Rainwater catchment and green roof - which is more appropriate in a permaculture design?

Hoping this will be in time for Ross to chime in, but if not, all input is welcome.

I've been grappling with an issue regarding the more appropriate technique for roof building and I've gotten to a pretty much 50-50 split of pro/con for both: impermeable roofing for rainwater catchment vs thick, insulative green roof.

I'm in central Maine, zone 4b or 5a (depending on the year) which is quite cold in the winter. Our growing season runs 4-5 months and our heating season runs the same. We receive lots of fairly evenly dispersed rain, about 3.5 inches per month year-round, and want to utilize rainwater catchment for all our water needs (drinking, washing and utility). At the same time, in building our zone 0 / house, we're finding the potential benefits of a thick, insulative green roof (18 inches to 2 feet thick of layered sod) to be very important for our heating concerns. It's also a cheaper solution which is highly important for our shoestring budget - all materials other than the waterproofing membrane can be harvested on-site, compared to the need for purchasing rather expensive roofing panels of some type in order to do rainwater catchment.

I've often imagined combining the two and just collecting the rainwater runoff from the green roof for at least utility purposes, but I'm concerned about the TDS (total dissolved solids) count being high and the potential for copious amounts of duck and other critter poop being present and causing the water to be unusable for anything but irrigation (which we almost never need here).

A few other green roof "pros":
noise buffering potential of a green roof would help cut down on noise pollution from the logging truck traffic in our area
the green roof would help hold a snow load in place over the winter, further enhancing insulation levels
roof area can be used for growing duck and rabbit forage (clover, purslane, etc)
reduced visibility from the air means less chance depts of making you sad will come poking around

But ... water is essential to life! We need clean drinking water catchment and it needs to be close. Setting up a large, separate structure just for rainwater catchment is definitely outside our financial capabilities and because of the site's limitations, the structure would have to be some distance away, making it necessary for us to transport our drinking water some distance or install electric pumps and buried piping to transport it. The frost line here is 4 feet deep, so that's some deep trenching through our rocky, rooty forest soils. Additionally, the framing for the structure itself can be built much lighter, using far less materials (all also being harvested on-site) if we are creating a more steeply sloped roof with so much less weight for rainwater catchment.

When a situation like this comes up - where there's no clear winner - there absolutely must be something that's missing from the equation. So what am I missing?

Neither is more appropriate in a permaculture design....meaning there is not a right or wrong answer. Permaculture is strictly a design system and about stacking functions. Your problem is you have too many choices. Don't get so worried about the department of sad. They are not necessarily the enemy and they can help us with our decision making, for instance if you lived in Colorado, catching rainwater might be illegal, so your decision is already made for you...boom green roof.

When we don't have any regulations on us and the design field is so completely open, the amount of choices you have can be overwhelming. I promise you this. I'm in the middle of building a homestead right now, constructing ponds, building structures, etc...

What I can tell you is you need to take a permaculture design course. Its a must.

Then trust your instincts! They have led you this far in life! Go with the one that resonates most with you, and then incorporate it in every way you can to your design. You don't need all the answers right now. I can promise you that. If you install your rain water roof it will lead to an "aha" moment and then your off designing something else like fish pond, but the EXACT same thing will happen if you choose the green roof. There will be more "aha" moments. You'll find so many different things you can do with each.

You will find permaculture is a very fluid design system and about careful observation. Its constantly changing. There is no wrong answer. Do what YOU like best.

Tristan Vitali wrote:

I've often imagined combining the two and just collecting the rainwater runoff from the green roof for at least utility purposes, but I'm concerned about the TDS (total dissolved solids) count being high and the potential for copious amounts of duck and other critter poop being present and causing the water to be unusable for anything but irrigation (which we almost never need here).

Could this fertile water be run through a series of aquaculture tanks until it comes out pure and filtered? I can imagine a whole series of gardens ending with a sand filter for fresh water.

Green roofs , especially one as thick as you are describing , require a very robust structure to support them,
Other than insulating living space, they seem to offer nothing a raised bed would not.
DIY insulation could be borax treated cardboard or papercrete, a foot thick, probably more insinuative than soil, and still cheap.




How about something like a flattish roof deck, and a sunspace/shade structure over that?
In the cold months, use it as a Low thermal mass sunspace for space heating
In the hot months, grow vines and or drape it in shade cloth or even Mylar faced insulation.
Of course, these uses are going to require a more sturdy roof than an insulated/rainwater catching roof, almost as much as a green roof, so why not just do that anyway?
Well, if I was going to invest in an extra strong roof deck, I would want o use it to store my water supply, so I could gravity feed my fixtures.

So, a strong, flat, insulated, roof deck,daisy chained water storage tanks and wicking containers distributed over it, and a canopy over top,to deflect heat in the summer, collect water year round, and siphon off heat in the winter.

I would also consider berming the structure on up to three sides, for thermal mass/ insulation and easy access to the roof.
This could complicate construction.

Thanks for the replies - it's definitely a conundrum. As Eric pointed out, there's not a clear restriction making the decision for me on this one. Available on-site materials, cost for what can't be sourced on-site, and the ease of construction (one guy doing everything manually - no heavy machinery available) are the only restrictions I'm working with on that end of things. I've gotten so used to the whole "define the problem to find your solution" methodology...multiple solutions drive me nuts!

Our current design utilizes a pretty stout roundwood timberframing with 14 to 36 inch diameter eastern hemlock and relatively short rafter spans (maximum 14ft spans between beams with most being just 8ft). The weight of the roof isn't something I'm too worried about...it's the weight of the logs and the joinery skills used to build it that are!

Tyler set my mind going with the idea of filtering the rainwater runoff, though, and something like that would almost certainly work. Our current shed/wrap structure has a thin (3" or so) green roof (experimenting with the technology) and we initially found a lot of clay and organic matter washing out last year, just after it was installed. This year, however, the water is almost running clear to the eye. We know the ducks are getting up there and doing their thing, and we have n-fixing clover, etc, up there putting more nutrient into the run-off, but it certainly isn't as bad as it was when first put in place. As the root mass builds up more, I'm sure this will improve further, and by running the water through at least a slow sand filter, if not a cattail/reed bed followed by the filter, we could definitely harness this water for utility purposes (hand washing, clothes, dishes, etc). The utility water, of course, makes up the majority of our water use, so this would be a huge plus. In that sense, we can consider that "con" to be removed for the green roof.

As for drinking water, another thought came to mind - a free-standing catchment system on the roof as well. A few roofing panels on a lightweight frame, sized appropriately to ensure apx 100 gallons of drinking water catchment per month, could be set up pretty cheaply and piped down to separate holding tanks. With decent anchoring and proper placement, it should hold up to the wind well, and with some fencing over/around it, should be able to keep the dirty duckers off it, too

Will have to noodle on this some more before I make a final decision, but with the water situation becoming moot, the green roof certainly takes the lead in the mental battle. Construction can't start until at least next year after we've done the site prep work and I've cut/limbed/peeled enough posts and beams, so plenty of time to play with ideas still.

Would it be possible to combine both?

A green roof with rainwater catchment?

Maybe something similar to this layering could work:
1. sky
2. plants
3. topsoil
4. sand
5. clay
6. membrane
7. wooden structure

A drainage pipe would be at the bottom of the sand layer (4) and connect to the storage tank.