How to Design a Roof Deck

If you are planning to build an addition / sun room with a deck on top, or planning to convert existing roof top into a roof deck, the most essential step is proper planning, so that you can enjoy this deck space all summer long, instead of fixing leaks.

image of Flat Roof Deck

Here are some essential planning steps you should take in the design stages:

Choose between Floating Deck and Waterproofing Deck membrane:

Floating deck is a wooden deck built on top of a flat roof material (typically a single ply roofing membrane such as PVC roof of EPDM Rubber Roof) and is installed loosely laid over “slippers” – hence it is called “floating” deck.

Advantages of a floating deck are:

– Roof membrane is protected form damages by people on the deck.
– Classic look of either natural wood or stained wood, or a multitude of composite decking materials.
– Your wife will like it 🙂

Disadvantages of a floating deck:

– COST! Building a floating deck requires having a roof membrane installed, and then the cost of deck itself, which can be 2-4 times more than a Waterproofing Deck Membrane such as IB DeckShield, depending on type of wood used.
– If roof below deck leaks, you need to take the deck off to repair it, and then put the deck back on. Essentially you will be paying twice for installation, and paying for leak damages and roof repair.

Waterproofing Deck Membrane is a product that will make your deck watertight and will allow you to walk on it, making a floating deck not necessary. Flat roof decking materials like IB DeckShield are a perfect example of such product.

Besides being 1/2 the total installed cost of a floating deck, IB DeckShield also eliminates extra wight on the roof, and gives you additional 3+ inches of clearance, often necessary for proper waterproofing at the roof-to-wall flashing.

Plan for rain – avoid roof leaks:

  • Design your roof deck with build in slope, which will allow water water to run off, instead of pulling on the roof.
  • A 1/4″ slope (1/4″ rise per 1 foot run) is more than sufficient for drainage.
  • Most roof decks have a door above it, so you can go in and out of the house. Have the clearance between roof surface and door sill at least 3″, so that any water on the roof will not go inside the house.
  • Flashing between roof and wall is essential. Siding above the roof deck must be removed at least 12″ up from roof level for proper roof-to-wall flashing.

Plan to install railings:

image of IB DeckShield

  • It is a law in most states, that all roof decks should have railing to prevent people from falling over.
  • Installing railings posts inside the roof (attached to the rafters) will make your railings stronger.
  • Single ply membrane such as IB PVC roof, come with pre-formed post flashings, so it is very easy to make these posts watertight. If you opt to use EPDM rubber roof, you will be faced with problems when trying to flash posts, and frequent flashing repairs down the road.

    Keep your deck design simple:

    Rectangular shape is best for any roof deck, because it allows for simple framing, easy drainage and fewer posts. Try to keep your posts about 6 feet apart – this will reduce costs, and a number of roof penetrations.

    Overall, with these simple planning steps, you can design a beautiful roof deck that will serve you for many years, without any roof leaks!

3 thoughts on “How to Design a Roof Deck

  1. ned miller

    Other than gutters, what’s an effective way to divert rain water flowing off a second floor deck, so it doesn’t run down the wall below? Seeking minimal appearance.

  2. admin Post author

    Ned, the only other way is to either have concealed gutters (custom fabrication) or roof drain.

    If you have a floating deck in top, hiding a drain is easy. If your deck is like DeckShield, then, it will be visible.

    Other than that, there are no other ways.


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    Building with recycled materials is a cost-effective way of making a hen house.

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