A constant problem that contractors see in siding installations is improper flashing or, even worse, no flashing at all. Relying on caulk alone to keep water from leaking into homes is not a good practice. Whether you're talking about wood, fiber-cement, or vinyl siding, three areas in particular can prove problematic:
Above door and window headers.
After installing window and door trim, either make flashing using a sheet metal brake or apply flashing pre-bent to the correct trim size. Nail your flashing to the wall, not to the trim, because if you put holes in the flashing, you've defeated its purpose. Flashing has to be intact to do its job, which is to keep water from contact with the wall. That goes for the weather barrier as well. Run the weather barrier over the flashing so that water doesn't get behind it. You want water to run over, not under, the siding.
Tough and tricky, because sometimes your weather barrier gets torn trying to fit it into corners at a 90-degree angle. A simple fix for this is bending flashing in an L-shape before installing it in the corner. Then, after flashing is installed, run weather barrier over the flashing to eliminate the worry of tearing the paper and getting a leak.
Roofs and walls.
Another overlooked detail is where roofs intersect walls. Relying on step flashing alone is usually not enough. Most siding manufacturers specify a minimum of 1-inch clearance from the roofing surface. This roof clearance typically results in a portion of the step flashing being exposed. An easy way to fix this problem is by installing Z-flashing. After installing Z-flashing, you can lay a 2x4 flat on the roof and run your siding to the 2x4. It will leave a perfect 1½-inch clearance every time. Don't forget to run your weather barrier over the flashing to keep water out.
With today's modern products being under warranty for up to 50 years, it's important to pay attention to flashing details to ensure that the home is weather-tight for the duration of the siding warranty and beyond. —Carl Sperry is a contractor who lives in South Carolina.