Now that New York is installing a lot of green roofs the main question is how much weight a green roof has. I had my ideas but two professors from Virginia Poly did some great research into the subject. Their findings were published in the Fall 2008 Journal of Green Building, which I’m only now getting to as I slog through the stacks of publications I read.

Their numbers range from1.6 inches to more than 20 inches of green roof. Most Brooklyn brownstones can’t handle more than 8 inches and that is if there has been some serious structural work. Larger Manhattan buildings usually have at least a foot of concrete as their roofing system and thus can hold up to 20 inches but again you have to get an engineer involved to be sure.

Here they are:
1.6 inches – 12psf (pounds per square foot)
2.4 – 16
4 – 27
6 – 41
8 – 54
10 – 68
20 – 206

So your average brownstone green roof of 4 inches has a green roof dead load of 27 pounds per square foot. Thats an extra 21,600 pounds over a 800 square foot roof (20×40).

Another factor they looked into is the sound qualities of green roofs. The fact that a green roof stops sound is a big selling point. But how much?

Here are their numbers.

2 inches of soil – 16 STC (sound transmission class) – Poor Acoustic Rating
3 – 31 – Fair
5 – 40 – Good
7 – 44 – very good
8 – 46 – Excellent

A typical interior wall of drywall and studs with no insulation is STC 33 and you can hear people talking on the other side. So a three inch roof is pretty useless for sound. A 5 inch roof reduces the voices to a muffled mumble. At 7 inches you can still hear voices but it is low. At 8 inches the voices start to fade away completely.