Dear Tiffany,
Thank you for letting me stay at your penthouse on this trip. The bedroom was very cozy and efficient. What I loved the most was the window seat, where I could watch the birds in the garden while enjoying the morning coffee, which was my favorite part of the day.
While I cannot thank you more for this beautiful roof garden, I discovered a few structural issues when picking up the newspaper downstairs. The staircase has several cracks, and I suspected they have something to do with this garden.
It is not unusual for concrete structures to have cracks, especially in Taiwan, which is in the seismic zone. Although your building is not newly constructed, most reinforced concrete structures can still have a life expectancy of about sixty years. Thus, I do not see these cement fractures as immediate structural failures.
On the other hand, I felt that giving you some heads-up analysis might be necessary. So you could begin considering how you would like to remodel your roof garden for more extended use. Here are a few structural principles that I use to analyze the overall landscape on the roof:
The column capital bears the most structure load.
The middle section of a beam is the weakest spot of the entire column span, which means it can carry the minimal structure load along the beam/girder as intended.
Extending the load-bearing area to one-quarter of the entire column’s span is still considered safe. That is because the compression and tension were still at balance without causing much deflection. (see graphic)
With these three principles, I started to review your roof plants’ locations.
Southwest side (between columns B-C): Two Japanese maples are on the street-facing side. You can keep the maple A but remove the maple B. Because planting location B is a cantilever situation, no supporting structure is underneath. With an added maple tree load on the top, it would be subject to a greater risk.
Southwest side (between columns C-D): The Bougainville facing the street side is also close to the middle portion of the entire column span. Thus, I suggest you remove or relocate it. Although most branches and leaves of the Bougainvillea are grown downwards, scrambling over the parapet, their roots tend to increase thicker and deeper proportionally.
Southeast side (between columns 1–2): The mandarin tree is also located in the middle portion of the column span, which is also a weak spot. Therefore I recommend removing or relocating it.
Southeast side (between columns 2–3): The Portia tree is about 155 cm from the capital. It also needs to be relocated.
Northeast side (between columns C-D): The lush cypress on the northeast side might also interfere with the waterproofing level below. (Based on its height, I suspect its root has already extended and penetrated through the waterproofing layer. The concrete slab below might have fractures.)
At the exhaust vent stack (outside of the bathroom): the Allamanda Cathartic surrounding the exhaust vent should also be removed. This lush cathartic was visually pleasing, but it blocked the opening of the plumbing chase. On top of that, Allamanda Cathartic is a plant that requires soil rich in organic matter and plenty of moisture. Watering them frequently might cause erosion of that plumbing fixture. Moreover, this plumbing chase also needs an access panel at the level below. Since you use tongue and groove-mill wall panels on the main floor already, it would be easy to create an access panel. The current painting there is also large enough to cover the opening.
All the plants mentioned above can be transplanted into a larger-size planter. I also recommend you replace the current lawn with artificial turf. As I have observed, some of the roots from well-grown trees probably already extended beyond the landscape rocks and the waterproofing layer. Removing and replacing lawns with turf will eliminate grass irrigation and provide a more resistant coating to protect the concrete slab. So you will unlikely have water damage on the main floor again.
“A plan is to some extent a summary, like an analytical contents table… it contains an enormous quality of ideas and the impulse of intention.” — Le Corbusier
You might find all this garden work is burdensome and regret your initial design decades ago. But I affirm you that, since you did not study architecture, your roof garden would be considered one of a kind in history, and you should feel proud that you are the owner of such a garden project. People who studied architecture all read about Le Corbusier’s five points in modern architecture, which he used in his influential work, Villa Savoye in the 1930s, to represent his ideology toward the new machine era. One of his five points was the Roof Terrace, which he intended to demonstrate “a functional roof serving as a garden and terrace, reclaiming for nature and land occupied by the building.” The original statement was written in French:
Cinq points
les pilotis
le toil-jardin
le plan libre
la fenetre enlongueur
la facade libre
Many scholars translated “le toil-jardin” as a roof terrace (rather than a roof garden). Yet, the straightforward translation was “garden canvas,” which means using landscape as the “drawing tool” on the floor plan. Your garden has nothing that looks like a “terrace.” Yet, in a sense, it is closer to its original intent merely because Chinese calligraphy paintings were distinct from Western abstract oil paintings. How you carve these views toward the surrounding buildings also reflects the “view-framing” techniques in ancient yuan lin (Chinese gardens), which was a huge breakthrough.
Thanks again for the breakfast; I hope you don’t find our conversation long and trivial. Or, perhaps, you are more found in your garden now than ever and have already pictured your next design for this roof garden in your mind.
Your affectionate guest,
Hepburn
If you are curious to see how the roof terrace of Villa Savoye, Le Corbusier's epoch house, looks like, you can check out this article, “Architecture Classics: Villa Savoye/Le Corbusier”, from ArchiDaily.