Green Roof?

I’m waiting for Tesla to install solar roof tiles. The project is at the Xcel power company application stage. The permit from the city was granted. We decided to upgrade the roof to solar tiles the day the president announced pulling out of the Paris climate accords. We needed to do our part and figured for twice the price we can get over 3 times the solar that we have now and get a new roof. But the solar roof does not cover the flat roof at the front of the house. It needs at least a 2/12 roof slope.

Flat Roof
Flat Roof

I investigated installing a green roof on the front that would help nudge the house toward net zero. That means the house creates as much energy as it uses. The growing medium on the roof has insulating value. A green roof also manages rooftop water by absorbing it and releasing filtered water to catch in rain barrels. Since we are in Colorado we can only collect 110 gallons of rain water at a time and that is a recent change in the law. When we first moved here rain barrels were illegal.

I learned about green roofs when we toured one in Denver during a USGBC conference in 2013. So I’ve been considering this idea for a long time. The roof we toured was built from shallow trays of sedums. That is an extensive style roof and the lightest type. An intensive style has a deeper soil bed.

Green Roof Systems
Green Roof Systems

Extensive roofs are lighter than intensive but they can still weigh 30 lbs per square ft. fully saturated. That in addition to snow load can be too heavy for a typical roof. Fortunately we don’t have a typical roof.

Flat roof structure
Flat roof structure

But the first step is getting the official calculations from a structural engineer. I contacted Andy Creath from Green Roofs of Colorado and the person who presented at that conference years ago. He discussed the green roof options with me and arranged for a structural engineer to run the calculations on our roof.
Andy’s green roofs are not in trays. He creates a monolithic growing medium above waterproof membrane roofing material like TPO. A parapet rises above the roof edge to hold the plantings. All the typical green roof layers are installed and a patio and walkways would be incorporated into the design.

Denver Botanic Garden Rooftop garden
Denver Botanic Garden

The layers of a green roof are made from differing materials depending on the vendor but the design is the same. It is a layered system that creates a barrier rooftop but provides drainage for plant material and a growing medium. It is referred to as growing medium because rooftop soils are specially formulated to be lightweight and well-drained.

Roof layers for green roof
Roof layers for green roof

The roofs feature a variety of plants from wildflowers to succulents and cactus or grasses. Of course the Midwest and East require different plants than more arid areas of the country. Colorado rooftop gardens also require an irrigation system. The research conducted for the EPA determined that a green roof cannot survive in this climate without periodic irrigation.

This Old House Green Roof
This Old House Green Roof

In Colorado the plants are often varieties of sedum a low growing succulent with low water and nutrient needs.

Sedum Green Roof
Sedum Green Roof

This sedum roof is similar to the roof on the Denver EPA building. It is planted with sedum and the plants are in trays that sit on top of the other layers. This is the lightest green roof.

Intermountain Green roof with trays
Intermountain Green roof with trays

We had the structural report created and as I suspected our roof is built solidly enough to hold a green roof. The engineer’s conclusion was for a medium weight green roof.

For the calculations of this residence roof, the green roof system dead load of 42 psf was utilized. Based on the revised loading and the existing spans of the wood joists, the existing 2×12 joists spaced at both 16-inches on center and 24-inches on center do provide adequate capacity to carry the additional proposed superimposed green roof loading.

Donald E. (Leo) Whiteley, P.E., Assoc. AIA, LEED AP

The next step is to decide whether to engage Andy’s company to create the green roof or investigate a tray system. He told me that the best time of year to install the green roof would be fall. That would give the structure time to settle before the growing season. I’m waiting for the Tesla solar roof to be installed before we make a decision.

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