All About Wall Rot

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All About Wall Rot

If your wall sheathing is rotten, the first question to answer is: where did the moisture come from?

Posted on Nov 23 2012 by Martin Holladay, GBA Advisor

Contractors who specialize in repairing rotten walls won’t run out of work any time soon. The epidemic of wall-rot problems that began more than 20 years ago shows no signs of abating. In fact, wet-wall specialists are often called to investigate problems in developments where most of the homes have rotting walls — and in some cases, these homes are only six years old.

You may be wondering, what does this topic have to do with green construction? The answer is simple: if a six-year-old house requires $40,000 of repairs because the walls have turned to mush, then the house wasn't very green — because rebuilding the house has an environmental impact.

Some people have called this an EIFS crisis, or a stucco crisis, or a leaky condo crisis. But wall rot isn’t restricted to homes with just one kind of siding or one kind of sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ; nor is it restricted to condominiums.

This isn’t just an OSB crisis. Admittedly, OSB rots very quickly. Because of the speed at which it rots, Mark Parlee, a builder in Iowa who provided many of the photos shown here, calls OSB wall sheathing “vertical mulch.” But plenty of walls with plywood sheathing and board sheathing have also turned to oatmeal. It just takes a little longer with plywood or boards than it does with OSB.


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This isn't just a stucco crisis. While it's true that some types of siding are more forgiving than others — vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding is the safest siding, while stucco and adhered manufactured stone are the riskiest — choosing the right siding won't be enough to keep you out of trouble, especially if your windows aren't flashed properly.

Scared? You should be

If you’re a builder, and you still think that siding keeps walls dry, study the photos on this page carefully.

These photos are like photos of gonorrhea and syphilis sores shown to military recruits: the idea is to scare you. Condoms and housewrap were invented for a reason.

How are these problems discovered?

A house can have seriously wet walls without the owner realizing that anything is wrong. One day, perhaps, the owner decides to call up a handyman to replace one or two pieces of rotten window trim. Or maybe the homeowner decides that it’s time for some new vinyl siding.

After a few pieces of trim or siding are removed, the handyman or siding contractor finds mushy sheathing. Houston, we have a problem.

At this point, the contractors usually just want to make everything look pretty again. So they replace a few pieces of sheathing and patch the wall back together. Three years later, the homeowner notices something odd, and the wall is opened up again. This time, the rot is even worse than it was the first time — because the water entry route had never been diagnosed, and corrective measures had never been taken.

Where’s the water coming from?

Here’s the most important advice for any homeowner with a wet-wall problem: before beginning any repairs, be sure that you know the source of the moisture that caused the rot. Once you have determined the source of the moisture, be sure to implement corrective action to ensure that the same problem doesn’t recur.

There are only two sources of moisture: it either comes from the exterior — in other words, it’s rain — or it comes from the interior. Of these two sources, by far the most common is rain.

Rain (or melted ice) can enter a wall through many mechanisms: bad window flashing, missing kickout flashing, bad roof flashing, ice dams, splashback, and inward solar vapor drive.

Although the phenomenon is rare, it’s also possible for a wall to get wet when interior moisture that is piggybacking on exfiltrating air gets trapped in a wall.

Cherchez l’eau

In the 1970s and 1980s, many builders worried that interior moisture would travel through wall assemblies, leading to wintertime condensation on cold wall sheathing. While this is a genuine problem, it usually does not lead to rot, for the simple reason that the damp sheathing usually dries out when warm weather returns in the spring.

The vast majority of sheathing rot isn’t caused by an esoteric mechanism like condensation or inward solar vapor drive. The rot is almost always caused by good old-fashioned rain.

Take off the siding and WRB

If you are investigating a wet-wall problem, you’ll need to remove the siding and housewrap or asphalt felt to get a good look at the sheathing. Keep removing siding until you’re sure that you have reached solid sheathing.

Once the sheathing is exposed, you can look at the pattern of rot. Because water flows downward under the influence of gravity, you want to look at the uppermost area of rot to help determine what’s going on. The pattern of the rot is the best clue to the water entry mechanism.

Rot due to bad window flashing. This type of rot shows up at the lower corners of the windows and spreads out from there. In extreme cases, the entire area under the window is rotten. To solve this problem, the window has to be removed and the entire rough opening needs to be properly flashed. Simply slapping some peel-and-stick tape over the window flanges won't prevent future problems.

Rot due to splashback. This type of rot occurs in a horizontal band along the bottom of a wall. In most cases, the siding is rotten as well as the sheathing. Rot due to splashback occurs under the eaves of a roof, so it usually doesn’t show up on the gable end of a house. The cause of this problem is simple: the grade is too high. While the traditional minimum distance between the grade and the lowest wooden components of the building is 8 inches, I think that 12 inches makes more sense.

Rot due to missing kickout flashing or missing step flashing. If the roof of a one-story addition or an attached garage runs alongside a two-story wall, it’s usually necessary to install kickout flashing at the eave of the lower roof. If the builder left out the kickout flashing, water can dribble down the siding and lead to rot. A similar problem occurs if the roofer omitted the lowest piece of step flashing. If sheathing stains originate near the eave of such a roof, you’ve probably discovered the source of the water.

The house in the photo below lacks kickout flashing:

The roofer who installed the step flashing in the photo below forgot to install the first (and most important) piece of step flashing — the one that conveys the roof water all the way to the eave:

Rot due to missing flashing at a deck ledger. This common problem leads to rot under the deck. If it’s a second-floor deck, the wall sheathing under the deck is often rotten.

Rot due to ice dams. When ice dams lead to rotten wall sheathing, you’ll notice that the water has entered at the top of the wall under an eave. In many cases, the signs of water entry occur directly under a valley. If you suspect this mode of water entry, ask the homeowners whether the house has a history of ice dams. To learn how to prevent this type of problem, see Prevent Ice Dams With Air Sealing and Insulation.

Rot due to inward solar vapor drive. While rare, this type of rot can be devastating when it occurs. The mechanism requires a so-called “reservoir cladding” — that is, some type of siding that can hold a lot of moisture. The most common reservoir claddings are brick veneer and manufactured stone veneer; more rarely, inward solar vapor drive occurs with stucco cladding. One important clue that you’re looking at inward solar vapor drive: the damage is worst on the south side of the house and almost nonexistent on the north side of the house. For more information on the causes of this problems and possible solutions, see When Sunshine Drives Moisture Into Walls.

Rot due to winter condensation of moisture piggybacking on exfiltrating air. Although this phenomenon is often proposed as a cause of wall rot, the problem is relatively rare. However, it happens; for example, this phenomenon was responsible for a cluster of wall rot problems on SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. houses in Juneau. It can be hard to pin this mechanism down, but look for the following clues: it usually occurs in homes with elevated indoor humidity levels (for example, in homes with humidifiers), and the rot usually shows up at sheathing seams near the top of a wall. For more information on solving this type of problem, see Questions and Answers About Air Barriers.

Why did the wall rot?

Walls get wet all the time, but they don’t always rot. So, if you are looking at a rotten wall, you may be wondering, “Why did this wall rot?”

There is just one answer to this question: “Because the rate of wetting exceeded the rate of drying.”

A little bit of moisture won’t destroy a wall, as long as the wall is built in such a way that it can dry rapidly. The best way to be sure that wall sheathing can dry rapidly is to include a ventilated rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap between the siding and the sheathing.

So if you don’t want to come back in a few years and fix more rotten sheathing at the same house, here’s what you have to do: fix all those flashing problems, lower the exterior grade, seal the air leaks on the inside of the house — and be sure to install the siding over a rainscreen.

Martin Holladay’s previous blog: “Passivhaus Practitioners Share Their Success Stories.”

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Image Credits:

  1. Photo with pipe staging: Will Smith — Rot under window: Mark Parlee (MP) — Splashback: Fairhope Farm — No kickout: MP — Missing step flashing: MP — Missing deck flashing: Everflashing — Ice dam: Ecduzitgood — Solar vapor drive: MP — Exfiltration: FHB