All About Flashing: Flashing Materials and Installation Tips

Types of Metal Used in Deck Waterproofing

For deck waterproofing, we typically use three different types of metal: copper, whose thickness is measured in ounces; galvanized sheet metal, whose thickness is measured by gauge; and, less commonly, stainless steel, which is also measured by gauge.

Common Flashing Locations

The location of flashing when waterproofing a deck is usually the edge metal or the deck-to-wall flashing. Edge metal is typically 3 inches by 3 inches, and deck-to-wall flashing is typically 3 inches on the deck and 6 inches up the wall. Door pans, which lap over the deck-to-wall waterproofing, attach up the sides of the jamb and run under the door itself.

The Role of Counterflashing

On some occasions, counterflashing is required. This may be needed after posts are flashed and waterproofed or in other details. Counterflashing finishes off the top of the post flashing to ensure no water intrusion.

Understanding Flashing and Its Limitations

One may think that the more flashing the better, but in truth, flashing is just to protect the waterproofing and has no waterproofing properties of its own. In fact, if the deck flashing is too large, the metal can expand and contract due to temperature changes throughout the day. Metal expands in heat and contracts when it is cool, and the temperature difference from day to night can actually damage the waterproofing membrane. This is particularly relevant when using copper, which expands and contracts more than other metals.

Sourcing Quality Flashing Materials

As a deck waterproofing company, most of my flashings are ordered from a sheet metal fabricating company because their gauges are thicker than what’s available off the shelf at a roofing supply house, hardware store, or lumber yard.

Stainless Steel Variations

Sometimes, blueprints will specify a particular type of stainless steel for the deck. Stainless steel is an alloy, meaning it is comprised of two or more chemical elements, at least one of which is metal. There are many variations of stainless steel, and sheet metal fabricators have access to several options.

Copper vs. Galvanized Sheet Metal

As we all know, copper is becoming very expensive, and while people often think it is better than galvanized sheet metal, that's not really the case. Air doesn’t come into contact with the flashing once the metal is encapsulated by the waterproofing membrane. Edge metal and counterflashing remain exposed and visible after completion of the waterproof deck and can benefit from being copper, particularly in a marine environment where corrosion is more of a concern.

Proper Waterproofing Membrane Termination

For deck-to-wall flashing, the waterproofing membrane should terminate on the wall above the top of the wall flashing and attach onto either the shear plywood or the blocking if no plywood is present. The door pan also should be fully covered and enveloped in the waterproofing membrane. The door pan has a kick at the back of the door sill, which is actually inside the building itself and stands up about 3/8”. The waterproofing should be a monolithic membrane that reaches from the top of that kick all the way to the edge of the deck.

Preventing Galvanic Corrosion

When using copper edge metal with galvanized sheet metal, it's important to create a bond breaker where they overlap. Copper, and to a lesser degree stainless steel, can produce “electrolysis,” as known in the trades—more properly called copper galvanic corrosion—when left in direct contact with galvanized sheet metal. If this happens, it will cause the nails and the galvanized flashing to rust, affecting the integrity of the deck waterproofing.

Proper Installation and Nailing Practices

Once the flashing for the deck waterproofing is chosen, purchased, and delivered, it needs to be nailed in place. To do so, there needs to be backing, such as solid blocking or framing. Nailing is typically done 3 inches on center, which may seem like a lot. The reason for this is to mitigate expansion and contraction as the flashing heats up from the sun underneath the waterproofing membrane.

It's important when nailing through plywood that there is framing or solid blocking beneath to allow for solid purchase of the nail into wood. That means when nailing the six-inch leg of the deck-to-wall flashing, there should be solid blocking or framing installed behind the plywood.

Flashing, Building Paper, and Finishing Materials

The waterproofing extends an inch or two above the edge of the flashing, at which point building paper is lapped over it. The building paper is then covered by either stucco or siding.

Siding should be a minimum of two inches above the finished surface of the deck once completed. If the deck is going to be tiled or if a floating wooden deck is going to be built over the waterproofed deck, the height of the finished deck needs to be accounted for so that the height from the top of the deck to the bottom of the siding or stucco is a minimum of two inches.

By leaving a two-inch gap, this helps keep the area from the bottom of the siding or stucco to the top of the deck clear, mitigating rot issues.

Importance of Building Orientation and Design Considerations

The building’s orientation can make this issue more important. If the deck faces the weather side, wind-driven rain can overwhelm the door pan. This may appear to be a deck waterproofing problem when it really is a design mistake.

Sometimes in the real world, this is not doable because there is often a height issue between the surface of the deck and the surface of the interior floor. If, due to preexisting conditions, the two inches at the door pan cannot be achieved, it can be beneficial to have a substantial eave. I've seen many hardwood floors get damaged, and the ceiling below be damaged due to this design flaw.

Managing Water Drainage on Decks with Parapet Walls

If the deck to be waterproofed has parapet walls, scuppers will need to be installed to penetrate the walls at their low points to redirect water from the deck. Also, a system of crickets will need to be designed to create positive water flow towards the scuppers. Other times, drains will be installed towards the center of the deck along with crickets designed to create positive flow towards them. The cricket system needs to be carefully thought out.

Waterproofing membranes are not designed to handle standing water. It is important to design the deck so that the flow of water moves off the deck to either a drain or a scupper.

Assistance from DWC

DWC can assist in drain, cricket, and scupper layout and design.