Building A Deck Over Concrete – Guide for Beginners

From cracks to discolouration, old concrete patios can be an eyesore. While you can cloak these backyard blemishes with outdoor rugs, fresh coats of stain, or other temporary fixes, building a deck over the concrete is one of the best solutions.

Adding a deck over a concrete slab is a permanent fix that can help refine your outdoor space whilst adding value to your house. Additionally, if this renovation is under $20,000, it does not require a registered builder in most parts of Australia, including Melbourne—making a DIY deck a viable option.

Whether you’re considering building a floating deck or simply exploring your home improvement options, the guide below takes you from start to finish on how to build a deck over concrete.

How to Frame a Deck Over Concrete

Framing a deck over concrete is done using sleeper panels, also known as sleepers or joists. These strips of lumber sit directly on top of the concrete, are evenly spaced, and run parallel to each other to support the decking boards. Additionally, sleeper panels raise a ground-level deck off the concrete, creating space for airflow and proper water drainage. Both of which are essential for preventing rot and building a long-lasting deck.

Building this type of deck framing requires basic woodworking skills, various tools, and materials. But before you go investing in any of those mentioned above, you should analyse your existing concrete to ensure it can adequately support an addition.

Analysing the Condition of the Concrete

You want to build a deck that lasts, and your deck is only as strong as its foundation. Therefore, it is essential to assess the condition of your concrete. Is it cracked? Is it shifting? Is it water damaged?

While you can repair concrete foundations with minor cracks, slabs with extensive damage or evidence of sinking aren’t worth building on. In addition, a damaged base will shift over time, inevitably taking your deck with it—damaging the structure and compromising its safety.

Tools & Materials Required for Deck Building

Once you’ve determined that your concrete is fit to build upon, it’s time to get to work. You will need various decking tools and materials to get the job done right.

Tools

• Hammer Drill
• Cordless Drill
• Miter/Circular Saw
• Drill Bits
• Hammer
• Level
• Tape Measure
• Pencil
• Marker
• Safety Glasses
• Ear Muffs
• Mask

Decking Materials 

• Self-levelling Compound
• 90mm x 45mm Timber 
• Plastic Shims
• Concrete Screws
• Timber Screws
• Decking Boards
• Flashing Tape
• Fasteners (optional)
• PVC spacers (optional

Steps Involved in Decking Over Concrete

Find below the steps involved in decking over concrete:

• Choose a Decking Type
• Calculate the Foundation Size
• Level and Repair
• Cut and Level Out the Sleeper Panels
• Secure and Connect the Decking
• Bolt/Screw Your Decking to Sleeper Panels
• Do Some Waterproofing
• Add Some Personality

Continue reading to know what each step is about.

Choose a Decking Type

When it comes to composite decking vs. timber decking, there are plenty of factors to consider. These include:

Price: Timber decking is great for those on a tight budget as it runs cheaper than composite (on average, $120 to $200 per m2 vs. $60 to $140 per m2). However, it is essential to think beyond the initial build and keep in mind that timber has more maintenance costs over time (sanding and sealing). Plus, it doesn’t last as long as composite.

Aesthetic: Since timber can be painted or stained any colour, it is highly versatile—you can achieve any look from rustic to refined. Contrastingly, composite decking comes in a set range of colours. But unlike timber, it doesn’t have to be regularly refinished or sealed to maintain its hue and structural integrity. 

Calculate the Foundation Size

Measure your concrete slab, so you know how much of each material you need. How big your deck is will determine everything from how much self-levelling compound you need to the size to cut your sleeper panels.

Level and Repair

If your concrete is weathered or unlevel, you will need to fix it with a self-levelling compound. Apply a thin layer to raise any low spots and even the surface. You don’t need to aim for perfection here, as your sleeper panels will also help even out the area. Instead, ensure you smooth out any large depressions.

Pro Tip: Sloping the slab slightly away from your home improves water drainage. However, this step can also be done when installing sleeper panels. 

Cut and Level Out the Sleeper Panels

Cut: Your sleeper panels will need to run the entire width of your deck. Precise measuring is essential to avoid wasted timber, so measure twice and cut once. Note that the cuts for this type of deck are relatively simple, making it one of the more suitable DIY decks for timber working beginners.

Mark: Sleeper panels should be evenly spaced, generally around 450mm for standard decks (this can vary depending on your build). You can use masking tape to map the layout on the concrete as a visual guide. Or, at the very least, mark out the positions using a pencil or marker. 

Level: You can place a sleeper panel directly on the concrete or on top of PVC spacers. Once in position, start by securing the highest point on the concrete that the sleeper touches. You may run these supports parallel to your concrete slab or on a slight incline away from your house. Either way, you will need to use plastic shims under any spots that need to be raised. Use your level to ensure the surface is a single flat plane—the goal is for the decking to lay flat perpendicular to the sleeper panels.

It is worth taking extra time to ensure your supports are level to avoid building issues and frustrations down the road. 

Secure and Connect the Decking

Double-check that your sleeper panels are level and in the correct position. If so, pre-drill holes with your hammer drill. You may drill down through the top of the sleeper into the concrete or use a joist fastener such as an L-bracket. Either way, you want screws that will drive a minimum of 25mm into the concrete.

Pro Tip: If you have difficulty pre-drilling holes into the concrete, try frequently lifting your drill bit out as you go. This helps clear dust that accumulates around the bit. Changing out blunted bits for new ones can also speed up the process.

Once your sleepers are in place, run flashing tape along the top. This joist protector is a quick addition that helps seal off cracks and waterproof the structure. 

Bolt/Screw Your Decking to Sleeper Panels

Lay your decking perpendicular to the sleeper panels and ensure that everything is level before you begin screwing down pieces. 

Note that you may need to pre-drill holes for solid composite decking (check the installation instructions). However, installing timber and composite is generally similar. You want to start outside the deck and work inwards, screwing the boards into the sleepers as you go. Aim for two screws per deck slat on each sleeper panel. 

Do Some Waterproofing 

By this point, your deck should be looking great. And to keep it that way, you will need to do some waterproofing.

Beyond using Trex Protect to protect your joists, timber decking will also need to be oiled. Wait 4 – 6 weeks for your boards to release their natural oils. Then, from there, apply decking oil. A decking oil allows a timber deck to breathe—unlike a heavier sealer. Additionally, it excels at accentuating timber’s natural lustre and beauty, all while protecting your deck.

Add Some Personality

Finish off your deck by infusing it with some extra style. This may be as simple as adding patio furniture and some built-in deck lights. Or many homeowners also opt for a pergola. If your deck is small, don’t forget what you put around it can also greatly enhance the deck itself. This may mean planting a few flower beds, adding shrubbery or laying a path of pavers leading to another backyard feature like a fire pit.

Enjoy Your Deck

A new deck is the perfect backyard addition to enjoy some fun in the sun! So don’t forget to celebrate your build when everything is done. Throw a backyard BBQ, dine outside with the family, or simply sprawl out to enjoy some fresh air.

If you can’t choose which one to do first, know that there will be time for all—a well-built deck will enhance outdoor living for years to come. And the fact that you built it yourself will make it all the more.