Installation & Maintenance

How to Install Composite Decking Over Concrete

December 14, 2020

Wondering how to install composite decking over concrete, or unsure if you even can? The good news is TimberTech® composite decking can be installed over concrete. While you can remove the concrete before putting in your composite deck, it’s often far more cost-effective to build the composite deck over the concrete if your concrete is in good condition.

howto install composite decking over concrete featuring TimberTech AZEK Vintage Collection in Coastline

Some benefits to installing composite decking over concrete include:

  • A beautiful, nature-inspired aesthetic, rather than a dull concrete slab.
  • Long-lasting performance instead of the potential for cracking and crumbling.
  • Long-term value with minimal maintenance as opposed to routine sealing and polishing of stamped concrete or full replacement of cracked concrete.

If you’re ready to learn how to make the switch from conventional concrete to innovative composite decking, you’ll want to follow these steps.

Step 1: Prepare the Patio Area for Decking Installation

Before you learn how to install composite decking over concrete, you need to prepare the patio.

  • Clear off any debris on your patio.
  • Inspect the patio for any problems.
Common issues to look for in 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 leveling 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 leveling compound to level the surface or use shims to level the joists when you install the sleeper system.
  • 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 2: Install a Sleeper System

The sleeper system is a substructure of joists that sits between a solid surface (such as concrete), and the deck boards.

A sleeper system allows for adequate drainage and ventilation under a ground-level deck which will help protect the traditional wood of the substructure from moisture damage. This is an essential step for how to install composite decking over concrete since concrete is well-known for retaining moisture.

Deck Joist Spacing Horizontal

Tips for Installing the Sleeper System

  • The sleeper system’s joists need to be at least 1-1/2 inches in both height and width to allow for effective ventilation.
  • Sleeper joists should lay parallel to the direction of the water run-off.
  • The sleepers will need to be fastened to the concrete for stability. Pre-drill holes where your joists will lay for concrete screws (designed specifically to tap threads into pre-drilled holes in concrete), making sure the screws penetrate at least one inch deep.
  • Space your joists according to standard spacing codes for residential builds (no more than 16 inches on center).
  • Joists need to be in plane with the house. Check your joists frequently with a tight string line.

Step 3: Protect Your Sleeper System

Since your sleeper system will be made of traditional lumber, you’ll need to protect it from moisture damage. Why? Because all types of traditional wood, including pressure-treated lumber and exotic hardwoods, are vulnerable to mold, mildew, and rot — all of which can decrease the lifespan of your deck.

To ensure your sleeper system lasts just as long as your TimberTech composite deck boards, protect your joists with TimberTech PRO-Tac™ Flashing & Joist Tape.

TimberTech PRO-Tac Flashing & Joist Tape

Why Joist Tape Is Important

A healthy substructure is essential for the overall health of your deck. Applying deck joist flashing tape to the tops of your sleeper system:

  • Seals it against moisture accumulation.
  • Prolongs its structural integrity by resisting mold, mildew, and rot.
  • Can prevent costly repairs and replacements of rotted sleepers.

Step 4: Install the Composite Deck Boards

Once your sleeper system is installed and properly protected with joist tape, you’re ready to install the composite deck boards. This is the final step for how to install composite decking over concrete.

Some Best Practices for Installing Composite Decking

deck joist string lines
  • Use a string line to ensure all joists are level and in plane.
  • Shim any joists that sit too low and use a power sander to level any joists that sit too high.
how to install composite decking over concrete power miter saw
  • Use the proper saw and blade. There are different recommended saw blades for different materials.
  • Cut boards with grain facing upward.
  • Measure twice and cut once.
Fasten deck boards with CONCEALoc hidden fastener

Fasten the Boards

  • Keep in mind that different decking lines and board profiles require certain fasteners.
  • Different decking lines will also require pre-drilling before applying fasteners.

Remember: These are just a few of the steps involved in installing composite decking. Always be sure to closely review the Installation Guide for your TimberTech decking line before building.

Looking for More Installation Guidance? The project guides in our Deck Building Resource Center can give you the helping hand you need.
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Find yourself with some lingering questions about how to install composite decking over concrete? Partner with a TimberTech-registered contractor for expertise and insight into building your perfect outdoor living space.

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