So, with two winters in a row where we have encountered more ice and single digit temperatures than normal, I have been thinking about ice dams a lot. And visiting clients who have them. A lot.
For those who are not sure, an ice dam occurs during periods of thawing and freezing after snow or ice has fallen and stuck to your building. They can occur in clogged downspouts, at angle changes on complex roof lines, and often in box gutters. The box gutter begins filling up with the melted runoff from your roof as the sun heats the building and temperatures rise. Then, as the sun goes down and temperatures drop, the runoff freezes again and begins to expand. This forces ice under your roofing material, through window sills, onto brick walls, and just about anything else that ice can stick to. When this happens, the warm air from your house slowly melt the ice that it is exposed to, causing puddling water wherever the warm air i coming from. It can be extremely difficult to eliminate or identify the source of the small amount of warm air from your house.
The preparation for ice dams is to make sure your roofing has been installed properly, with a full application of underlayment under your roof material. This underlayment should extend from the roof edge to the ridge. In some cases, it is appropriate to install a thicker, self-adhesive 'ice and water' guard under your roofing along edges where snow and ice accumulation is probable. this is especially true of box gutters- snow and ice ALWAYS accumulate in box gutters and hanging gutters. Often, the warm air from the attic circulating in the cornice under the box gutters causes the ice to melt. This same warm air also has a wicking effect, leading the melted ice towards susceptible points in your roofing.
There is no fail safe method for preventing and Following a simple checklist can help to eliminate ice dams during these periods:
*Make sure there are not large accumulations of debris in your box gutters
*Make sure your drainage lines are clear and that there are not clogs underground if you tie into a daylight system
*Make sure your box gutter seams are in good repair; if your home is at a stage where caulk is used to maintain the box gutters, be vigilant about it
*make sure the edge of the roof deposits runoff into the box gutter at a point at least 2 inches higher than the front edge of the gutter; this allows the ice an escape route other than going up your roof
*Install as much insulation as you can to minimize drafts tot he roof edge; this will prevent warm air from drawing that ice inside
*Inspect the conditions of chimney flashings and other flashings around other roof penetrations when you have your gutters cleaned
If any of these items cannot be accomplished by you or if you're not sure of the condition of your gutters this winter, it is recommended that you call a qualified tradesmen before next winter to get on the right track. One ice dam can make an awful mess of ceilings and walls- a lesson no one wants to learn twice.