Designing to Avoid Mold

[Guest guest]

Designing to Avoid Mold

Source: BUILDER Magazine

Publication date: 2005-05-01

http://www.builderonline.com/industry-news.asp?

sectionID=28 & articleID=127525

By Wardell

Q: What design details make a home more susceptible to mold and

mildew problems?

A: U.S. BUILDERS SPEND ROUGHLY $9 billion a year on call-backs, with

most of those dollars going to fix moisture damage. Law journal

articles estimated about 10,000 mold-related lawsuits in 1992, and

most legal experts agree that the numbers have been growing. What's

going on here?

To keep moisture out of walls and roofs, you need proper cladding

and flashing details, along with wall assemblies that will dry if

any moisture does get inside them. But design also plays a role.

According to Steve Easley, a partner in Chicago-based Building

Media, a consultant and trainer, some designs are more prone to mold

problems than others. Many of today's homes require complex flashing

details that are easily executed by a master craftsman but that can

lead to disaster when put in the hands of the average framing

crew. " Many designers still design as if it were 30 years ago, when

crews were better trained, " says Easley.

Proper flashing is beyond the scope of this article (Easley travels

around the country giving day-long seminars on the topic), but you

can protect your reputation and your clients—and even reduce your

liability—by designing homes that don't require complex flashing.

That doesn't mean shortchanging aesthetics, but it may mean making

changes to certain problem areas.

Here are a half-dozen of the most common design flaws Easley sees,

along with some suggested fixes.

1. Horizontal Valleys And Other Water Traps

Avoid horizontal valleys.

Flaw: One of the common details is a water trap created by two

sloping surfaces, such as a horizontal valley where two roof slopes

meet. The tremendous moisture load at this intersection almost

guarantees moisture problems. An even worse spot is the valley

formed where a roof slopes into a vertical wall, because you have to

flash the wall. If the valley is only open on one end, the

intersection of the back and side walls and the roof is a leak

waiting to happen.

Solution: Design structures so that all roof runoff is carried away

in gutters anddownspouts. Avoid hard-to-flash details like

horizontal valleys and other water traps, or use crickets and

saddles to divert the water away from these areas.

2. Dueling Roofs

Keep the gutter above the roof.

Flaw: A ridge that intersects an eave can create a flashing

challenge where the gutter dies into the roof slope. Easley

estimates that a 1-inch rainfall on a 2,000-square-foot roof will

deposit 1,250 gallons of water in the gutter—that's a lot of water

to deal with.

Solution: Lower the ridge or raise the eave. The ridge at the gable

should run underneath the eve of the adjacent roof, so you can run a

continuous gutter across over the top. This eliminates flashing

difficulties by eliminating the need for flashing.

3. Little Or No Roof Overhang

Roof overhangs keep walls dry.

Flaw: On a house with no overhang, water is more likely to drip down

the siding and then work its way into the wall. According to Easley,

a Canadian study found that in 80 percent of homes with moisture in

the walls, there was no roof overhang.

Solution: You guessed it. In fact, the same study found that only 25

percent of homes with roof overhangs of at least 2 feet had wall

moisture problems.

4. Too Many Roof And Wall Penetrations

Every penetration is a potential leak.

Flaw: Every house has at least one plumbing vent punching through

the roof. Many also have other vents, chimneys, plumbing exhaust

hoods, and plumbing and electrical penetrations through walls and

roofs. Although they must all be correctly flashed to keep water

out, often they're not.

Solution: Minimize the number of roof and wall penetrations. Put the

chimney on the gable end, use air-admittance valves to reduce the

number of plumbing stacks, and put exhaust vents for fans and

combustion appliances through the wall rather than the roof.

5. No Building Wrap

Siding alone won't keep a house dry.

Flaw: Easley estimates that over half of all homes built in the

United States have no building wrap. Their builders assume that

siding will keep water away from the sheathing, but it won't. " All

cladding systems leak. It's not a matter of if, it's a matter of

when, " says Easley.

Solution: This is a no-brainer. You wouldn't think about building a

roof without some type of underlayment, but you need it just as much

on the side walls. And you need to make sure that when water does

get behind the siding, the flashing will keep it from getting into

the wall.

6. Poor Site Drainage

Get water away from the house.

Flaw: Poor design can include the landscaping as well as the house.

If the ground slopes toward the house, it's obvious that water will

flow toward the slab or foundation. " Oftentimes, site drainage

problems become interior moisture problems, " says Easley. " If

there's a high moisture load on the basement wall, that moisture

will often end up inside the house. "

Solution: Site the house well enough above grade so that water can

easily drain away from the foundation. If that's a problem, install

foundation drains.

While the above design mistakes are common, they're by no means the

only offenders. One key to avoiding moisture problems is to

carefully consider how a particular detail will shed water. But once

you learn to think in terms of water management, it becomes a lot

easier to spot a design that potentially could get you into trouble.

Wardell is a freelance writer based in Vineyard Haven, Mass.

Steve Easley is a consultant and trainer who helps builders solve

building science–related problems.

Second Opinion

Do you have a question, a comment or advice about effective design

to combat mold? Consultant Steve Easley will return in June to offer

his responses to reply to your postings in the BUILDER Online Talk

section. Post your opinion here