How an Ice Dam is Created
An ice dam forms when heat escapes from interior spaces to an unconditioned attic space, causing rooftop snow to melt. Meltwater sheds from the area of escaping heat towards eaves and or gutters, where colder temperatures cause it to refreeze, creating a dam.
As snow continues to melt, the ice dam prevents additional water from properly draining from the roof, allowing it to pool. The pooling water is then able to work its way behind shingled roofing materials by capillary action, damaging roof sheathing, joists, and other roofing materials.
How to Prevent Ice Dams
Several aftermarket products treat symptoms of ice damming, such as heat-tracing elements for gutters or roof edges, which create a directed path for meltwater to flow. Service companies will also physically remove built-up ice.
The most effective long-term approach to prevent damage from ice damming, however, is multi-faceted: preventing heat from reaching the roof plane; ventilating and insulating attic spaces to maintain a uniformly cold temperature in the roof plane; and installing self-adhering membranes at eaves.
Properly insulating the overall ceiling plane at roof level is fairly straightforward, and additional insulation can usually be added easily. Special attention should be paid, however, to interfaces between exterior building walls and the roof. These locations are particularly challenging because they are small, narrow, and hard to reach, and because the number of intersecting building elements reduces the area available for installation of insulating materials.
To be most effective, insulation must be properly installed. If too loose, warm air may may migrate from below living spaces. If packed too tightly, insulation fibers become compressed. It is also important to extend the insulation over the top plate of the wall to insulate the wall cavities below, and additional insulation should be installed throughout as space permits.
Installation of corrugated insulation baffles is also recommended. These baffles are installed at the underside of the roof sheathing, typically at the area where exterior wall construction meets roof framing, essentially separating insulation from the roof deck. Insulation baffles work in conjunction with soffit and attic venting to create a continuous path for air circulation below the roof deck, keeping the roof plane uniformly cold.
The final strategy employs a self-adhering polyurethane or bituminous membrane, commonly referred to as an ice-and-water shield, to compensate for the inherent difficulty of insulating attics at the wall/roof interface, and providing a waterproof layer to protect wood decking and framing from any moisture that might migrate behind the finish roofing and roof paper.
The ice-and-water shield is installed on the surface of the roof deck, prior to roofing paper and finished roofing material, at building eaves and extending at least 2’-0” beyond the roof-wall intersection.
When taken together, the measures described above produce a highly effective defense against wintertime weather-related roof damage.
We are proud to introduce a new team member in our San Francisco office!
Ashley A. McDuffee, PE, REWC, LEED AP BD+C brings a strong background in building enclosure inspection, design and investigation. She has a Master of Science in Historic Preservation from the University of Pennsylvania and a Bachelor of Science in Civil & Environmental Engineering from Bucknell University.
Line Item Closeup: Winter Conditions
In the midst of one of the colder and snowier Midwestern winters in recent memory, our Chicago office has encountered some unusual field adaptations intended to cope with weather conditions in the course of construction. A few examples:
At a payment-request meeting, a general contractor produced a $54,000.00 invoice for propane, used as a fuel source for temporary heat for the single month of December 2013.
A developer in central Illinois considered replacing specified asphalt pavement with concrete in order to ensure completion of work and to obtain occupancy permits before spring. (Asphalt batch plants in the Midwest close by Thanksgiving and don't reopen until spring.)
A contractor delayed installation of gypsum wallboard and finish work until temporary heat could be supplied to the building interior.
The Winter Conditions line item is generally dedicated to the financial impact of continuing construction work in weather otherwise considered unsuitable. Such costs might include, but not be limited to, the following:
- • Providing temporary heat using propane, electricity, or another resource
- • Installing temporary enclosures
- • Producing material admixtures
- • Providing alternates and substitutions
Fuel costs for temporary heat are difficult to predict in advance of the season, but are unavoidable when cold weather sets in. With an owner's cooperation, however, such costs may be mitigated slightly if base-building mechanical systems have been installed and are operational.
Temporary enclosures could range from simply infilling window openings to constructing temporary structures that enclose weather-sensitive exterior work, such as masonry. Depending on the enclosure methods chosen, costs will vary significantly.
Admixtures introduce chemicals designed to improve cold weather performance to certain standard materials, typically concrete and mortar. Chemical admixtures introduced during concrete mixing, for example, can improve concrete resistance to freeze while offering higher initial compressive strength.
Alternates and substitutions using materials more suitable to cold-weather installation sometimes become necessary as temperatures fall. Shifting to concrete paving instead of asphalt, as in the above example, might permit an otherwise behind-schedule project to finish on time, since concrete, unlike asphalt, is generally produced year round.