Today's re-siding industry has improved from the bad old days when vinyl- or aluminum-siding contractors hacked off window sills and brought J-bead right to the edge of the sash. Now, best practice typically calls for stripping the wall down to the sheathing. This not only allows siding to butt the casing but exposes any existing water problems.

“We assume that every type of siding leaks,” says Robert Criner, owner of Criner Construction of Yorktown, Va. “The real water barrier is what's underneath.” Criner, like many remodelers, relies on a combination of plastic housewrap or asphalt building paper, plus peel-and-stick flashing tape, layering these materials to ensure that any water that leaks through has a chance to drain out.

Housewrap Debate The great debate seems to be whether plastic housewraps perform better than black paper. It generally boils down to this: Plastic housewrap stays more flexible in cold weather and resists tearing far better than asphalt felt, making housewrap easier to install. Non-perforated housewraps are much more water resistant than cross-woven polyethylene or needle-punched products. However, all plastic housewraps are susceptible to damage from extractives that can leach out of wood siding. Remodelers who regularly install cedar and redwood siding, which are rich in extractives, often gravitate toward black paper to avoid problems.

A sill pan may be the most important piece of window flashing of all. However, it's rarely part of a re-siding unless extensive rot demands replacing the window unit, as well.
Tyvek A sill pan may be the most important piece of window flashing of all. However, it's rarely part of a re-siding unless extensive rot demands replacing the window unit, as well.

“The take-home message is not what housewrap you use,” argues Tom Roche of Excelsior Siding in Rochester, N.Y. “It should be about how you use it.” Housewrap must be layered shingle fashion so water drains out. “You'd be surprised how many walls we've torn apart where this wasn't done. Those are the ones with real problems,” says Roche.

Problem Spots Stripping the siding exposes other troubles, especially around headers, sills, and the rim joists for open decks. “If there's no sign of water [around an opening], existing trim can be capped or repainted. But if there's obvious damage, we rip it off and start over,” Roche says. “The most important detail is getting flashing under housewrap at the head.” The worse cases involve leaks at a door or window sill. The unit must be yanked and an effective sill pan installed (see photos).

Decks are also problem areas if the rim joist wasn't well flashed. “It's so common when the deck is retrofit,” Roche says. “No one wants to take the time to pull the wall apart and get a flashing high up on the wall. Then snow sits there and seeps in. By the time we come along, the band joist is fine because it's pressure treated. But underneath, the sheathing, even studs and joists, are mush.”