Our Little Green Roof Shows Amazing Potential

Finished green roof. Mason bee boxes nest in the gable.

Of course, now that the green roof is finished there hasn't been much of any rain in the forecast. Only the night after it was planted, a gully washer tested its merits. It passed muster with flying colors. Water was pouring off of the other, more conventional roofs here, but scarcely a drop came off the green roof. After all was said and done, only a few drips emerged out of the drain pipe. That means it was doing its job; that is, mitigating storm water run-off. I'm looking forward to seeing how well it does in a prolonged period of rain.

Although installed more and more in commercial developments, green roofs are underutilized on residential structures. The benefits of a green roof are many and include: mitigating and cleaning storm water run-off; reducing the urban heat island effect; providing summer cooling and winter insulation, thus lowering energy costs; providing habitat for beneficial insects and providing noise insulation, just to name a few. Green roofs have been used by different cultures for hundreds of years, but we are just starting to understand how well they function as a viable roofing system. Being that we live in the Norwegian ghetto, having a green roof keeps up with Scandanavian tradition as these roof systems planted with turf on birch bark water-proofing were quite common in that part of the world, once upon a time.

The perception of green roofs being expensive and high maintenance is unfortunate. In reality, most green roofs require little to no maintenance. Just the occasional pulling of unwanted seedlings or top dressing every few years with compost is all that’s usually required. Like any type of garden, it’s about the soil. A low-nutrient, free-draining soil will cause much less of a weed problem than a soil rich in organic matter and nutrient levels. Dead heading is not necessary, and the use of drought tolerant plants such as succulents, eliminates the need for constant irrigation.

This roof is called an ‘extensive’ roof system, meaning that the substrate is less than 6 inches thick. ‘Intensive’ roof systems have much deeper substrates, allowing for larger shrubs, grasses and even trees. Extensive systems are lighter weight (around the weight of a tile roof) and the plant materials used most often consist of succulents which can grow well in a shallow, rocky soil. Last fall, I picked up 9 sedum tile flats for half price at a big box store.

This roof system caused Roland to exacerbate an eye-rolling habit he usually expresses around me. It started with my initial proposal of installing a green roof the minute he announced that he was going to build a garden shed, followed with, "How do I do that?"

Installing the front drainage gravel.

So, after showing him many pictures from my green roof books, the first thing he did was build a bowl on top of the structure. I liked the rafter tail details on the green roof pavilion at the Arboretum, so I had him replicate that. Of course, it couldn't be a simple shed roof; he had to put a dormer on it to make it more appealing. That only complicated the water proofing part. Tar paper went on and sat there for two years whilst he figured out in the back of his mind what to use. Pond liner would have been an inexpensive solution for a straight up small shed roof, but having that gable complicated things a bit. The liner needed to be weldable and regular roofing materials often used in large commercial installations is very expensive, even though we needed so little of it. However, while at the big box, he came across the solution - PVC shower pan liner. It's a heavy gauge, weldable and only somewhat expensive instead of extremely expensive ($85 vs. $400 for the roofing stuff - eye roll). So, he picked up some scraps at a discount and then stashed them until he could get enough of the stuff to do the job. A year went by and still no green roof. Sedums over wintered in their trays. There the roof sat until the Edible Garden Tour. Now I had leverage. Deadlines make the world go around. 

Installing the liner.

Roland came home with the rest of the liner and we proceeded. He laid down new tar paper, then the liner which he glued together with PVC cement. We had an argument about laying down some landscape fabric over the liner. I felt it would help keep the fines from flowing down and protect the liner. Roland saw it as unnecessary and slippery to walk on (more eye rolling between whining). I do know that if you use a separate drainage layer, having a geo-textile is extremely important to keep the fines from clogging it up, but I'll admit, it may have been unnecessary here. I like to fault on the side of caution. Roland built a wooden grid system out of pine to help keep the substrate from shifting down to the bottom. It is designed to last long enough for the plant roots to get established. A metal screen set back 6" from the bottom end of the roof held back the substrate and allowed for a row of pea gravel to go into that area for added drainage. He installed conventional flashing for the edges.

The substrate mix.

Next, came the substrate, or growing medium. On this roof, the drainage material consisting of lava rock, was mixed in with compost and a small amount of perlite. I needed 12 cubic feet of material, so the ratio was 7 cubic feet of lava rock, 4 cubic feet of compost and a cubic foot of perlite. I mixed it all together on a tarp and schlepped it up to Roland in 5 gallon buckets. He now complained about the treacherousness of the round lava rock - like walking on ball bearings - while he walked around the roof distributing the materials (more eye rolling ensued). 

The plant material - sedums with Hens and Chicks.

The next day Roland took it upon himself to pull out the largest chunks of lava rock claiming that he was going to break his neck and couldn't get the sedums into the medium (more eye rolling). He divided the succulents and planted the whole roof while I was at work. I came home to a lovely roof all planted up and a crate full of large lava rock chunks. He had added a bit more compost too. A drain hole leads down a chain to a rain barrel to capture what storm water runs off there is, being slowly released off the roof thanks to the mitigating properties of the roof materials. I'll periodically water it during our dry season while the plants get themselves established.

A lot of folk who came through the tour commented on the green roof. They thought it was a great idea and looked really nice.

I proclaimed to Roland, "I think you should convert the roof on the front porch of the house into a green roof when you re-build the front porch deck."

His response? More eye rolling.