Creating a Floating Deck Over a Flat Roof

During my time in the decking industry, I’ve seen firsthand how rooftops can be transformed into extraordinary outdoor areas. The trend of upgrading flat roofs with stylish and functional floating decks is rising, offering a smart way to reclaim space, embrace outdoor living, and enjoy panoramic views.

Whether you’re a city dweller seeking a private oasis or a building pro looking to maximise function, a floating rooftop deck might be the perfect solution. Here, I’ll share the basics to consider when creating a floating deck over your flat roof.

What Is a Floating Deck Over a Flat Roof?

A flat roof floating deck is a self-supporting structure built without penetrating the underlying roofing material. It rests on a support system that elevates it off the roof surface to prevent water accumulation and ensure even weight distribution.

Rooftop floating decks are becoming popular in the UK’s architectural landscape, particularly in modern and urban settings. Here are a few reasons you may want to consider one, too:

Expanded Outdoor Living: A floating deck transforms unused space into a functional outdoor area, ideal for homes with limited garden space.

Seamless Modern Design: With clean lines and a sleek, stylish appearance, floating decks can enhance the aesthetic appeal of contemporary architecture.

Added Property Value: A rooftop deck is an attractive feature that can enhance your property value and help your home stand out in the real estate market.

Now, let’s get started!

Considerations for Flat Roof Decks

The first thing I tell homeowners: Transforming your flat roof into a functional space involves careful planning. While floating deck systems are simpler to install than traditional elevated decks, there are unique aspects to consider:

• Roof Strength: Ensure your roof can support the weight of the deck, furnishings, and people. To assess the weight capacity, consult a qualified professional or check with your local council for resources.
• Water Management: Ensure that your current roof membrane—such as EPDM, PVC, or built-up bitumen—is intact and provides effective waterproofing. I find there’s generally no need to add an additional moisture barrier specifically for the deck. Instead, focus on proper drainage to prevent water accumulation and ensure effective water management through gutters and downpipes.
• Emergency Exits: Plan for two escape routes in case of emergency. One exit can lead back into your home, and the second should be external, like a fire escape staircase or roof ladder.
• Building Regulations: Comply with all regulations for rooftop structures and railings. Consult your Local Planning Authorities for required permits and Local Building Control for approvals before starting your deck.

Floating Deck Systems

There are several systems that are used to elevate floating decks above the roof surface. All create a needed gap of about 50 mm to 75 mm beneath the deck. The gap promotes ventilation, prevents moisture build-up, and maintains the integrity of the roof.

Pedestal System

Pedestals are supports with adjustable top plates and wide bases placed across the roof to support deck joists. This is my choice for slightly uneven roof surfaces because the heights can be tweaked to establish a level platform.

Tip: Pedestals may also be sloped to improve water runoff.

Grid System

Grids feature interlocking plastic squares that create a stable deck base. They’re lightweight, easy to install, and allow water to drain through. The weight of the decking materials holds the grid in place.

Sleeper System

Wooden or lightweight aluminium sleepers are bearers installed in parallel, creating a sturdy frame for the deck boards, which are fastened directly to them.

Tip: Rubber or plastic pads underneath the sleepers can help distribute weight, reduce friction, and improve drainage.

Deck Blocks

I don’t recommend concrete or Evo-Crete blocks for rooftop decks due to the risk of roof damage, potential instability, and wind vulnerability. However, lower-profile polyolefin plastic “deck blocks” spaced across the roof can offer effective joist support if they provide adequate clearance.

Lightweight Materials for a Floating Rooftop Deck

Flat roofs are accessed by specific entry points or even hatches, and experience has taught me to plan with those limitations in mind. I advise pre-cutting your deck materials and choosing lightweight options for easier transport and to stay within your roof’s weight capacity.

Framing Materials

Save weight (and your back) by choosing these framing materials for your joists and beams:

• Pressure-Treated Softwood: Often pine or spruce, affordable pressure-treated softwood (PTS) is heavier than other options here, but is still manageable.
• Engineered Lumber: Laminated veneer lumber (LVL) and parallel strand lumber (PSL) offer strength and stability with a lighter weight.
• Composite Materials: Composites aren’t only for decking; some are designed for framing and offer a balance of weight and durability, including resistance to moisture, rot and insects.
• Aluminium: Aluminum framing offers a significant weight reduction, corrosion resistance, and high durability.

Whichever material you choose, position your deck beams and joists perpendicular to the roof framing below to evenly distribute the load and enhance the overall stability of your deck. Additionally, apply butyl tape like Trex Protect to the top surfaces of your deck substructure to waterproof your deck and prevent moisture from penetrating and reaching the roof below.

Decking Materials

For your new rooftop deck, choose decking that suits your style as well as your decking cost budget, but figure weight into the equation, too. There are lots of beautiful, lighter-weight decking options:

• Composite Decking: A mix of wood fibres and plastic, composite decking is generally lighter than solid wood and offers durability with low maintenance.

Tip: Look for hollow or scalloped deck boards to further reduce weight.

• PVC Decking: PVC decking is usually lighter than composite or wood, and it’s highly resistant to moisture and decay, lasting 50 years or more.
• Softwoods: Softwoods like cedar and redwood are naturally lighter than other woods and also resistant to rot and insects.
• Lighter Hardwoods: Denser hardwoods like Ipe and Teak offer a good balance of weight, strength, and durability, including resistance to weathering.
• Bamboo: Strong and durable, bamboo is also lightweight and eco-friendly.
• Aluminium Decking: Very lightweight and durable, aluminium decking can deliver a modern aesthetic.

Maintain a consistent gap of approximately 3-6 mm between your decking boards to accommodate expansion and contraction due to temperature changes, prevent warping and buckling, and aid drainage.

Maximise Your Rooftop Potential

A flat roof deck offers an opportunity to transform underutilised space into a valuable outdoor living area. By carefully considering factors like weight distribution, moisture management, and material selection, you can create a beautiful, functional space that enhances your home’s value and your enjoyment of it. With thoughtful planning, I know your new floating deck will become a favourite destination for relaxation, entertaining, and enjoying the view!