Living Roof / Battery Shed

Green Building Systems

The summer we built our house we also built a small straw bale outbuilding to house our batteries, inverter and controller. In 2006 we demolished this structure to make room for the new apartment/office/garage but we learned some valuable lessons from our little straw bale shed.

We wanted to store the electrical system outside our house because the batteries contain toxic lead and acids, and the inverter gives off a high level of EMFs. To store the batteries in an unheated outbuilding, we needed to find a way to prevent the batteries from freezing (if the batteries get too cold, their ability to hold a charge is greatly reduced, and if they stay frozen too long they can be permanently ruined). The straw bale walls, coupled with an insulating living roof, provided some shelter against the cold, though not enough to prevent the batteries from freezing in the winter. Therefore, we enclosed them in a straw bale box. The heat generated by the batteries is contained within this box, and this has proven sufficient to prevent freezing; we did not have any problems with our batteries holding their charge until they were about 10 years old and starting to degenerate.

One objective in building the battery shed was to experiment with a variety of alternative building styles, including a circular structure, a living roof, and a minimal foundation.

With the foundation, we wanted to see just how minimal we could go. The quicker and cheaper the structure, the better it could serve as a model for temporary shelters, like those found in fairs or those required for emergency housing. Therefore, the battery shed foundation was constructed by digging out some earth, dumping in some gravel, and covering the gravel with boards of naturally moisture resistant black locust.

The main structure consisted of circular straw bale walls. There was no frame to support the roof, so the walls were load-bearing. Because the battery shed was made in a weekend as part of a workshop, it wasn’t sturdy enough to support the roof without the addition of metal rebar for support. Unfortunately, the circular shape, though intriguing, has not proven to be very useful for electrical and other equipment storage. A rectangular shape would have allowed for more storage per square foot.

For the roof, we wanted to try a living roof.  Living roofs are wonderful insulators that provide a space on which plants and animals can grow. We were concerned about the roof leaking, especially during the winter months, so we experimented with a sloped living roof that could shed snow and moisture. Traditional living roofs in Europe and the U.S. tend to be flat. Only in Greenland and Iceland do living roofs tend to have a fairly steep pitch.

To help prevent leaking, most living roofs use a waterproof petroleum-based liner. As a natural alternative, we used bentonite clay, which swells to several times its normal size when wet. A layer of this clay serves as a self-sealing membrane; a hole pricked in it will quickly close itself as water causes the clay around the hole to swell.

The problems with our roof began even before it was built. The bentonite clay came sandwich between 2 layers of plastic webbing that’s very slippery, and so the living material slipped right off. Therefore, Juliet wove a layer of saplings into an upside down basket,and placed this basket over the clay to provide textured pockets to stuff dirt and straw into. This proved successful at first; over time the field plants seeded themselves on the roof and their annual breakdown created a rich and heavy compost that fostered further growth. However, after about 5 years the soil and living material began to slip off the roof, revealing the black plastic surface over the bentonite clay. We suspect that some combination of heavy spring rains, the degradation of the sapling basket, the weight of the accumulating soil, and the pitch of the roof were to blame. After a while the roof began to leak.

Overall, the building did not hold up very well. The walls cracked and sank perhaps because of the weight of the living roof, perhaps because of the instability of the foundation, perhaps in part because of both.  While building a “temporary” structure is fun at the time, it really does make sense to do the work to make things water and airtight. 

The battery shed was demolished in 2006 to make room our new straw bale structure. At the present time (Winter of 2008) our new building is still under construction, but our love for living roofs continues and we have constructed the roof of the connector between the two buildings to receive a living roof in the Spring of 2009.  We have also constructed the roof of the carport to hold a living roof.  These two living roofs will function in slightly different ways and will give us an opportunity to learn more about maintaining a living roof in our moist, snowy Vermont climate.


The remains of the straw bale battery shed are breaking down like a pile of compost. Truly natural building is also biodegradable, cutting down on the tremendous amount of construction debris that goes into our landfills.

The connector roof is pitched to the North and sheds moisture and snow to the north.  The 40 foot long connector under the roof is unheated but extensively glazed on the South  (ie. we have a lot of windows), the space (still unfinished) during the day heats up to at least 40 degrees above the outside temperature.  We have insulated the ceiling of the connector with blown in cellulose and the underside of the waterproof rubber roof is insulated with foam board.  In the Spring of 2009 an 8” “pocket” will receive soil and plants.  This is how a roof on a home might be constructed.

The living roof over the carport is part of a large deck off the south side of the new structure.  The deck will have locust boards as well as a large square of grass filled living roof.  This  4” deep living roof is pitched slightly to the South and  has a drain in it as well as a built in soaker hose.  This is the type of living roof that might be built on top of an urban building.  We are looking forward to seeing the results of our living roof experiments.

  PROMOTING SUSTAINABLE DESIGN TECHNOLOGIES
  VERMONT, USA info (at) earthsweethome.com