Evolution of Deck Waterproofing

Waterproofing, particularly waterproofing decks and decks over living spaces, has come a long way since I started in construction in 1972. In those days, a guy would drive around a hot mop truck (tar), waterproofing shower pans, decks, and other items. It wasn't until the 1980s that I even heard about a thing called a door pan. Today, waterproofing decks is a specialty, and door pans are a necessity.

Advances in Waterproofing Materials

Materials have advanced tremendously as well. The best materials that have been developed are monolithic membranes, as opposed to sheet goods or even sheet metal. Monolithic membranes are liquids, usually a type of synthetic rubber such as neoprene or urethane, or epoxies that come as two-part liquids. The beauty of a liquid membrane waterproofing system for a deck is that there are no seams, and they are fully adhered.

Reinforcing Fabrics in Deck Assemblies

To ensure these systems are robust, reinforcing fabrics are used in the build-up of the waterproofing deck assembly. At a minimum, plywood joints are stripped with a reinforcing fabric, as are the top of the deck-to-wall flashing and the bottoms where the flashing attaches to the deck. Any penetration in the deck, be it where pipes come through the deck or where drains and scuppers come through parapet walls, should be reinforced with fabric. Also, door pans to deck-to-wall joints, and cricket hips and valleys should ideally all be reinforced with fabric. With certain deck assemblies, I install reinforcing fabric over the entire deck, running from the top of the deck wall to the end of the deck.

The Three-Course System

Most reinforcing fabrics that are used on the waterproofing of decks are applied using a three-course system: a coat of the liquid membrane, then a layer of fabric, followed by another coat of liquid membrane. Once those coats are cured, an additional coat of the liquid membrane is applied. On a urethane deck, two additional coats would be applied: a sand coat and a color coat. The sand coat provides texture to make the deck slip-resistant, and the color coat is simply for aesthetic purposes.

Protection Boards and Drain Mats

Deck membranes that will tile, a pedestal tile system, a floating wooden deck, or a concrete slab installed on top of it, all require an asphalt protection board laid over the top of the waterproofing. The protection board, like its name suggests, protects the waterproofing while the decking system above the waterproofing is being applied. Waterproof decks that will have mortar beds installed, in the case of tile or concrete, will also require drain mats on top of the protection board. Drain mats reduce the hydrostatic pressure from the weight of the mortar bed or concrete on the waterproofing of the deck, applying less pressure to the waterproofing membrane itself.