You've seen it many times - a mass of heavy ice collecting at the eaves of a roof. But did you ever wonder why these hazardous (and dangerous) ice masses and icicles form? The answer is simple: Trapped heat in your attic melts the snow on your roof. The melting snow then trickles downward to the edges of the roof and re-freezes, over and over again, continually adding more ice mass, and eventually results in extreme havoc to the structure of your roof. You might not know the extent of the damage until it is way too late: Damage such as warping and detachment of eave troughs, fascia board warping, roof wood rot, and melting ice leaking into your house resulting in drywall and plaster stains. Why does this happen? You might be surprised by the answer. The power of ice is tremendous. When water freezes it expands and creates a huge force that pushes against anything in its way. This unstoppable force can bend steel like bubble gum. Imagine this huge level of pressure pushing against soft materials such as wood, aluminum, asphalt shingles, or bricks, concrete, and stone walls (all easy prey for ice dams). Clearly, the damage could be enormous.
What is the answer? Here it is: The only lasting solution to prevent roof ice damming is proper roof ventilation. Why? Because proper roof ventilation removes the trapped heat in your attic (the very heat that causes ice dams) and studies prove that a cool attic during winter stops the thaw/re-freeze cycle and thus stops the melting snow from re-freezing at the roof edges. But how does roof ventilation specifically prevent ice damming? The answer is to create an attic temperature that is close or the same to the temperature outside of the attic using specific roof/attic ventilation procedures such as soffit roof ventilation and ridge roof ventilation. Let's expand on these two types of roof ventilation. Soffit ridge ventilation is a system of openings along the perimeter of your roof. These openings are referred to as intake vents. Their purpose is to allow air to easily enter the underside of your roof which will then travel skyward to the top of your roof directly beneath the roof boards. Ridge ventilation (at the top of your roof) will now complete the process. Ridge ventilation is a continuous vent that is installed along the entire ridge (or top) of your roof, referred to as exhaust vents.
Using the wind, ridge ventilation pulls air out of the attic using an energy-free power source, namely Mother Nature. Wind creates a negative pressure that effectively pulls a constant stream of new air into the soffit vents, along the entire surface of the attic roof boards, and is then expelled out through the ridge vents. The outcome is substantial. With fresh air constantly being pumped throughout your roof's attic spaces, any trapped heat is removed. When this heat is removed, the temperature of your attic becomes close or equal to the outside temperature. The result is no melting snow that will dribble down and re-freeze at the eaves of your roof. As a final word of advice, be sure that the ridge vent is baffled. A baffle is simply a small curve on the ridge vent itself that creates the negative pressure that sucks the trapped heat out of your attic much more efficiently than ridge ventilation without baffles. Without this baffle, the effect will be minimal. Lastly, the ratio of soffit and ridge ventilation should be 50/50 for optimal results, meaning 50% soffit intake, and 50% ridge exhaust.
From: William Woolsey, EzineArticles.com