Why a Rain and Ice Shield is Necessary on a Roof

ice dam calgary — Ice Dam Formed on Roof Edge

Roofs are generally-speaking intended to control the flow of water traveling down the slope. Hence the lower edge of an upper tile or sheet overlaps the top of the next tile or sheet to a necessary extent.

However the system does not work as well if a gust of wind drives the water upwards. That’s because moisture can travel up the inside of the lower edge of the upper tile or sheet, and drip down into the roof space.

The Added Complications of Ice Damming

The air inside the roof space of a heated building is generally warmer than the atmosphere outside in winter. This is due to a combination of some warmth finding its way past attic insulation from the living space below. The area under the roof is also largely shielded from icy blasts when wind gusts strongly.

Fitting vents to soffits under eaves encourages warm attic air to find its way through to outside. This helps balance the outside roof temperature from the rooftop all the way down to the eaves troughs.

However, this can prove ineffective during extremely cold weather such as the recent polar vortex shifting south and causing air temperatures as low as -30C. The roof above the eaves may remain frozen in daytime, while the sun causes ice melt higher up the roof. When this happens, the melt flowing down the roof re-freezes over the colder eave surface.

As a result, the water flowing down above dams up against the ice barrier. It can then find its way through the overlapping tiles or sheets in much the same way as a driving rain might exploit them. The result is the same: high humidity in the roof with the possibility of moisture dripping onto the ceiling.

How a Rain and Ice Shield Barrier Helps Prevent This

The building codes provide for rain and ice shields in areas with a history of ice dams forming over eaves and causing water backup. These replace normal underlayment and comprise either (a) at least two layers of underlayment cemented together or (b) a self-adhering polymer modified bitumen sheet.

This normally extends from the lowest point on the roof to two feet inside the outer barrier wall. Flashing eaves beneath tiles and sheets in this way can help prevent ice dams forming on lower roofs of heated buildings, and causing melt to find a way inside.

Valiant Exteriors recommends you find an experienced contractor because fitting rain and ice shields is a specialist task requiring extensive experience.