Planting Our Roof, Part II

Read Part I here.

We constructed a roof garden, complete with drainage system, soil, raised garden beds and grass on top of our garage when we built our Minnesota home in 2005. At first, the design idea seemed unusual, then the process became complex, but the resulting space now feels natural and almost expected.The details of construction are somewhat technical, but show how we can plant in soil on our roof and water it without damage.

Garage Construction. The foundation of the roof deck is the garage. The walls are built of ICF’s or Insulated Concrete Forms. They’re like large, hollow, Styrofoam Lego blocks filled with concrete.

Roof Deck Planks and Topping. Pre-cast hollow core concrete planks form the roof of the garage and floor of the roof garden. The most exciting part of constructing the roof deck was the day the crane came to lift the seven 28-foot concrete planks into place.

After the planks were in place, the construction crew filled them with high-strength grout. Then, they built a ledge for the roof with concrete blocks and covered the roof deck with 2″ of concrete as a structural topping. It adds strength to the planks, allowing them to carry slightly greater loads. A standard concrete truck with a conveyor attachment pumped the concrete up to the top of roof deck.


This summer I accidentally tested the waterproofing systems. I turned on the water to the garden at 8 p.m. We have a system of soaker hoses buried in the raised beds. I forgot to turn off the water. The next morning, at 10 a.m. I heard water rushing outside the house and remembered! I immediately turned off the water, but worried about the damage to the garage and house. Everything was dry. The water I heard was not water pouring to the house and garage below but excess water pouring out the three drains across the back of the roof deck. The rocks below received a good deal more than necessary and my zucchini plant has mildew, but I didn’t cause any permanent damage.

The waterproofing system is as much about structure as it is about material. For the overflow of excess water the roof deck concrete slopes towards 3 scuppers (or drains) on the north side. The slope is about 1/8″ per 1′-0″. The concrete at the south end is approximately 5″ thick, 2″ at the north end. The corners are also sloped to keep water from pooling.

Over the concrete topping is an asphalt coating and then two inches of rigid foam insulation. The final step of waterproofing is the rubber membrane roof system or EPDM with several layers of drainage mats on top of the rubber. Finally, a filter fabric and felt complete the layers. The filter fabric keeps the dirt and sand from filtering through and being drained. Here’s an architectural section of all the layers that comprise the roof deck.

Next week, in the final post of this series, I will write about how we delivered the soil to the roof, created raised beds and planted it.

1 Comment

  1. Michelle at Scribbit
    Aug 13, 2008

    I love this–I’m excited to read the next installment.

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