Dreaming My New Home, cont.

A Green Roof. Green roofs and living roofs have taken on a bit of a buzz-word populism among televised home-improvement shows (HGTV) and the budding green-building movement overall. I'd like to draw a minor distinction between a couple "green roof" strategies that should help define what I mean by Green Roof.

Most commonly, green roofs used in commercial applications - like the roof of the new Ford factory - and green residential developments are lightweight applications using a very thin layer of soil and utilizing a manufactured drainage mat made of corrugated or dimpled plastic that provides a place for the little plants to take root. The plants must be extremely hardy and drought resistant (read: non-native) to survive the rooftop environment. These roofs provide runoff remediation (less hardscape in an urban environment), some minor passive cooling benefits to the building, return O2 to the air and are nearly maintenance free. In addition, the building roof support system need not be over-engineered or rebuilt because these thin mats are comparable in weight to more standard gravel over asphalt flat roofing materials.

On the opposite end of the spectrum are folks who build earth-bermed or underground homes. These buildings often have a soil depth of 20-24" on the roof and are blended into the surrounding landscape. That much soil presents a real professional-grade engineering challenge to support! When the soil gets wet and then it snows, think 500lbs per square foot live load. Pre-stressed concrete panels and beams or steel girders are really the only materials suitable for this application. Very expensive.

In my particular case; there are a couple things I'm trying to gain from the use of a green roof:

Insulation value. OK, so we can all agree that earth itself is not a great insulator. But it is a great thermal mass and tends to help damp out wild temperature variations even a few inches below the surface. However, to keep the house warm in the winter, a sufficient layer of insulation will need to exist between the home interior and the soil. Additionally, earth roofs retain snow cover. And snow IS a comparatively good insulator. Fluffy dry snow accounts for roughly R1 per inch. So, 18 inches of snow is an additional R18 on the roof when you need it most!

Passive cooling. Soil deep enough to retain some moisture in the summer months can act as a whole-house sized evaporative cooling system. Moderately deep soil also ought to be able to keep the attic area from overheating in the sun - imagine the cool you'd find 8" down in shaded soil on a summer day...

Aesthetic harmony. A soil covering will help keep a quiet house and (if properly built) should require absolutely no maintenance in my lifetime. Plus, it'd look really cool.

So, my ideal green roof is one with roughly 6-8" of soil supporting a growing matrix of native grasses, wildflowers and other shallow-root plants.

To support this considerable load will definitely take some planning. With responsible spans, I believe a roof substrate of 2x6 tongue-in-groove planking over frequent joists and girts ought to do the trick. On top of this plank surface will sit alternating layers of waterproofing membrane and closed-cell (doesn't absorb water) rigid insulation. Next, a gravel drainage layer allowing water leaving the soil to drain away to the edge of the home to daylight. Next a filtration layer of fiber matting to prevent the soil from homogenizing with the underlying gravel. Lastly, the actual soil layer on top of it all. In my minds eye, I picture all these materials retained on the rooftop using Trex-style decking as batten boards - this material is not prone to rot when in contact with the soil.