May, 2007

Preventing Water Damage and Roof Leaks

A North Carolina builder asks, “What is the best way to reduce the potential for water intrusion at our roof wall intersections?”

By Steve Easley

Sloping roofs that intersect walls create one of the most common and troublesome areas for water leaks, since these areas can experience quite a bit of water loading. You might be surprised to know that 1 in. of rain over a 2,000-sq.-ft roof area deposits 1,250 gallons of water. That’s a lot of water.

The first priority for good moisture management is to deflect away as much rainwater as possible from vulnerable areas such as roof wall intersections.

I regularly see builders making a mistake when they “reverse shingle” the building wrap, placing it behind the roof flashing at these areas.

You wouldn’t tuck your raincoat into your pants, so why would you reverse shingle the weather-resistive barrier and the step flashing?

A wall is nothing more than a vertical roof in a wind-driven rain, so we need to build and detail walls as we would a roof, making sure that all water is diverted downhill and away from—not into—the structure. In this case, the building wrap should be placed over the top of the step flashing.

As a backup, I like to take a piece of peel-and-stick flashing, adhere it to the step flashing, and onto the OSB. This turns the sheathing into a tertiary drainage plane, creating an additional layer of protection against water intrusion.

In this example, your first layer of defense is the siding to deflect most of the rainwater away. But in the real world, all cladding systems leak, so the second layer of defense is a properly installed building wrap. Any water that gets behind the siding will hit the wrap and drain out. This is what we call a secondary drainage plane.

The third layer of defense is the peel-and-stick flashing that bridges the gap between the step flashing and the sheathing. I prefer a butyl-based peel-and-stick because they perform better and longer.

Another problem area is the termination of the step flashing at the edge of the roof, which causes a huge moisture load at the wall roof intersection. Notice how much water pours over the top of the window. You also can see that there is no effort to “kick out” the flow of water at the fascia board area to divert it away from the wall. The result is often water intrusion into the wall at this interface of roofing, flashing and siding. Fortunately, this is an easy fix.

Kick-out flashing is simple and economical to install.