There’s growing interest in living roofs or green roofs. The difficulty is deciding on the waterproof membrane. Rubber pond liner (EPDM) is the most waterproof and durable material, and the most common choice due to proven performance, but it’s also very expensive and made from nasty petro chemicals.
I think a number of less toxic, more affordable materials are possible, including recycled materials in good condition. One reader suggested pool covers. Heavy duty trucker tarps are another option. You could add 6 mil poly (plastic sheeting) underneath and/or above these other materials for extra moisture protection.
For my dome, I used 2-1/2 layers of 6 mil black poly and have had no leaks so far after about three years in a rainy climate. (The half layer is a small piece on the very top.) You need to cover 6 mil poly carefully to avoid punctures. Some use old carpet or cardboard against it while adding soil. (Screen out rocks.) In our case, we simply packed soil on the plastic, starting at the bottom and working up, with no protective cardboard, etc.
Recycled vinyl billboards are another possibility, but I wouldn’t use them because of the health hazards of leaching chemicals into the soil around the house.
Roof pitch is another consideration. Steep roofs shed water faster and are less prone to leaks. But you need to strike the right balance or top soil and nutrients will wash away, and your roof will dry out too quickly.
Another consideration is the value of the structure. It’s no big deal experimenting with alternative waterproof membranes on a simple $2,000 guesthouse. But you might want to use better materials or multiple layers on expensive homes.
Photo credit: Heartwood Homesteads
Beautiful and amazing green roofs in Slovakia https://www.daibau.sk/clanok/1178/zelene_strechy_a_fasady
I have been looking at different pondliners. I find epdn very expensive and is now considering a material called flexiliner. Anybody who has experience with flexiliner?
In the old days shipbuilders would use pine pitch to chink the gaps between the slats in boats. I haven’t worked with pitch yet myself, but I wonder if that could do the job? Someday in the future when I actually get the chance to build my own house, hopefully I’ll be able to experiment with that.
It could work. Just realize water will find a way through the tiniest cracks. And over time most every waterproof sealant will dry and crack. This leads to constant maintenance headaches in rainy climates. You can’t ignore it or your roof structure will gradually rot and fail, and all that heavy soil would collapse.
We have used pondliner – which is PVC based but with a tear resistant cloth. It is weldable. On top of that is a recycled wool bases carpet underlay and then coir geotextile 600gm psm.. Locally sourced sods are 80mm thick. All our roofs are at 26 degrees.
Thanks Owen. I’ve ordered the movie you spoke about.
Owen, what is your position on using recycled vinyl billboards as an underlayment that is then covered with cob – in a non-living roof situation? I live in a humid area with lots of rain fall (+60″ year) and ample signs. I was planning on using same as a vapor barrier for the earthen floors, walls and roof but am now apprehensive based on your comment.
You don’t need moisture barriers in walls. In fact, they can trap moisture in the wall and cause problems. Far better to build wider roof overhangs to protect walls.
For floors, it’s best to raise the building site so water drains away from the structure. I use road base for this and drive a truck over it back and forth. Then build on rubble trench foundations.
For the roof, vinyl is an option. I don’t trust it after watching the movie Blue Vinyl. This stuff is really dangerous, and yet it’s everywhere in plumbing and so on. Our culture is sick. Who would willingly poison themselves and their environment and not really care?
But there may very well be unhealthy side affects from truckers tarps and swimming pool covers. Any synthetic material could cause problems. I was thinking of using recycled materials in good condition where most of the nasty fumes have already offgassed, but I can’t say for sure these things are totally safe.
The simplest solution seems to be layers of 6 mil plastic sheeting that is separated with scrap cardboard. Work as carefully as possible to avoid punctures. (Note: this plastic lasts a very long time if not exposed to sunlight.)
The goal is to build as sustainably as possible. Most living roofs are outrageously complex and extremely expensive. I’m in favor of low-cost, low tech like this cob garden shed: http://www.busyboo.com/wp-content/uploads/2008/11/cob-garden-shed.jpg