Wintertime Water-Intrusion Blues

By: Casey M. McKinley, P.E.

While shovels, snowblowers, plows, and deicing agents are used to clear snow and ice from driveways and sidewalks during the winter, removing frozen precipitation from the roof of a house is a bit more challenging.  As snow and ice piles up on the roof and subsequently thaws out, the melt-water can sometimes result in damage to the interior of the structure.  In those areas of the country that are so “fortunately situated,” ice damming, roof-deck condensation, and water intrusion around roof penetrations are as perennial as the common cold.  Fortunately, barring catastrophic levels of snow and ice buildup on a roof, these problems can be prevented or corrected by using proper construction and maintenance techniques on the structure.

A roof that otherwise does not leak can develop leaks following a winter storm as a result of ice damming.  Ice dams form when heat rises from the occupied space below, warming the underside of the roof deck and melting the snow.  The lower portion of the roof (at the eaves and above the gutters) is typically colder, and the melt-water refreezes, progressively building up a dam of ice and trapping any water resulting from further melting.  Because ice dams, like snow or sleet accumulation, build up on the top side of the shingles or slates, the roofing is rarely damaged by the accumulation of ice.
Because shingled roofs are designed to shed water but are not actually waterproof, water pooled behind the ice dam flows under the shingles and onto the roof sheathing (Figure 1).

Figure 1: Illustration of Ice Dam
Source: University of Minnesota Extension

Having breached the weather protection normally provided by the roofing, the water disperses down walls and through ceilings into the space below.  Installation of a self-adhering ice and water barrier membrane, which is required by the 2012 International Residential Code in much of the northern United States, can prevent melt-water from reaching the roof sheathing.  However, this is only effective if the ice dam does not exceed the height of the membrane.

Ice dams, and the resulting water intrusion, can be a one-time event caused by very specific conditions or can be a recurring problem aggravated by poor insulation and poor ventilation.  Proper air circulation within the attic reduces the likelihood of ice damming.  Insulation often blocks the free flow of air along the roof slope, allowing for the buildup of heat and the eventual warming of the roof surface.  Keeping a consistent temperature in the attic and maintaining proper insulation and ventilation are key in the prevention of ice dams.  Replacing or improving the attic insulation can keep warm air from escaping, preventing ice dams while reducing energy costs.

If the roof deck’s temperature is cold enough and the air in the attic has sufficient moisture (in the form of water vapor), condensation on the roof sheathing can also cause water to leak into the house.  Often, this is mistaken for a breach in the roofing material due to strong wind gusts, hail, or other phenomena.  In actuality, the water vapor in the interior air has condensed on the cool wood sheathing and/or the roofing nails.  Interior damage from this condensed water vapor can range from an annoying dripping and stains on the ceiling to much more severe water intrusion problems.  If the condition is severe enough, left unchecked over an extended period of time, it can result in mold growth and wood rot.  These conditions can be combated by proper insulation, ventilation, and humidity control.

Wintertime water intrusion can also occur around various penetrations through the roof, including chimneys and vents.  The potential for water intrusion around these penetrations is greater if these areas are not properly flashed and sealed, or if the flashing or sealant has deteriorated.  For this reason, homeowners should have these flashings and sealants inspected periodically to verify their integrity.  Lower-sloped shingled roof surfaces, where the residence time of melting snow and ice is often longer (particularly on areas of the roof that are shaded or do not receive much direct sunlight), are more prone to water intrusion during the winter months.  Many lower-sloped roofs are found to be leaking during snow-melt and rainfall events because they do not meet basic drainage principles, i.e. “get the water off the roof as soon as possible.”  When the installation conditions warrant the use of a low-sloped roofing material, single- or multi-ply roofing membranes should be considered in lieu of asphalt shingles.

Building codes and manufacturers’ specifications address minimum roof slopes for different roofing materials, attic ventilation requirements, and other criteria which collectively have an effect on the structure’s ability to withstand Mother Nature’s colder side.  These codes and specifications guide the builder’s initial construction of the house and must also be considered during any remodeling or reroofing efforts.  Once the structure is occupied, periodic inspections, routine maintenance, and common sense are a homeowner’s best weapons against the wintertime water-intrusion blues.