A Watertight Second-Story Porch
With a rubber membrane and careful flashing, you can create an outdoor room above and shelter the living space below.
Synopsis: The arched opening of this upstairs porch dramatically changed the look of the house, but waterpoofing the deck was the key to making it work. The article details the framing of the substructure, the application of an EPDM membrane, and the sleeper/deck on top.
With Long Island Sound closer than a short fly to the outfield in Yankee Stadium, this house was begging for a porch to take advantage of the views. A roof that skirts the second floor offered a choice of locations. But it was obvious that the porch should be above the entry and adjacent to the master bedroom.
Waterproofing is an issue anytime you attach a new feature to a house. On the ocean, strong windswept rain is a good reason for concern. And because the porch is over an entryway designed to shelter the front door, the area below the new porch needs to stay dry.
Meeting these challenges is a lot easier today than it used to be, thanks to seamless rubber membranes and the underlayment, flashing, and adhesives designed to work with them. The key to creating a watertight porch lies in careful detailing: installing each layer correctly and in the right sequence.
Drainage starts with the subfloor
To drain water away from the house, I built a sloped subfloor. Adding new joists strengthened the porch and offered a simple way to create a pitched surface. It takes only a slight pitch to drain water. But to be safe, I pitch the joists 1⁄2 in. for every foot of subfloor.
I sheathed the floor first with 3⁄4-in. plywood, then with a high-density fiberboard (www.gp.com/build) designed to be used with rubber membranes. The fiberboard has no inherent strength. It is used because it is smooth and won’t damage the membrane. It also is…