What is an ice dam?

            An ice dam is a ridge of ice that forms on the eaves during the winter.  This ridge of ice can become so high that it forms a puddle on top of your roof.  Even though there are good shingles on the roof, this puddle can get deep enough to “back up” over the top of a shingle.  Remember each asphalt shingle is only 13 or 14” tall, and they do not seal together to form a sheet.  Hence, when water puddles up deep enough it’ll “leak” into the home.

            What causes an ice dam?  Many things.  Basically, if there is insufficient insulation in your ceiling the roof stays warm enough to melt any snow that falls.  This water drains down towards the edge of the roof.  Since your roof extends over the edge of your house (to form the eave), you can understand that the edge of the roof is colder since there is no heated space below it.  Therefore, as the water runs down the roof, it reaches the eave and freezes.  Potential ice dam problems often start as a row of icicles.  Slowly but surely, this repeated action forms a tall ridge of ice until it occurs over the heated portion of the roof.  Here it isn’t cold enough for it to freeze, so the puddle just gets deeper and deeper.

            Lack of insulation is the primary cause.  Typically, over a wall the insulation is the thinnest or the most compressed, and hence it has a lower R-value.  Cathedral ceilings are a big cause of ice dams, because they inherently have warmer roof temperatures.  This leads to another primary cause of ice dams, ventilation, or lack thereof.  No matter how good your insulation is, there is residual heat that dissipates through it.  If this heat isn’t dealt with, you’ll have an ice dam.   The best way to remove this residual heat is by building in proper ventilation, which includes soffit vents, ridge vents and possibly gable vents.   You also need a free air space for these to work properly.  Typically rafter mates (proper vents) are used to maintain this space from soffit to ridge.   Once you have all of these features, the residual heat naturally rises up the roof slope until it exits the roof, hence creating a constant, passive convection current.  The only thing to note is that if there is a lot of snow on your roof, it creates natural insulation AND blocks the ridge vent.  Hence the residual heat lasts longer, causing more problems.  Adequate gable vents can combat this problem.

            Another factor that shouldn’t be overlooked is inside humidity.  Oftentimes builders will install kraft paper covered insulation on the ceiling.  This is seen as okay because kraft paper is a vapor retarder, not a vapor barrier.  Hence, indoor humidity can pass through it, enter the ventilated cavity and be removed form the house.  This is seen as better than allowing it into a wall cavity, because there it can’t escape without ventilation.  But you have to remember the force that drives this activity.  Cold air cannot hold much moisture, but warm inside air can.  This higher moisture content wants to diffuse into the lower moisture content to achieve equilibrium.  This creates a “pressure gradient” of sorts that forces hot, moisture laden air out of a house during the winter.  If you do not have exhaust fans in your shower room, for instance, you are making this predicament much worse. 

            Do gutters cause the problem?  No, but they don’t help.  Oftentimes the weight of an ice dam can rip them off your roof.

            Do skylights cause the problem?  Yes, at least in that immediate area.  Remember that there is a lot of heat loss through glass, so any snow that falls on the glass will melt, starting the whole process.  To top it off a skylight typically blocks the important ventilation cavity, compounding the problem.   We do not recommend any skylights on any structure that will be heated in this climate.  Period.

            What can be done?  If you have an ice dam already, there isn’t an easy fix.  You can install a heat tape in a zigzag pattern on your eave, but this is a problematic band-aid to say the least.  If you are planning a new roof, you can use an “ice and water” shield such as “Grace” underneath the shingles.  This is a 3’ wide product that adheres directly to the roof sheathing.  Manufacturers and NYS code now advocate that it should be used on the eaves of a building such that it extends two feet inside the inside plane of the wall.  It should also be used on all valleys and low slope roofs (less than 3/12).  This product differs from normal felt paper because it is a self-sealing, continuous membrane.  This product can prevent leaks, but it won’t prevent ice dams from forming.

            Another solution is to use ice pans.  They are typically aluminum or copper “pans” about 30” square.  They are installed in an overlapping manner to create a standing-seam appearance. 

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