Barn-style gambrel roofs have two, usually-symmetrical sides, with gables at each end. This style is simple to construct, and maximizes the potential usable attic space. Mansard roofs, by comparison, are gambrel styled on all four sides. We discuss ice damming on gambrel roofs, and the best way to counter this.
Principles of Gambrel Roof Design
The gambrel roofs we see on farmyard barns, and houses designed with generous attic space, are near-vertically-sloped at their lower ends. However, from shoulder height upwards the roof takes on a gentler angle creating a sloping ceiling in the roof space.
Gambrel design therefore offers several advantages, which are worth considering when constructing or purchasing a home.
- Maximised headroom and usable space in the upper level.
- A double storey without the added height of a roof on top.
- Shorter span of rafters compared to a conventional building.
- A lower construction cost without sacrificing living space.
This style of construction became popular in North America, after New York City introduced the first building code in the United States. This favoured mansel roofs atop tall buildings, because they admitted more light and air to the streets below.
Their twin-pitched roof style spread to suburban areas where nearby residential land was at a premium. This ‘old colonial style’ became part of North American vernacular. Some folk believe there is nothing to beat a twin-pitched roof with dormer windows peeking through the sides.
The Big Issue: Ice Damming on Gambrel Roofs
Ice dam potential begins when snow packs on a gently sloping roof. All is good as long as it stays there, for it reduces the likelihood of the attic losing heat, and cooling any non-insulated space below.
However, if the attic becomes too warm then some of the snow can melt, and slide down the sloping roof. But when it encounters cooler space on the edge of the roof, it can freeze again creating a ‘dam wall’.
Preparing a gambrel attic for occupation involves ‘boxing in’ the sides and ceiling to create near-rectangular living space. The air in the resulting ‘side attics’ remains relatively warm from the low winter sun. If this air escapes into the ‘ceiling attic’ above, it can warm the snow on the roof.
This can lead to ice damming on gambrel roofs that can leak past the underlying insulation and damage walls and ceilings below. Upper attic venting may alleviate the situation. However, at Valiant Exteriors we believe that insulating and sealing the upper attic is more likely to succeed.
More Information
Three Flat Roof Drainage Options
Attic Insulation Baffles and Their Importance
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