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How to Install a Deck on a Flat Roof

How to Install a Deck on a Flat Roof

Posted by Deckorum Admin on April 25, 2023

How to Install a Deck on a Flat Roof: A Comprehensive Guide

Learn how to install a deck on a flat roof and enjoy the beauty of outdoor living space with this comprehensive guide.

Author: Adam King is in the Design and Planning team of Deckorum. A dedicated team player and boasts experience in the building industry from property renovations to new builds. Adam’s passion in Deckorum is to help build spaces people can create memories in.

Installing a deck on a flat roof is a fantastic way to add some extra outdoor space to your home, an outdoor deck adds value and make the most of your available space. We believe once familiar with the process you will be able to use this article as a guide on installing a deck properly on a flat roof, preventing any damage, by using some helpful tips and advice along the way.

Table of Contents

Assessing the Roof

Composite decking can be installed on a variety of roofs including:

  • Inverted roof systems
  • Mastic asphalt
  • Layered asphalt
  • Concrete

The first step in installing a deck on a flat roof is to assess the roof to determine if it can support the weight of a deck. You will need to check that the roof joists used are at 150mm minimum, so it is structurally sound and suitable to carry the load capacity of composite decking. It is important to consult a professional if you are unsure about the load capacity of your roof.

Once you have determined that your roof can support a deck, you will need to decide on the size and shape of the deck, and the materials you will use. Consider if you want to install lighting or railings. Measure the area of the roof where you want to install the deck. This should include and note the height limit, personal preferences of decking profiles to the style of your home.

Materials to consider for this are the composite joists, a lightweight composite material designed to suit a terrace roof installation. At 30-40mm in height they make an excellent product when height needs to be kept at a minimum.

Plastic Lumber joists are a tough and lifetime solution. Never to rot and made in multiple profile sizes the strength and durability is unmatched. From 50mm – 150mm height options the plastic lumber matches well with composite decking.

Treated timber is the long-standing construction superhero. With versatility and countless sizes and lengths, timber is still a great material to make installation easier.

To post or pedestal, both options to create height of to level the deck area. Flat roofs, contrary to their name are often unlevel or have a gradient to allow for drainage.

Preparing the Roof Surface

Once decided on the materials the next step is to prepare the roof surface for the deck. You will need to clean the surface thoroughly, removing any debris, dirt, or vegetation. A pressure washer can make quick work to clean the surface effectively. Once the surface is clean and dry, Check and ensure the membrane to protect the roof is waterproof.

Installing a deck on a flat roof will need ensuring that there is proper drainage to prevent water from pooling on the roof. Consult if you need to with a professional to help design an effective drainage system or make sure you allow sufficient drainage routes.

Building the Deck Frame

Building the deck frame can be made easier by pre-planning a design. This will help note what additional changes need to be planned if lighting or railings are included. Also, make sure to follow local building codes and regulations choose the material to suit the location considering cost, height, and durability. You may also want to consider using adjustable pedestals to raise the deck off the roof surface. Additionally,  this will allow for proper drainage and ventilation and will help to protect the roof from damage caused by water.

Use the chosen materials that are suitable for outdoor use and additional supports that may are needed. Build the outside frame to the size and shape of your deck area, making sure the frame is level and square. Use metal brackets to securely attach the frame to a wall or other non-moving structure. This will ensure that the deck will not shift or move over time. Install the joists opposite to direction of the decking boards every 300mm apart adding supports where needed. Add additional joist supports if railings are included and drill holes for wiring if lighting is part of the deck.

Installing the Decking Boards

Once the frame is installed, you can begin installing the decking. You will need to choose the type of decking material that best suits your style. Composite decking comes in a variety of styles and colours to choose from. Choose a high-quality decking material that is durable and weather resistant. Composite decking is a popular choice for flat roof decks as it is low-maintenance and has a long lifespan.

The boards are installed perpendicular to the joists, starting at the outer edge, and working your way inwards the boards should be securely fastened using starter fasteners and t fastener clips specific to the deck board. The hidden fastening system gives the deck a clean and seamless look without screws placed through the boards.

Adding Railings and Finishing Touches

Once the decking boards are in place, you can add railings and other finishing touches to complete your deck. Choose the style and material of your railings from composite, glass, or steel. Ensure that they are securely attached to the deck frame.

You may also want to add lighting that creates an inviting outdoor living space at all hours and seasons of the year. As always it is important to ensure that any additional features do not add too much weight to the deck or compromise the structural integrity of the roof.

Questions and Answers

Can any flat roof support a deck?

No, not all flat roofs are suitable for supporting a deck. It is important to check the roof joists are over 150mm or assessed by a professional to ensure that it can support the weight of a deck and that it is structurally sound.

Do I need planning permission to install a deck on a flat roof?

It depends on where you live. Some councils require permissions for deck installations, while others do not. Checking with your local building regulations department to determine what or if planning permission is required is important.

What is the best material to use for decking on a flat roof?

Composite decking creates the outdoor low-maintenance option. The best material for your substructure will depend on your personal preferences, budget, and the climate in your area.

How can I ensure proper drainage on my flat roof deck?

You will need to allow an effective drainage system for your flat roof deck to prevent water from pooling on the roof. This may involve allowing channels for the water to drain in the correct direction without obstruction and follow the natural slope of the roof surface or adding a drainage system such as a downspout.

What should I do if I am unsure about the installation process?

If you are unsure about the installation process, it is best to consult a professional. A roofing composite deck company/builder can provide you with guidance and ensure that your deck is installed correctly and safely.


Installing a deck on a flat roof can be a challenging task, but with the right knowledge and guidance, installation can be done successfully. By assessing your roof, preparing the surface, building the frame, installing the decking boards, and adding the finishing touches, you can create a beautiful outdoor living space that adds value to your home. Remember to consult a professional if you are unsure about any aspect of the installation process, and always prioritize safety and structural integrity.

Deckorum have a start to end support team that make any questions easily overcome. An in-house team provides technical and design information so be sure to reach out to kick start your composite decking project. By following the proper steps and considering the right factors, you can install a safe and durable deck that will provide years of enjoyment for you and your family.