Garden with a view: Rooftop can expand your space
A 4-foot ladder leaning against a narrow, shoulder-high deck takes you face to face with Bob and Fran Clarke's roof garden.
Take a few steps to the left, and another, taller ladder leads you up onto the roof of the house and under the massive limbs of a 43-year-old pin oak, where you get a bird's-eye view of the wildflowers and succulents that inhabit the 4-by-16-foot shed roof garden. A catwalk on the lower side gives Bob room to walk the length of the garden and tend it.
The garden originally was intended to be a succulent collage. Several types of creeping sedums vie for space. For a splash of color, the Clarkes tossed in 26 packets of wildflower seed last winter. The masses of orange California poppies have since given way to baby blue eyes and bright pink clarkias.
"It's called an 'intensive' green roof," Bob Clarke explained. "The soil depth is only 2 to 4 inches, so the plants have to be shallow-rooted ones." Another type of green roof is called "extensive," he said, because the soil depth is greater and plants are self-sustaining.
The Clarkes built the rooftop garden because, well, it sounded pretty. Fran, who works as a stewardship and education coordinator for the Sacramento Tree Foundation, had returned from a conference in Washington, D.C., five years ago where she learned about roof gardens.
"That's for me," she said. Bob agreed, and welcomed the task as a challenge.
But where would they put a roof garden?
Their home has a gently sloping roof with good sun exposure. It is ideal for a roof garden, Bob said, but it would have been too costly to build one on such a large scale. Besides, the Clarkes, both enthusiastic gardeners, already have a large backyard planted full of lilies, foxgloves, geraniums and citrus trees, all crisscrossed with paths that lead to surprises and hidden rooms.
The roof of the storage shed was one of the few places left for a garden. The shed roof has a slight pitch, so drainage was not as complicated as it would have been if the shed had a flat roof.
The first step, Bob explained, was to build a test garden in the backyard. He built a simulated roof garden, experimenting with slope, soil mixes, moisture barriers and plants. His main concern was the weight of the soil on the roof, especially when it was wet. Then Bob discovered a Danish product called LECA (light expanded clay aggregate), which resembles small clay marbles. Added to the mix, it lightens the soil and enhances its moisture-holding capability without adding weight.
Another thorny problem was devising a moisture barrier between the roof and the garden bed that would last for years. He looked at all sorts of plastic sheets and pond liners. He settled on 60-mil thick plastic liner that he special-ordered. The pieces were slowly coming together, Bob said.
Next he reinforced the shed roof. He enlisted the help of a strong neighbor to help carry and spread the plastic moisture barrier over the roof.
He constructed the garden frame in two 4-by-8-foot pieces in the garage, took them apart and hauled them up to the roof to be reassembled. Inside the wood frame he spread a layer of LECA, then a filter cloth to prevent the soil from washing away. Then, bucket by bucket, he hauled up sand, bags of loam, LECA and, finally, the plants.
The Clarkes weed and water the garden about once a week. They'll let the wildflowers reseed themselves, and the garden will be a constantly changing palette through the year. Aside from Chinese pistache and oaks reseeding themselves in the bed, and a few digging squirrels, there haven't been any problems.
Four years after planting the roof garden, "the drainage is great," Bob said. "The moisture barrier works perfectly."
The garden is as pretty as Fran envisioned it would be.