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  • Lorianna Kastrop

Consider a Roof Garden for Safer Gathering Space (Part 2)

By D. Michael Kastrop, A.I.A., Principal Architect, The Kastrop Group, Inc.

Being on top of the roof exposes you to weather, including sun, wind, and rain. Sunlight can be controlled using trellis structures and umbrellas. Note that umbrellas and furniture need to be well secured to the roof so that they do not lift and blow off the roof in a big storm. Consider heavy-duty commercial grade outdoor furniture that has attachments for bolting it to the roof. We do not recommend using standard residential patio furniture. Sunbrella ® fabric wears well in sun and rain. It is easy to clean. We recommend that any cushions used have sewn-on loops so that the cushions can be zip-tied to the furniture. (The loops can be added if the cushions don’t come with them built-in, but if you add them be sure to use thickly-corded loop material and heavy-duty thread that is rated for being in the sun, such as used in curtains. Any fabric store can recommend what you need.)

The wind generally blows in one prevailing direction (and maybe a secondary direction during parts of the year), so it is possible to include partial glass partitions in the design to block the wind from the prevailing direction while still maintaining the rooftop views. These need to be placed far enough from the roof edge to allow for cleaning.

Photo of the roof garden we recently completed for a San Mateo office building which included glass partitions and an accessible ramp.

As mentioned in the last blog, it is important to replace the roof before creating the Roof Garden.  It must drain well to roof drains and overflow spouts and be completely watertight.  Of course, you don’t want people walking directly on the roofing surface, so pavers should be installed for a good, durable-wear surface.  We recommend a grid system mounted on adjustable feet for paving.  There are many choices of finish pavers that can be installed.  The important features to look for are durability and light weight.  The weight of the pavers should be included in determining any structural reinforcing that is required. 

Construction progress photo of the pavers and grid mounting system for our roof deck project in Burlingame.

The color of the roof material will be a big factor in the comfort of the outdoor Roof Garden. We recommend light-colored roofing as well as a light-colored paver material to reduce the summer roof heat absorption. Some of the materials we have used include IPE wood, porcelain pavers and planks, concrete pavers, and composite wood planks. Again, be sure to include the thickness and weight of the paver material in your structural design.

Make sure that you include a water spigot for drip irrigation and cleaning and waterproof electrical outlets. Plant selection is best done with the advice of a local master gardener or horticultural specialist. Trees can create issues with weight and toppling in high winds. We recommend that you select full-sun, drought-tolerant plants to keep the watering to a minimum. Set up a drip irrigation system Plants that lose their blossoms or drop leaves in a heavy afternoon breeze should be avoided, for less sweeping or leaf-blowing clean-up. Putting in a spreading groundcover between the larger plants will help to keep the soil from blowing as well.

See photos here of a completed Roof Garden project.  Thank you for reading, and as always, we are Designing for Your Reality.

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