Eating your roof

Producing food is a critical need for any community project. Your first thought might be traditional gardens or farm plots, neatly ordered into precise rows, tucked away on a back corner of your yards. That’s certainly one vision of personal food production, but is far from the most efficient. Why not let your food production aid in other areas of your design?

A key point of many advocates of sustainable design is using a green roof because it saves energy (and money) you might otherwise use for heating and cooling. However, many designers of green roofs often miss a key point of overall efficiency, which is using edible plants for green roofing. If you’re to go to all the trouble of planting greenery on your building, why not plant stuff you can eat?

Of course, installing a green roof isn’t as simple as throwing some dirt and seedlings on your roof – it takes careful design to understand how a green roof affects your structure, internal temperature, rainwater runoff, and so forth. So let’s practice running some numbers. (Note: equation key is at the end of the post.)

Heat flux is a measure of the rate of heat energy over an area. It is the primary measure of energy losses and gains for home construction. For a traditional shingle roof in the Midwest, summer heat flux is 2.37 W/m^2 compared to 1.57 W/m^2 for a green roof, which is an improvement of 33% (summer heat flux is used here because in winter, everything’s white!). For a simplified calculation, let’s assume a flat roof for a square house 10 meters x 10 meters.

For asphalt shingles, H = (2.37)(100) = 237 W
For a green roof, H = (1.57)(100) = 157 W

The net energy savings over the summer months (June-July) can be calculated as follows.

E = (237 – 157W)*(90 days)*(24 h/day) = 172.8 kWh

This can help drastically shrink your estimates for energy and insulation requirements. (There is also an insulation effect from the soil I will roll into another post targeting earthen insulation.)

Using native soils, the typical loading on a roof could be 2300 – 7000 Pa, as opposed to under 230 Pa for a shingled roof. This is a significant weight addition and many buildings need structural reinforcement before they can support green roofs. A discussion on calculating structural requirements is upcoming, so I’ll save it for that post, but keep this in mind during your brainstorming.

Also, many green roof projects today use plastic barriers to prevent water intrusion into the building. One sustainable post-peak alternative might be using clay instead, but remember this would add significant weight and require greater roof reinforcement.

Green roofs may seem like a lot of work, but consider this: most shingles are made from asphalt, which is made from oil. As oil prices and scarcity increase, how will you protect your roof? A well-planned green roof seems like a good solution. (But there are others…)

Key:
W=watt
m^2=square meter
H = heat flux
kWh = kilowatt hours
Pa = Pascals (unit of pressure)

References: www.greenroofs.com
ASHRAE Journal