Siding in the Rain

Working in Rehoboth Beach, Del., Patty McDaniel of Boardwalk Builders is never far from the salt spray of the ocean. Her crews contend with heavy moisture and wind-driven rain year round.

“I have a very fast feedback cycle,” McDaniel says. “If I make a mistake, I usually know about it after the first good rain.”

Though not every remodeler works in such an unforgiving climate, everyone can learn something from the best practices of those who do.

It's important to understand that siding is just the first line of defense. Proper flashing at seams and penetrations and a well-installed moisture barrier are essential components, without which the installation is almost certain to fail. Eventually, experts say, water will find its way past siding, no matter the material, and through seams around windows, doors, and penetrations such as vents.

“The siding's job is to look good,” McDaniel says. “It's the flashing and the weather barrier's job to keep water out.”

Because the layers between the siding and the sheathing must be properly installed to secure a house from moisture, best practice requires that old siding always be stripped, even when working with vinyl designed to be installed directly over wood. Gino Streano of Lifetime Remodeling Systems in Portland, Ore., says he would never risk relying on another contractor's flashing and moisture-barrier installation, especially given the poor quality of construction he finds in more recently built houses.

A rain screen system provides a ventilated air space that equalizes air pressure behind the siding to minimize wind-driven leaks. It also promotes drying and creates a way for any water that does get behind the siding to escape.

Photo Credit: The Journal of Light Construction

“New construction in the '80s is about when it starts,” he says. “For whatever reason, installation just hit the toilet. I would say 95% of the time, if not more, the siding has been improperly installed — whether it's gaps caulked over, over-nailing, nailing too close to the corner of the siding, improper flashing, or even not using flashing on windows and doors.”

New siding applied over existing also stands proud of window and door trim, creating a difficult joint to protect, McDonald says. Once the old siding is off, the flashing and moisture barrier go on, and it's at this stage that many installers make crucial mistakes. The first essential, Streano says, is to never tear off more old siding than can be replaced the same day. Leaving a portion of wall exposed to the elements can sabotage a job before it's finished.

The most important rule — one that is often ignored — is to lap every piece of the moisture-protection system, starting from the bottom up.

“You have to think like water — following the logic of the path of water and the way it's going to get in through what you're building,” says Joseph McKinstry of McKinstry Construction in Seattle.

Each overlap should be several inches long. In climates where heavy wind and rain are common, experts recommend overlaps of about 6 inches at horizontal joints and 8 inches to 12 inches at vertical joints.

“You're always thinking of shedding water,” says Marty Schirber, of Castle Building and Remodeling in Minneapolis, Minn. “You're always thinking of moving the water away from the building.”

Though the logic seems simple enough, plenty of installers get it wrong.

“That's my main rule,” McDaniel says. “Everything goes on lap-shingle style. I've never seen anyone put shingles on backward but I've seen people make all kinds of mistakes putting on weather barriers.”

Proper lapping is also essential when flashing around rooflines, doors, and windows, vulnerable areas where leaks most commonly occur.

“Over and over again we see people not doing proper perimeter flashing prior to putting in a window or not doing proper head flashing,” McKinstry says.

On a window, the housewrap or moisture barrier should be laid first (see drawing below). Though some contractors argue for installing side flashing before laying in the window, McKinstry and others who work in climates with heavy wind-driven rain argue for installing the window first.

“It's very important to flash the sides right,” McKinstry says. “Where we have seen penetration is on the bottom along the outside edges. That's where windows fail, at the corners.”

It's also essential when installing the window to avoid caulking underneath the bottom flange. Caulk there only traps moisture inside.

The head flashing is the final piece to be installed. This piece must be installed underneath the housewrap, so any water that gets behind the wrap will shed out and over the window flange instead of behind it.

“That's one thing lots of guys don't understand,” McKinstry says. “You have to tuck the head flashing underneath something.”

Flashing is also required at joints between two pieces of siding. This is an area where installers often err, particularly when installing fiber cement or vinyl. The most common error, Streano says, is the failure to account for contraction and expansion.

Make sure all window head flashing is overlapped by building paper or housewrap. This may mean having to slice the paper to insert the flashing behind it. In severe climates, tape the paper to the flashing.

“Guys typically just butter the joint with caulk and don't leave a gap,” Streano says. “But you have contraction and expansion, so the caulking will fail and that gap will open up.”

Rather than caulking the seam, Streano's crews first apply a back flashing using 60-minute tar paper, then butt the two pieces of siding together.

“You're going to get some wind-driven rain in there,” he says, “so now there is an additional piece of flashing that gets that rain out of the wall immediately.” As added moisture protection, some contractors will create a rain screen before installing siding (see drawing on previous page). A rain screen is a system in which the siding is separated from the moisture barrier by attaching furring strips at regular 16-inch intervals. This separation creates an air cavity that equalizes air pressure behind the siding — which deters wind-driven infiltration — and also encourages drying and drainage of any moisture that condenses behind the siding.

“We know that moisture is going to get in there and we want to manage it,” Streano says. “We want to facilitate, not prohibit, airflow. Mold likes a stagnant, moist climate, so you want to get air flowing through the space between the wall and the siding.”

(For more information about using rain screens, click on “WebXtra” under the “Magazine” tab at www.remodelingmagazine.com.)

There's little consensus among remodelers as to which moisture barrier works best. McDaniel prefers a combination of 30-pound felt paper and Tyvek FlexWrap. McKinstry favors impregnated black tar-paper, having followed the lawsuits that resulted from failures in first-generation synthetic wraps.

But Streano sees an advantage in new synthetic wraps such as Tyvek DrainWrap, GreenGuard's Raindrop, and Valeron Vortec. Designed with channels to facilitate air flow and drainage, these products provide an effective alternative to the more costly rain-screen approach, Streano says.

“These products are all moisture barriers,” Streano says, “but they're also able to drain moisture and facilitate the drying of the system. They don't have the same capability to drain as the [rain screen], ” he adds, “but they do a very good job, relative to a regular moisture barrier, of draining excess water as well as facilitating air flow. And they're only a fraction of the cost of installing a traditional rain screen.

McKinstry says he's finding increasing demand for rain screens but, either way, “ventilation is so key in this area. If you have good ventilation, even if you get moisture, it dries out.” —David Zuckerman writes frequently on construction topics from his home in Brooklyn, N.Y.