What slope changes

Steeper roofs drain quickly and tend to hold less snow, since gravity helps it slide or settle and melt off. Lower slopes drain more slowly and rely more on the covering and underlayment to keep water out. Each roofing material carries a minimum slope below which the manufacturer no longer warrants standard performance.

Slope categories in practice

  • Steep slope: sheds snow and water readily; suits shingles, slate, tile, and metal.
  • Moderate slope: the common range for residential shingle roofs.
  • Low slope: needs membranes or seamed metal and careful drainage design.

The freeze-thaw mechanism

Water expands as it turns to ice. When meltwater seeps into a hairline crack, a lifted shingle edge, or a gap in flashing and then freezes overnight, it pries the opening slightly wider. In a climate that crosses the freezing point many times each winter, that cycle repeats often, and small defects grow into leaks.

Why eaves are vulnerable

The overhang sits beyond the warm interior, so it stays colder than the rest of the roof. Meltwater from above refreezes there, building the ice dams and icicles that signal heat loss higher up the slope.

Detailing that resists freeze-thaw

  1. Eave and valley membrane. A self-adhering layer seals around fasteners and blocks water that backs up behind ice.
  2. Sound flashing. Metal flashing at walls, chimneys, and penetrations directs water onto the surface rather than into the joint.
  3. Ventilation and insulation. Keeping the roof deck cold limits the melting that feeds ice dams.
  4. Clear drainage. Gutters and downspouts that stay clear move meltwater away from the eaves.

Material weight and structure

Heavy coverings such as slate and clay tile last a very long time and resist weathering, but they require a structure designed to carry their weight in addition to snow load. That structural demand, together with their steep-slope preference, is why they appear less often than shingles and metal on ordinary homes.

Red clay roof tiles
Clay roof tiles. Source: Wikimedia Commons.
Authoritative reference

Slope, snow load, and ice-protection requirements are addressed in the codes published by the National Research Council Canada.