you inspect the crime scene of your patio in an attempt to deduce what led to your injuries, you find the culprit to be the concrete slab itself. Or rather, your neglect for having failed to properly care for the slab these past few summers.

The root systems of nearby trees have forced the slab to hump and crack and buckle upward. That must have been where you stubbed your toe. And over there, you can see where the slab has subsided slightly over time thanks to the movement of earth beneath the concrete itself. This has caused the concrete to fissure into long crevices, which were likely the cause of your fall.

You know that you could have corrected the problem had you acted sooner,  but your failure to do so has amounted to the uneven, hazardous and downright dangerous surface which is now the back patio of your home.


How to rescue my patio?

So, what should you do now that you’ve recognised that your patio is in a state of disrepair?

Well, you could contact a contractor and engage their services to replace the slab… but that’s too expensive and likely to consume too much of your time. Or maybe you could have a ground level deck put down… then again, wooden decks can lead to splinters and are fire hazards when built in such close proximity to the home.

Sigh. It seems there is no end in sight to your patio perils.

Wait. Don’t lose hope just yet… there may be one last solution to your problem that you haven’t yet considered: composite decking. It is fire-resistant. Friendly to bare feet (which means no splinters!). Looks like real wood. And, best, of all, it can be installed right over the top of an existing concrete slab.

But first, a little bit of honesty…


The truth about decks built over concrete slabs

Although it is not impossible to install a deck over a concrete patio, it is uncommon.

This is because concrete patios rarely offer enough space over which to install a deck. Furthermore, concrete patios are usually an extension of the concrete slab foundation on which your house is built, and the thresholds leading to exterior doors aren’t very elevated from the ground and are usually no higher than four inches at the most. This means there will be little clearance available to create a deck and you’ll need to expand the size of the patio outwards from the house in order to create enough clearance over which to lay the platform.

If you’re one of the lucky ones and your patio already has a suitable surface area and is at an acceptable height to your door threshold, happy days! If not, the process can be much more challenging… but that is not to say it can’t be done.

To remedy a height/threshold problem, you can lay boards down with the wide side down to fashion a frame. A standard two-by-four, for example, is only an inch-and-a-half wide on its shortest side. With their widest faces towards the slab, these boards will now be at the right height to build a deck. Voila! You may get started!

However, if your threshold is fewer than three inches, you might find yourself in a bit of a pickle. To get your boards to the right height you may need to rip them — an industrial term for sawing the boards lengthwise or shaving some height off them — to make the frame at the right height.

Furthermore, this process makes it difficult to build a level deck as it can be extremely difficult trying to rip every board at an identical height.


Making sure the slab is even

Okay, so you’ve done some preliminary investigations. You’ve figured out that you have enough clearance to build your deck and gathered all the boards and planks necessary for creating a nice stable substructure over which your platform will be built.

Now the hard work begins — making sure that your frame will be level so that the resulting deck will also be level.

Why might this be difficult, you ask? The answer is because few concrete slabs are perfectly even. In fact, most of them are characterised by little dips and rises that are usually imperceptible to the naked eye, but would make for a wonky substructure if you were to just slap one down over the top of it.

This is why you must smooth out any rises and dips, and even out the slab as much as you can before installing the frame over the top of it. The best way to do this is through using a concrete mixture such as:

  1.   Levelling compound. This is a quick-dry cement that can even out rough surfaces. Make you get an exterior grade product.
  2.   Ready-mix. This is similar to a levelling compound although it dries and hardens at a much slower rate, making it the better option for DIYers who have little experience in the way of working with cement.


Regardless of which one you choose to use, make sure that you first identify the divots and dips in your slab. Pour into each low point a generous amount of mixture then use a straight edge, such as a two-by-four or a ruler, to smooth out the contours so they match evenly with those that characterise the rest of the slab.

Don’t fret too much about using primitive tools such as a length of wood here to even out the low points; it may not look pretty, but it is more practical as it will extend your reach and allow you to level out a surface area in a smaller amount of time. In addition to this, the slab will soon be covered by a deck so any blemishes or ‘ugly spots’ will be hidden from sight.

As you work your way across the slab with your mixture and straight edge, puttying up those dips and cracks, make sure you apply a level at regular intervals to ensure the concrete surface is levelling out and becoming even.

However, you don’t want all parts of your slab being perfectly even.

You’ll want to create a slight slope in the slab to ensure that water drains away from your home, preventing any water damage to the foundations of your residence. To do this, apply a small measure of mixture to the slab nearest your home, and pull it away, outwards from the home towards the opposite edge of the slab.

As you do this, make sure that you check with a level that you are creating a slight downwards slope that will be imperceptible to the eye, but help effectively with water run-off and drainage.


Creating the substructure

In most instances, two-by-fours aren’t recommended for creating a frame for a deck. They are too flexible and will result in a platform that feels slightly like a trampoline.

However, when two-by-fours are installed across a hard surface such as a concrete slab, they are much more functional. This is because the hard surface area over which they’re laid absorbs the weight rather than the bouncy two-by-fours themselves.

Nevertheless, it is advised that you should lay your sleepers (joists on a surface) no more than a foot (or twelve inches) apart. By placing them in such close proximity to one another, you will increase the amount of support to the platform and minimise the amount of flexing to which it will inevitably be subjected.

To further reduce flexing, you may wish to add a reinforcing sleeper every couple of intervals, which will bolster the platform and absorb more weight.


A step by step guide on installing the substructure

  1.   Measure up. Get your tape measure out, a lengthy piece of wood (preferably one that can span the width of your slab), and a piece of chalk. Begin measuring and marking out on the slab the pre-designated spots where you will lay your sleepers.


(Note: pre-drilling will be essential for ease of access and simplicity, so it is a good idea that you make sure that the holes on each board will correspond with matching holes drilled into the slab.)


  1.   Drill. Use a powerful drill to bore into the spots you pre-marked on the concrete (and pre-drill the corresponding pieces of timber at the same time). Make sure you use a drill-bit that is slightly smaller than the fasteners you’ll be using to create the most stable substructure possible.
  2.   Attach the sleepers. Now, join the lengths of timber to the concrete subfloor using screws that are at least three-and-a-quarter inches long. If you have pre-drilled each length of timber and assigned it a spot on the slab, this should be a relatively easy process.
  3.   Check with a level. As you work your way across the surface area of the slab, fastening the lengths of wood to the concrete, use a level to check that each one is level with the direction that parallels your house.
  4.   Chock up any dips. If you find a low-point that wasn’t evened out when you were levelling the slab, you may need to chock it up with a shim (smaller piece of wood cut to size) to compensate for the gap and even it back out. If this occurs, you’ll need to use a longer screw that can penetrate both the two-by-four and the shim, and still secure the deck to the concrete.


Installing the deck

Congratulations. Now that you have attached all the sleepers to the house, and effectively created your substructure, you can be begin laying the composite decking boards that will soon become the platform on which you sit, walk and enjoy the outdoor surroundings of your home!

When you are cladding the substructure with decking boards, it’s important to take into consideration the height of these boards. They are really quite small, meaning you will be limited in the length of fasteners or screws you use to attach them to the sleepers.

At the most, you will have just over one and a half inches of wood to work with as you burrow through it with a screw, joining it to the sleeper.

Please keep in mind that should you choose to drill directly through the board (which most people are likely to do), you are better off buying a composite board that is fully capped and thick enough to resist moisture. This will prevent water seeping through microscopic pathways created by your fasteners, to get to the core of your decking boards, where they might cause rotting and degradation.

On the other hand, you might be intent on using hidden fasteners, which clip directly onto the decking board and is drilled into from the sides, giving the outward appearance of a deck where nails and screw heads don’t exist at all.

If this is you, make sure you plan ahead, and order a hidden fastener system that uses smaller screws that won’t damage or compromise the integrity of your substructure.


Final words

Whether you’re hoping to get outside for a spot of gardening or simply want to recline in a sun-chair in the comfort of your backyard, nobody should be at risk when heading out onto the patio.

This is why, with a little hard work and gumption, you can transform your old concrete patio from a warzone into a comfort zone, by installing a composite deck over the top of it.

It isn’t an easy task, but the end result is most definitely worth it. Composite decks are visually appealing to the eye. They’re friendly to the feet. They will create an even surface underfoot that will never deign to trip you up, and they’re available at affordable prices in a range of great colours and designs.