Q&A – Rooftop deck build

This Washington homeowner came to AskARoofer for some advice about a rooftop addition to his new home.

William, a homeowner in Washington, was looking into building a deck on their roof and wanted some advice on the plan: 

I recently purchased a house in Seattle with a rooftop deck that uses IB Deckshield as its waterproof membrane. I want to start adding PVC interlocking deck tiles and turf (with PVC drainage mats underneath) to the roof, but I'm worried the feet from the tiles and drainage mats may damage or puncture the IB Deckshield membrane. The heaviest object that will go on top of the turf/tiles will be a three-seater sofa.

Should I be adding a protective mat underneath the deck tiles and drainage mats, or is the waterproof membrane strong enough to withstand it? Any guidance here would be greatly appreciated.

The interlocking deck tiles that I plan to use are these 12'' x 12'' Interlocking Plastic Deck Tiles. The drainage mats I will put under the turf are these VEVOR 12" X 12" Drainage Mats.

One of our experts, Jim from RoofSmart, responded to William's question:

Congratulations on the new home! Protecting your waterproof membrane is a smart move. Maintaining the integrity of a rooftop deck's "hidden foundation" is essential for long-term home health.

To address your concerns about the interlocking deck tiles and the potential for punctures:

• Pressure distribution: The "feet" or grid patterns on the PVC tiles and drainage mats you linked can create localized pressure points. A three-seater sofa adds concentrated weight that could accelerate wear or indentations over time, especially during Seattle’s wet and freeze-thaw cycles. I have seen this create punctures opn other homes.
• Essential airflow and drainage: One of the primary causes of membrane failure is trapped moisture. Your plan to use drainage mats is a great start — maintaining 360-degree airflow ensures both the tiles and the membrane dry uniformly, helping to mitigate mold or rot beneath.
• Serviceability: A major advantage of modular tiles is that they are easily "pop-up" serviceable. This allows you to inspect the underlying membrane or clear debris without a major construction project.

My recommendations for your setup:

1. Yes, use a protective layer: I highly recommend adding a slip sheet or protective mat (like a crumb rubber mat or geotextile fabric) between the IB Deckshield and your tiles. This acts as a cushion to distribute the load from the sofa and prevent direct abrasion.
2. Maintain drainage: Ensure your overall roof slope remains clear and that the mats don't block scuppers or drains.
3. Load management: Consider placing larger rubber pads specifically under the legs of the sofa to further spread that weight.

RoofSmart resources for your project:

I know you plan to do all the work yourself, and that's great. Here at RoofSmart, we've handled many IB Deckshield-style decks in the Seattle area. You might find these specific guides from our blog helpful as you plan the install:

• How Are Rooftop Decks Installed? — This covers the technical steps for waterproofing and overlay considerations.
• Common Problems with Rooftop Decks in Seattle — Useful for understanding leak prevention and long-term durability in our climate.
• RoofSmart Rooftop Deck Services — For more on how we approach custom deck designs.

If you’d like us to do a professional walk-through and check the condition of your membrane before you cover it up, feel free to reach out. It’s always better to ensure that foundation is 100% secure before the furniture goes on!

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