If you're an avid reader of this column, you know I'm a fanatic about preventing water from penetrating walls and floors. And if you want to live in a coastal climate, where wind-driven rain is a constant, you'll probably develop the same obsession?especially when you consider the costs of replacing rotted subflooring and joists.
Flashing is a time-tested technique we use in Nantucket for building extra protection into door and window openings. Besides being effective, the process has two other giant advantages: It's simple and cheap.
I'll describe the technique for exterior doors. You can adapt the steps for windows.
The problem. The most dangerous effects of water accumulation occur where you can't see it happening?beneath layers of siding or flooring. If you're building inland, caulking may seal out most moisture. But on the coast, you need flashing.
The material. I've learned through trial and error that the best flashing material is lead or lead-coated copper. Usually, I put about 600 pounds of lead into flashing around windows and doors. Unlike copper, lead is soft and a little lumpy to the eye. But it's easy to shape on the job site without special tools. Once installed, it's concealed by trim or siding or molding. It costs less than $20 in materials, and it could save thousands in repairs later.
Creating the pan. For most doors, a 14-inch by about 4-foot sheet does the trick. Place the lead sheeting, long side forming the width of the door opening, directly on the subflooring. The 4-foot length of flashing is enough to cover the threshold and bend up the sides of the studs. Its 14-inch width lets you extend it about 2 inches beyond the threshold on the interior and the same on the exterior. Crimp up the edges of sheeting on the inside so that about 1 inch sticks up from the subflooring. When the threshold is installed, this edge will be sandwiched between the threshold and finished flooring. On the outside, bend the flashing down the side of the wall. Bend it over the top of the felt so it forms a downward lip.
How it works. What we have now is a three-sided moisture trap running beneath the threshold, up the sides of the rough door framing, and against the flooring. The fourth side is the lip on the exterior, dumping any water that accumulates beneath the door back outside. It's a self-draining pan. If I need to secure it, I nail the top edges of the lead bent up the sides of the studs (not in the bottom) to prevent any leaks.
Final trims. On the interior side of the door, the finished flooring is butted right to the upturned lead sheeting against the threshold. The sheeting that sticks up over the flooring is trimmed flush with a sharp knife. I'll use a 3