Does Seattle Need More Shade?

Excessively tall buildings cast their neighbors into a lot more shade!  Is that “environmentally friendly” in our far northern climate?

At noon on the winter solstice, the length of a shadow cast by a building will be 2.9 times the height of the building, assuming flat ground.  If the average street frontage of a typical Seattle building lot is ~50 feet, then:

  • A 30 foot building will cast a shadow 87 feet long (i.e. across ~1.7 adjacent lots).
  • A 48 foot building will cast a shadow 139 feet long (i.e. across ~2.8 adjacent lots, or more than half a city block).
At 2.5 hours on either side of solar noon on the winter solstice, the shadow cast is 5.4 times the height of the building.  Using the assumptions described above, then:
  • A 30 foot building will cast a shadow 162 feet long (i.e. across ~3.2 adjacent lots).
  • A 48 foot building will cast a shadow 259 feet long (i.e. across ~5.2 adjacent lots, or nearly one city block).

Note that rooftop structures are becoming more common since the 2010 legislation, because they are not counted in the total floor area ratio of the building nor as part of the building’s allowed height.  Effectively the city treats rooftop structures as if they don’t exist; and yet, these can be quite tall and add considerably to the shadow that is cast.

    

This picture of the Sheffield building in Capitol Hill shows how much sunlight it gets in the early morning on a December day.  Much of this sunlight will be lost thanks to the shadow cast by a new  5 story development that is scheduled to be built at 133 18th Ave. East (Ruth Court) in the coming year.

This picture of the Sheffield building in Capitol Hill shows how much sunlight it gets in the early morning on a December day. Much of this sunlight will be lost thanks to the shadow cast by a new 5 story development that is scheduled to be built at 133 18th Ave. East (Ruth Court) in the coming year.