Roof Overhang

A roof overhang is the part of a roof that runs out past the wall beneath it. On a sun-facing wall it can be sized to block the sun when it rides high, in the hot half of the year, and let it through when it is low, in the cold half. The overhang makes neither heat nor coolth; it only decides when the window and the heavy walls behind it are allowed to work.

Its size is a proportion, not a fixed measurement, because the sun rides at a different height at every latitude. The depth of the projection and the drop from the top of the window to its underside are both multiples of the window's own height - near 40 degrees of latitude, about 0.53 and 0.32 of it - and both are set level and plumb, not measured along the roof slope. That second dimension decides where the line between sun and shade falls, and it is the one usually left to chance. The wall must face the sun's path as well: facing sunrise or sunset the sun arrives low all year, so a projection deep enough to shade in summer shades in winter too.

A fixed overhang is symmetrical about midsummer, treating the two dates either side alike - but air temperature lags the sun by about a season, so the sun it welcomes in late winter is the same sun it welcomes in late summer, when the building is already hot. Sized for midsummer noon alone it over-shades a cold climate and under-shades a hot one. A deep overhang also dims the room on overcast days, best answered with pale interior surfaces. The real risk is wind: an overhang works like a wing, and uplift tears rafters off the wall plate. At a gable end simple framing suits about 200 to 300 mm (8 to 12 in); deeper than that it must reach back to the second rafter and be strapped down, and guidance for high-wind areas caps a gable end at 200 mm (8 in). Upkeep is checking straps and fixings after a storm, keeping the soffit closed against driven rain and the gutter clear.