Honey, why is water dripping from the ceiling when it's bright and sunny outside?

January 01, 2015
January/February 2015
A version of this article appears in the January/February 2015 issue of Home Energy Magazine.
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This past February I received the following e-mail:

Hi, Guys,
We've had trouble with water spots on the ceiling since late last fall, and our builder tells us it's the result of melting snow—he said it can't be helped. We've got a few new ones today, and he's sticking to the same response. Does anyone have issues like the one pictured when it rains or the snow melts?

—Miffed in Midwest

I responded:

Roofs should not leak and snow should not get into the house below the roof. If it does, something is wrong. Your builder is incorrect—it can be fixed, it must be fixed. Now as to the cause of the ceiling “leaks”:
Assuming there are no water pipes in your attic, the leak is either from outside moisture or from condensation of inside moisture that has gotten into the attic. Condensa-
tion could be from within either heating ducts, dryer vent, or bathroom vents located in the attic. Except under very cold conditions, the moisture in these pipes would not condense. However, when it does it can flow through the seams of these pipes and drip onto your ceiling. Take a look in your attic to see if there are any ducts or vents in proximity to where you are seeing the water.
Another leak source could be from ice dams. Usually these occur within 3-4 feet of the exterior (perimeter) of your house. From the pictures you provided I don't think they caused your wet areas.
If you don't have this problem except when it is cold, it is likely that your roof membrane (shingles) and flashing are OK. Assuming it happens only when it is very cold, windy, and snowy, it is likely that fine powered snow has blown into your attic through the soffit vents (or gable vents). Once the outside temperatures moderate so that the top of your attic insulation is above freezing, this snow will melt and drip down to the joints in the ceiling. Snow on a lower roof was probably blown by strong winds through the soffit vent of the higher roof and into the attic. This typically occurs at two intersecting roofs with one overlapping the other. The problem can be fixed by addressing the soffit vents in offending areas.

fe_Wendt_photo3(Bob Wendt)

House 1

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House 2

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House 3

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What Causes Winter Ceiling “Leaks”?

These leaks through the ceiling are usually caused by snow entering the attic and melting. They are not leaks through the roofing and then the ceiling. That kind of leak is associated with damaged or defective roofing or flashing and moderate to heavy rains.

The environmental conditions that contribute to this problem are

  • very cold periods with significant snow—typically dry, powdery snow;
  • high winds associated with or closely following the snow event; and
  • a modest warming period after the snow event; outdoor temperatures may remain below freezing.

In short, a typical winter in the northern Midwest.

The building characteristics that contribute to this problem are

  • house completed within the last ten years;
  • complex roof configuration;
  • soffit and ridge vents for attic ventilation; and
  • few or no gable vents.

Some houses with these characteristics have no problems while others have leaky ceilings. Why? (Also see “Physics of Snow Infiltration Through Soffit Vents” for details on p. 6.) Let’s look at three houses.

House 1

This house has a soffit-and-ridge vent system. It experienced ceiling “leaks” in the winter several years ago but remained completely dry during torrential spring and summer rains.

The soffit was made of alternating perforated and solid plastic panels. (See photo 1.) Note the close proximity of the perforated portion of the soffit to the underlying roof.

The homeowner experienced ceiling “leaks” above the windowed portion of the family room. Powdery snow fell on the lower portion of the intersecting roofs and was blown and sucked into the soffit vents by high winds. (See photo 2.) The orientation of the offending area is northeast. Strong winter winds blow in this direction.

The builder corrected the problem by replacing the perforated panels on the upper soffit with solid panels where the snow had entered. The lower soffit was not changed. Note that removing the upper perforated panels could decrease ventilation of the attic above the family room, which might cause problems in the future.(See photo 3.)

The front dormer of House 1 also has solid and perforated soffits for ventilation on the attic above the bedroom. There have been no ceiling “leaks” in the bedroom. (See photo 4.) The orientation of this potential problem area is southeast, which may explain why there has been no problem. Strong winter winds do not blow in this direction.

House 2

This house has a soffit-and-ridge vent system. It has experienced many ceiling “leaks” in the winter, but it remained completely dry during torrential spring and summer rains.

The orientation of the front façade of this house is to the north east, which corresponds to the direction of strong winter winds. (See photo 5.) The façade also steps back and intersects the garage roof in such a way as to block the movement of air over the house. This increases the pressure of the wind. Note the close proximity (18–36 inches) of the soffits to the roof over the front porch.

Ceiling “leaks” occurred in the room with the shuttered window as well as in the hall (center window). (See photo 6.) Another “leak” occurred in a room to the right of the pictured area.

This shows the very close proximity of a significant portion of the roof to the perforated soffit vents. Strong winds blow powdery snow off the roof and into the second-floor wall. The snow has no place to go but up against the soffits and into the attic. Note the gutter discharge onto the adjacent sloped roof. (See photo 7.)This location appears to have the potential for wind-driven rain to be blown into the soffit vents as well. However, the prevailing winds during rain events are from another direction. The homeowner has not reported any rain-related “leaks.”

An alternative to the use of soffit vents on the front façade of House 2 would have been a conventional gable vent on opposite ends of the house. As you can see, no such vents were installed, possibly because they require more labor to install than the soffit vents. (See photo 8.) However, the builder did use a gable vent over the garage—probably for aesthetic reasons. (See photo 5.)

House 3

This house has a soffit-and-ridge vent system and has experienced no ceiling “leaks.” (See photo 9.) It is located next door to House 2 and across the street from House 1.

The orientation of the front façade of this house is to the north, which corresponds to the direction of strong winter winds. Note the distance (~4 feet) between the porch roof and the soffits of the upper roof. Also note that there are few pockets on this façade to trap the wind and increase its pressure against the house.

The rear of House 3 separates the lower roof from the soffit vents by about 4 feet. The relatively simple façade also minimizes the trapping of the wind that might blow powdery snow into the soffit vents. (See photo 10.) Many houses in this neighborhood have characteristics similar to those of House 3 and have not experienced any snow-related ceiling “leaks.”

Physics of Snow Infiltration Through Soffit Vents

fe-wendt_figaFigure A. This drawing shows the physics of how snow infiltrates through soffit vents.

For snow to enter the attic through soffit vents, it must be a dry, powdery snow with small granules that resemble fine sand. This type of snow occurs at temperatures well below freezing, typically in the teens. Wet snowflakes, on the other hand, are usually much bigger and heavier and occur when the temperature is closer to freezing. This dry, powdery snow is fairly common in the northern United States, while wet snow is common in the lower Midwest and South. Dry, powdery snow can easily become airborne in moderate to strong winds—much like blowing sand at the beach. And like beach sand, once airborne, it tends to stay within several feet of the surface from which it was blown. These are the characteristics that enable this snow to enter through perforated soffit vents. See Figure A.

The shape and orientation of the house also contribute to the entry of snow through soffit vents. The sloped roof of a house acts as an airfoil (like an airplane wing) in the wind. This creates a negative pressure at the ridge, and through the ridge vent, a negative pressure in the attic. Wind blowing into the house wall creates a positive pressure on the wall and at the soffit vent. In addition, wind trapped and channeled by the house configuration (see House 2) increases the velocity and pressure at that location. This is like the effect of walking between two large buildings that are close together on a windy day. The wind-induced positive pressure on the walls and at the soffit, coupled with the negative pressure in the attic, causes the airborne snow to literally be pushed and sucked through the soffit perforations and into the attic. The snow particles ultimately settle on the top of the insulation, where they remain until the attic temperature gets above freezing. This can occur on a sunny day even when the outside air temperatures are in the teens or 20s. Once melted, the water drains to the top of the ceiling drywall, where it collects and runs to the joints. It becomes visible as wet marks on the ceiling as seen in the first photos in this article.

How to Fix “Leaky” Ceilings

Here are some suggestions for fixing and avoiding ceiling leaks in existing and new homes.

Existing homes. Because these fixes deal with existing homes, the options are more limited than they would be for homes that are yet to be built. First, identify all the potential problem areas where the perforated soffit vents are less than 4 feet from any lower roofs (especially those on the windward side of the house in winter). Remove all perforated soffit panels that are 2 feet or less from any lower roofs and replace them with solid panels. Place the relocated perforated panels in portions of the soffit that are more than 4 feet from any lower roof. For perforated panels located between 2 feet and 4 feet from the lower roof, cover the inside surface of the panel completely with a 4-inch-thick unfaced fiberglass batt. This batt will inhibit the intrusion of snow and still allow some water vapor to be vented from the attic. The thickness will reduce the likelihood of strong winds displacing the batt, permitting snow to enter. If these changes reduce the venting of the attic below code minimums, add a good-quality gable vent to each end of the attic.

New homes. Do not use perforated soffit vents where a distance of at least 4 feet from lower roofs cannot be maintained. Avoid house designs with façades that can trap winter winds and increase the amount of snow blown toward soffit vents. Use good-quality gable vents whenever soffit vents are inappropriate.

The Bottom Line

This problem may seem small and isolated, but the builder of House 2 (built in 2012) noted that he had a number of houses with snow infiltration problems. Homeowners who have experienced “leaky” ceilings in recently built houses are not happy and do not consider the issue a minor inconvenience.

The solutions proposed for existing homes should eliminate the problem of winter ceiling “leaks” and can be implemented with reasonable effort, albeit not by your average homeowner. The solutions for new construction are simple and easy to implement. While gable vents require more labor than soffit vents to install, the added cost is very low when compared to the cost of identifying and correcting these “leaks.”

Bob Wendt is an architectural consultant specializing in the building science investigation of residential buildings. He retired from the Buildings Technology Center at the Oak Ridge National Laboratory in 2008.

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