What is the best way to make a deck water proof?


What is the best way to make a deck water proof?

Asked by Janice Stewart

We would like to have an open deck area with living space underneath. It would be a sunroom and garage area without heat and air. We have considered concrete and tile. Is there something better?

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Polly Osborne, FAIA, LEED AP's picture

Dear Janice,

Open decks above living spaces have had many failures, but they don't have to.

The secret to a successful deck/roof is a good system of waterproofing and drainage, correctly installed and maintained.

  • It can be a finish in itself or topped with a finish material, such as concrete or tile.
  • The devil is in the details, and the craft and knowledge of the installer.

Five layers

A deck waterproofing should ideally have the following layers:

  1. A good, clean continuous subsurface, such as plywood or concrete.
  2. A water proofing membrane layer designed specifically for the decking surface to which it is applied.
  3. A protection layer, protecting the water proofing from sun during installation and from the scuffing that comes from laying a finish product over it.
  4. A drainage layer- for allowing water to drain off that seeps through the finish layer, before it has to be repelled by the waterproofing layer.
  5. A top finish layer, which can be supplied by the water-proofing manufacturer as a specific top coat or be your own finish tile.

All the layers must be compatable and designed to go together. Some companies have systems that have layers applied in a few steps by the same installer, keeping the process simple. If you use their topcoat, it will have a rubbery finish, which is cheaper than your own tile but not nearly as pretty. The same companies usually also have a system designed to go with the finish of your choice.


Of key importance is the flashing and waterproofing around the edges where the deck meets the second floor walls and/or parapet. The metal flashing is interlaced with the waterproofing to be sure there is no leakage at this weak link.

Another point of both waterproofing and aesthetic concern is the exposed edge of the deck and its corresponding railing and /or wall. The railing or wall is going to be a key visual component as well as an essential safety mechanism.

Other issues to consider

Here are several other issues you should keep in mind:

  • Roof decks must be structurally designed to carry people and snow, if any (live load) and the weight of the materials (dead load).
  • The slope of the deck must comply with a minimum slope for a flat roof.
  • Scupper and downspouts must be kept clean so water doesn't ever build up on the deck.

Waterproofing systems to consider

Some name brands of deck water proofing systems that have been scratching their heads over these problems for a long time are:

A reputable builder with good references at tackling this kind of deck is a must.

I hope this helps.

For more information:

Read "What are waterproof roof decking options?" a Q&A answered by Gary Shlifer.