If El Nino’s younger sister La Nina blows into town this winter, we can expect lots of cold and snow. Beware that these conditions are perfect for spawning a potentially destructive condition known as "ice dam".

Ice damming occurs when melting snow runs down the surface of a roof and re-freezes. As run-off re-freezes, the ice builds up and begins to dam the subsequent run-off, causing it to pool. Unfortunately, shingles are designed to shed water with the help of gravity and they don’t perform well against the standing water. Some of the pooled water finds its way under shingles to hidden seams and fastener holes. Some of it re-freezes under the tabs and lifts the shingles, literally opening a floodgate.

The phenomenon is caused by warm air in the attic space, heating the undersurface of the roof deck, melting snow. Run-off makes its way down the slope of the roof and eventually comes in contact with the deck over the eaves -- which is not exposed to the heat from the attic. It is here the ice begins to form. A heavy freezing rain can also trigger this cycle even when snow is not present.

Additionally, water that reaches the gutters may freeze before the draining at the drop outlet. This is especially likely to happen if the gutters and downspouts are full of debris.

If the eaves overhang is less than two feet, interior leaks may result, damaging ceilings, walls, and furnishings. Regardless of how much overhang, soffits and trim are left vulnerable to the effects of the seepage. Further, ice build-up in eaves troughs can lead to damaged gutters, downspouts, and fascia boards. Hidden behind all this, and likely to result from smaller recurring events, are the destructive effects on roof decks, trusses, and structural members. Eventual infestation of carpenter ants may compound the damages.

Aside from leaks, stains, and damaged ceilings or walls, there are several tell-tale signs that ice dams have occurred or may be imminent:

1. Large icicles hanging from the gutters during cold-snaps following snow storms are an indication that internal heat is melting the snow from beneath the roof deck. This is especially so if the icicles form on the north, where the sun is a stranger.

2. A thick blanket of snow downslope of bare shingles points toward trouble. This is especially indicative of trouble if on the north side or during a period of overcast skies.

3. Water dripping from the soffit panel is a hint that a dam has already formed. Stains, peeling paint, or soft spots on the soffits are something to look for year ‘round.

The industrious might try measuring the temperature in the attic on a cold winter night: if it is above freezing, the potential for ice damming is strong.

One way to prevent damage from ice dams is to install water and ice shield on the lower reaches of the roof deck. This relatively new genre of products is self-adhesive (applied directly to the wood deck), self-sealing (around nailheads), and water-proof. It must be installed under the shingles, for adhesion and due to its vulnerability to ultra-violet radiation. Since water that gets under shingles does not breach the membrane of water and ice shield, its installation is preferable to tar paper. It is not a panacea, however, because it does not address the cause of ice dams.

The culprit is an imbalance between the temperature of the attic space and the ambient air. The ideal deterrent is known as "cold roof", a condition wherein the temperatures on both sides of the roof surface are equal. Proper insulation and ventilation are the keys.

In addition to saving costs on heating and cooling, proper insulation keeps the attic from assuming the temperature of the structure’s interior. Upgrading to current R-value standards is an important step in the process. Blown-in insulation over existing blanket fiberglass is the most effective way to add protection while avoiding heat-robbing spaces around truss webbing. Note that thermal "bypasses" need to be defeated. These are caused by oversized holes through which wiring and fixtures, plumbing, ducts and vents, and chimneys pass into the attic space. Sealing with caulk or expansion-type foam keep the warm air from drawing out of the living space. Insulation alone does not eliminate the drafts. And don’t ignore the potential heat leakage at attic access doors. Insulation should be firmly attached to the top of hatches and weatherstripping should be installed around the bottom.

All heating ducts and vent pipes should be wrapped properly with insulation to keep the heat within them from escaping. Also, kitchen and bath exhaust ducts should be wrapped and vented directly to the atmosphere. The humidity that is introduced into the attic is a problem unto itself.

Ventilation is probably the most overlooked factor in the equation. To achieve "cold roof", ambient air should wash over the underside of the roof deck. Generally speaking, a 1:150 ratio of free ventilation area to attic area (or about 1 square inch of unimpeded ventilation for every square foot of ceiling) is required. The vents should be split equally between the lower and the upper areas of the roof to promote air flow. More ventilation will be needed if the upper vents are not at least 3 feet above the lower (as measured vertically, not along the slope of the roof). The optimum solution involves a continuous ridge vent across the top of the roof combined with continuous venting under the eaves. Where under-eaves vents are not practical or desirable, combined drip-edge/venting systems are available. (They slip under the shingles and behind the gutters.) Ridge and eaves vents can be retrofitted without re-roofing, and should run the length of the roof, even if not required by shingle warranties. This allows the entire roof to benefit from the cooler air from below drawing behind the warmer air exiting at the peak.

Care should be taken in the attic to keep insulation at the top plate of the exterior walls from coming in contact with the underside of the roof deck. This blocks the soffit ventilation and creates a thermal bridge to the roof surface. Cardboard, plastic, and foam channels are available to ensure that these areas remain clear. And do not overlook the obvious: soffit vents that have been painted shut are useless!

Cathedral ceilings deserve special mention. There is little ventilation clearance designed into these systems. Careless installation of insulation at any point above the ceiling may have resulted in blockage of the ventilation space. Particularly where recessed lighting has been used, as there is little space between the cans and the roof deck. Better that recessed lighting were left out of cathedral ceilings also because of the additional heat that it introduces between the trusses. Recall that warmer-than-outside temperatures directly under the roof deck initialize the ice dam process.

Due to differences in architecture, it is impossible to look at all of the problems and solutions in this space. Have your maintenance contractor assess your individual situation with you. If a dam should occur, do not rush to remove the snow and ice: this could severely damage the shingles and pose a serious personal danger.

And don’t forget to have your gutters cleaned before La Nina sambas your way.