As inviting as this field might look on first glance, it’s not a great place for a bare foot walk. That’s because what you can’t see is very possibly the most advanced and comprehensive engineered wind uplift solution for a high-altitude green roof in use anywhere in the world.
Needless to say, there are a number of differences between conventional landscape design and green roof design. One such consideration is that, the higher the roof, the less protection it will have from wind – above a certain height there are no trees and fewer other buildings to baffle or reduce the wind’s impact. It’s not simply a matter of wind eroding away parts of the growing media, in fact the largest concern with wind is the vacuum effect created when wind blows across the parapet. Air turbulence above the parapet creates a vacuum on the other side of the parapet (two websites that illustrate the principle nicely can be found here and here). That vacuum – called wind uplift – can pull apart roofing systems.
Naturally the key to dealing with the vacuum effect of wind uplift is to resist lifting. One way to do this is simply to add more weight – adding heavier elements to the growing media to make it harder to lift. This is a workable solution in general, but it has it’s limits.
Take, for instance, the roof pictured above – a roof on the 29th floor of a building that sits only 500 meters (1600 feet) from the 14th largest lake in the world. There are few green roofs in the world that face challenges akin to this one, so there aren’t a lot of best practices in place for how to secure such a roof. Clearly though, conventional thinking would require that the weight of the growing media be increased dramatically to compensate for the increased uplift power – but increased weight would require significant alteration of the growing media and would add costs for structural reinforcement. Obviously some “out of the box” thinking was needed: that means, of course, that it was a perfect job for Living Architecture!
In a consultation between Living Architecture Principal and green roof guru Rick Buist, and the project architects, a plan was arrived at which, rather than weigh each individual point in the system down, would integrate the system more securely, so that a heavy wind uplift at any point on the system would have to overcome the entire system’s weight, rather than simply pull apart isolated sedum mats.
The first step, seen in the foreground above, was to lay down a thin layer of concrete ballast in the form of paving stones. Partly this was done to add some weight to the system, but also, with the ballast comes the skeleton that will be used to combine the elements of the system in place… Looking at the foreground of the image above, you can see that each of the ballast pavers has a steel rod anchored in place in it’s center. These rods in turn protrude through the retention boards (and here are capped with orange safety caps to protect in the event of falls during construction).
Once the remaining layers are in place (for more details on the layers of this green roof system see our previous blog entries) it’s time for the growing media, seen above.
Once the growing media is in place, it’s time for the ‘skin’ of our wind uplift system:
Above you can see the stainless steel mesh that is laid over the top of the growing media and tucked within the edging. The steel mesh will keep the media and the roof layers in place… once it’s been anchored to the skeleton. But as with any structure, the skin needs to be integrated with the skeleton:
Here we are at the almost final step. The stainless steel rods visible in the first picture are now submerged in roof system layers and growing media, but are still visible. You can also see, looking at the rods in the foreground, that each one has had a washer and nut fastened to the top of them (that’s what those two gentleman in the middle of the frame are engaged in). Once tightened up, this attaches the layer of mesh to the paver underneath, capturing the growing media and the green roof components in a ‘stainless steel sandwich’ and turning all the otherwise loosely connected components into a single entity – pavers, components, media and mesh – which will resist wind uplift along any one of it’s edges with the weight of the entire system.
Now the system is in place and safe, but you will recall I wrote ‘almost final step’ above – one more thing remains to be done. Looking at the image above, the sunlight on the mesh is quite telling – mostly what is visible is the stainless steel and the roof now has the look of something metallic rather more than natural. Somewhat disappointing from an aesthetic point of view. But luckily, time is on our side:
It doesn’t take long for the grass to start sprouting enough that the sheen on the mesh is obscured. A matter of weeks later and we’re back where we started:
Now that we know what to look for, we can see the ends of some of the rods peeking through – but by far the best part of the system is now hidden beneath a lush layer of green.
You’d never know you were looking at cutting edge design and engineering. That’s what we do at Living Architecture: make the impossible look only natural.
