Roof harvested rainwater is the primary (and often by design the only) source of water in an Earthship. One of the defining features of Earthships is therefore a sloped roof designed to collect rainwater. Water is accumulated in large underground (or sometimes indoors) cisterns, passed through a series of gradually refined filters and is then pressurized with a relatively small, simple and low-energy-consuming pump. This entire system can be complicated and expensive and is an all or nothing deal. There is no point in having a rainwater harvesting roof if you can’t store the water. There is no point in storing the water if you don’t or can’t use it.
We are questioning including this feature of Earthships in our plans and are considering in its place a living roof (earth and plant cover) as a preferred solution.
The primary function of a roof is shelter. It is so obvious that it is often compromised and overlooked. Most modern roof systems are actually very poor when it comes to shelter … they require maintenance and too often complete overhauling. Our architect took us on a day-trip which included very old houses with thatched roofs (once a common roofing practice, today a rare art) – If I recall correctly this roof was over 80 years old,s built of a natural and insulating material (straw) and can outlast the structure beneath it. Most modern roofs don’t come anywhere need this kind of longevity and require major maintenance every 5 to 10 years.
Earthships (especiall Global Model) seem to most frequently use something called “Propanel” roofing … which is basically a sheet metal roof usually made of steel with various protective (and rainwater safe) coatings. Some Propanel roofing even comes with 45 or 50 year warranties which is impressive. But the sheet-metal itself is just one part of the roof and even if, for arguments sake, they were to last 50 years, the longevity of the roof depends on the behavior of all the other roof elements.
The roof is subjected to some of the fiercest forces of nature – moisture, temperature, wind, etc. Assuming it is installed well (won’t blow off in the wind) and is properly insulated against moisture (won’t let moisture in and won’t trap moisture between its layers) it is left to the attacks of temperature. Here in Romania that includes a very hot summer and a freezing cold winter but most importantly it includes drastic temperature variations over a short period of time. Hot summer days can be followed by cool nights and both fall and spring bring intense freeze-thaw cycles.
Even though the sheet metal may be able to withstand these changes and variations it does not isolate the inner roof layers from them. What more, it may actually amplify them – it will reach much higher temperatures then the ambient air temperature in the summer and will freeze very fast in the winter and it will conduct those amplified variations to the roof layers beneath it. These layers will decay BECAUSE of the behavior of the metal roofing.
The metal roofing may last a long time but may contribute to destruction of the roof many times during its lifespan. A roof that needs to be fixed every 5 or 10 years is, in my mind, a failed roof. Or, put another way, I aspire for a roof I can forget about for the rest of my life.
The second most important function of a roof is insulation. Since warm air rises from below (inside the house) and falls from above (outside the house) the roof is the most vulnerable escape of heat.
This insulation can be achieved by:
- Brute force – industrial insulation solutions – such as the insulation suggested and often used in Earthships.
- Natural Materials – materials such as sheep’s wool or hemp can be used as insulation when properly prepared/treated.
- Nature itself – a living roof offers (in our climate) three important layers of insulation: earth, plants and snow.
Of the three options I trust nature more then the others because it is a dynamic system that adapts to climate conditions:
- Earth – though it is a poor insulator it has good thermal mass. As such, it absorbs ambient changes and dampens the effects of those changes from the layers underneath. In the summer it heats slowly and depending on its depth will usually stay much cooler then the ambient temperature. In the winter, it again accumulates “coolth” before passing it through to the lower layers.
- Plants – in the summer, plants (assuming they have enough water) provide cooling – through transpiration – release of moisture to the air (sweating). In the winter they die back into a naturally insulating later. That layer will decay in the next spring/summer and nourish new growth.
- Snow – is actually an excellent insulating layer (insulation is typically created by materials that have pockets of air). The combined effect of snow, on top of dead plants on top of earth provides substantial insulation for the under-layers of the roof. In contrast, Earthships include a hot water system to melt snow and ice to harvest water – that generates water at the expense of insulation.
All this boils down to the one most important feature our architect pointed out when he introduced us to living roofs. A roof with an outer layer that absorbs climatic shifts and creates relative stability for the under-layers.
Our main source of water is a well with a surface pump. However I do believe that water may potentially be a challenge in the future (I am thinking on a scale of 20+ years). I would love to be able to incorporate an independent water supply such as rainwater harvesting can provide BUT:
- The entire system (roof + drains + cisterns + filtering) is a very expensive part of an Earthship build. Since we are trying to create an Earthship that we CAN afford to build – letting this system go is very tempting.
- Harvesting rainwater while compromising and/or complicating the two core roof functions of shelter (see longevity) and insulation doesn’t make much sense and is not very appealing.
- I believe the best (and surely more affordable) way to filter water is through the ground itself (though we do have to deal with hard water issues).
- I believe that the best (and surely more affordable) place to store water is in underground aquifers and not in plastic containers.
- The way we, as humanity, are treating the atmosphere worries me to the point that I am not convinced rainwater can be a reliable long term source of water.
- I have doubts about the quality of rainwater as drinking water (the quality of the water is effected by all the materials the water meets on its way to the cup and can change its characteristics when stored over time).
- Our vision for our home goes beyond our house and we hope to create an ecosystem where more water is retained in the earth.
- We have drastically lowered our water consumption and continue to be very vigilant about it.
- We intend to build an outside shower for the warmer months of the year which will include rainwater harvesting and solar heating – so that too will reduce the “water load” in the house itself.
Rainwater harvesting from the roof simply doesn’t appeal to us. The lower cost, simplicity (though it needs to be done right to work) longevity and insulation performance of a living roof make it a more appealing solution.
We are considering some kind of cistern (1000-2000 liters) to both improve electric efficiency and if we manage to incorporate the cistern indoors and near the front glazing we may be able to bring up its temperature before it goes into the water heating system.
An extra bonus is that the structural strength of rammed tires seems superbly matched for the load requirements introduced by a living roof. The original combination of all-tire U’s and east-west orientation of root rafters make for an out-of-the-box-ready structural solution for a living roof.
I am assuming that we will need an additional structural face element to support the weight of the living roof above the greenhouse and corridor. I am thinking that beautiful natural wood posts will do the trick. And, ironically, to keep it simple, we may also embrace the raised front lip design of the original Earthships.