Designing for weight and water — the physics that turns a roof into farmland

On the ground, design can begin with the crop. Decide what you want to grow, build soil to suit it, bring in water if you must. There is rarely any need to ask what lies beneath the soil. On a roof that order is reversed. The first question is how many kilograms this slab will permit; the second is how the waterproofing will be protected; the third is where water will enter and where it will leave. The choice of crop comes last of all. Yesterday's opening instalment described the roof as farmland stripped at the outset of the most basic freedom of all, depth of soil. Today we open up the contents of that constraint, following the numbers and the order of the layers.

What happens when that order is broken is quite clear. Pile the soil too deep and the structure will not hold. Tear the waterproofing at a single point and the floor below gets wet. Block the drains and water ponds on the roof, and that water becomes weight in its turn. Leave out irrigation and the whole thing dies within days of midsummer. The failures of rooftop farms arrive as failures of equipment rather than failures of agriculture. This article is therefore written as a piece about design, not as a growing guide. Its subject is four physics — weight, water, wind and heat — and every one of them is tied to the others.

The way they connect often runs against intuition. Weight, for instance, is the single greatest constraint on a rooftop farm, and at the same time almost the only defence it has against wind: the lighter the medium, the more readily it is carried away. Water is the lifeline of the plants and the gravest threat to the building, and soil that has drunk its fill is soil at its heaviest. What produces the cooling is not the soil itself but the water the soil is holding. Lighten one thing and something else grows weaker — designing a roof is nothing other than deciding where to draw the line in that tug of war.

In Japan, what a roof may carry is set out first by the table of imposed loads in Article 85 of the Building Standard Law Enforcement Order. Row (8) of that table names roof plazas and balconies and directs that their values follow row (1) — residential rooms, and bedrooms or sickrooms in non-residential buildings. The row (1) figures are 1,800 newtons per square metre when calculating the floor structure, 1,300 newtons for girders, columns or foundations, and 600 newtons when calculating seismic force. For buildings used as schools or department stores, however, the values of row (4), the sales floors of department stores and shops, apply instead: 2,900, 2,400 and 1,300 newtons.

Divided by gravitational acceleration, 1,800 newtons comes to roughly 184 kilogram-force. Up to 180 kilograms per square metre sounds generous when put that way, but the meaning of the number must not be mistaken. An imposed load is a statistical allowance for loads that move and change — people, furniture, stock. The soil, drainage material and trees of a green roof do not properly belong there at all. Saitama Prefecture's published material on rooftop greening likewise states that rooftop greening is limited by the building's imposed load and refers to the Enforcement Order. The allowance does indeed constrain what is possible; yet the soil itself belongs to a different category of load.

For a new building the matter is simple: fold the weight of the greening into the design conditions from the outset and, if need be, add beams and columns. The difficulty lies with existing roofs. How much margin was assumed at design time, and how much of it may legitimately be used, cannot be known without consulting the drawings and the structural calculation documents. One cannot say there is a roof, so it can be a field; one should say there are structural calculations, so it can be considered. When citizens set out to start a rooftop farm, the first obstacle is neither soil nor plants but a single set of documents.