Making Weather Work

You've just completed a new addition with a beautiful pitched roof and high-end windows. After all that hard work, are you ever proud of the job. If you don't block air, manage water vapor and prevent bulk water infiltration, your proud accomplishment could turn into an embarrassing callback.

June 30, 2006

5 Need-to-know tricks to installing house wrap

You've just completed a new addition with a beautiful pitched roof and high-end windows. After all that hard work, are you ever proud of the job.

But will your addition be mold- and water-free in five years? It will if you take the time to weatherproof the roof and exterior walls.

In windy areas, roof coverings and underlayment can blow off. Roof decking can protect if it is sealed with a self-adhesive asphalt/rubber tape.

"For us, weatherproofing is a 5-star, priority issue," says Tommy Strong, project manager with Brothers Strong remodeling in Houston. "In the last few years, it has become more and more of a priority for our clients, especially with growing concerns over mold."

If you don't block air, manage water vapor and prevent bulk water infiltration, your proud accomplishment could turn into an embarrassing callback.

When it comes to weatherproofing, the first line of defense is a strong, durable roof. Add properly installed housewrap or building paper, flashing and overhangs to keep out moisture, and you can stand by your project for years to come.

Always start with dry materials. Use wet supplies and you're gambling with future moisture problems.

Weatherizing the roof

Having a waterproof roof is essential. Here are some tips:

Start by minimizing the number of penetrations in the roof. Use baffled ridge vents to prevent airflow and wind-driven rain from entering the attic through the ridge vent. Soffit vents with perforations concentrated near the outer edge also minimize the area that rain can enter, and they cost about the same as standard vents.

To withstand strong winds, it's best to use plywood or fiber-cement soffit materials and attach them securely to framing. A lumber baffle securely installed over the framing inside a soffit will greatly reduce water entry into the attic.

Extend the fascia board below the underside of the soffit by building the fascia from a 1x6 instead of the standard 1x4, which creates a drip edge and prevents water from leaking into the eaves.

Extend the fascia board below the underside of the soffit by building the fascia out of a 1×6, instead of the standard 1×4. This creates a drip edge so that strong winds can't drive rain across the soffit surface and into the eaves. The added cost is no more than the difference between the price of 1×4's and 1×6's.

Choose roof sheathing that is a minimum of 19/32-inch thick for added strength. Thicker sheathing can also offset sensitivity to overdriving.

Use 8d ring shank nails at 6 inches on center or as required by code. Also, make sure that sheathing is properly spaced. Allow a ¼-inch gap between sheets for thermal expansion. H-clips can provide this spacing while adding strength to the horizontal connections.

Add synthetic, tear-resistant roof underlayment, which is an integral part of the roofing system and a second layer of moisture protection. For the best performance, closely follow the manufacturer's directions and use approved fasteners.

In high-wind areas, roof coverings and underlayment can blow off, leaving the roof decking to act as the water and wind barrier. But decking can still provide an effective level of protection if you've sealed it with a self-adhesive asphalt/rubber (modified bitumen) tape that is at least 4 inches wide. Another alternative is to use a peel-and-stick roof membrane over the entire roof deck.

Paper or house wrap?

Weather-resistant barriers — either house wraps or building paper — shed water in bulk, protecting the house from driving rain. They also both allow the wall to breathe, letting water vapor pass through so the wall can dry out when needed, but house wrap generally does it faster. Determining which one is right for your addition depends on where you're building.

The first step to weatherproofing is minimizing the number of penetrations in the envelope, particularly in the roof.

Because building paper slows the flow of water vapor, it is recommended for areas where humidity levels outside the wall sheathing is typically greater than inside: hot and humid regions, behind a brick veneer, or under unsealed wood siding.

In most other applications, where walls dry from the inside out, house wrap is the best choice. House wrap is unique in its ability to stop both bulk air and water leakage while allowing water vapor that gets into the wall cavity — where it could do damage — to get out.

However, for house wrap to be effective, remodelers have to install it right. If you see holes in your house wrap, it's flapping in the wind, or it's installed top to bottom rather than bottom to top like a shingle, you can bet you didn't.

"There are only a handful of tricks — three or four — that you need to know to install housewrap, and those are intuitively logical," Strong says. (See list of tips below.)

In all, house wrap costs about $150 more in materials and labor than building paper to cover a 2,500-square-foot home.

Weatherize in a flash

Flashing is one of the longest lasting building components, so be sure to do it right. Take your time — it could save a callback.

"Flashing is a crucial element in weatherproofing a home — and not just the metal flashing that is commonly found above windows," Strong says. "This is where water and air can infiltrate the walls."

Here are a few tips:

  • Water runs downhill, so remodelers should overlap flashing like the scales of a fish to provide repetitive layers of resistant material.
  • Wind can move water uphill, so be sure to install the flashing with the amount of overlap the manufacturer recommends.
  • If flashing is not sufficiently lapped or it is placed on a low-pitch roof, water may move into the joints between the flashing. Take extra precaution in these situations.

Brothers Strong also recommends using flexible flashing tape, a rubberized tape that easily curves around windowsills, door frames and custom shapes. This eliminates the time-consuming and often flawed process of cutting tape pieces to fit around curved and rounded openings. While a little more expensive than traditional flashing tape, the flexible variety makes the process quicker and easier.

Hanging in there

Overhangs may seem like an extra to keep out the hot summer sun, but a properly installed overhang will go a long way toward protecting the home from rain. The larger the overhang for windows or doors, the fewer moisture problems will occur on exterior and foundation walls.

The local climate will determine the minimum size of overhangs. As a general rule, the wetter the climate, the larger the overhangs you'll need to install. Use 12-inch eaves and rakes in moderately dry climates; 18-inch eaves and 12-inch rakes in moderately moist climates; and a minimum of 24-inch eaves and 12-inch rakes in very wet climates. As a bonus, properly sized roof overhangs over south-facing windows will also block unwanted summer sunlight but allow heat from sunlight in winter.

More tips on proper weatherproofing can be found in "Durability by Design: A Guide for Residential Builders and Designers" available at under the Resources and Publications tabs; and "Moisture-Resistant Homes: A Best Practice Guide," in PATH Tech Set No. 2, Building Envelope, located under Tools.


5 Need-to-know tricks to installing house wrap

  1. Follow product manufacturers' instructions to a T. Many people don't.
  2. Tape all seams.
  3. Tape tears or holes in the house wrap. A small tear makes a big difference in air and water infiltration.
  4. Make sure the wrap covers gables and corners without ripping because water entering at these points goes directly to the structure and can lead to extensive rot. You may want to add a layer of tar paper over these areas as well.
  5. Install flanged windows over the top of flashing paper at the bottom of the window while having flashing overlay the flanges at the top of the window. The flashing paper can then be slotted in the house wrap and secured with flashing tape.
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