Spain's building height debate hits a snag: the drain stack

A 15-storey building: 840 flushes a day through one drain stack

Stacking homes solves the land problem and complicates something else: the drainage. That is the crux of a calculation circulating these days on a housing forum, putting numbers to an old hunch. If a four-storey block with four flats per floor generates 224 daily flushes through the same pipe, raising it to fifteen storeys sends the figure up to 840. Everything flows down a single drain stack.

From one flush every 4.2 minutes to one every minute

The exercise starts from a simple and somewhat debatable premise. It assigns 14 residents per flat, multiplies by the 16 flats in a low-rise building, and comes up with 224 flushes a day. Spread over 16 hours of activity, that works out at 14 per hour: nearly one every 4.2 minutes for whoever lives on the ground floor. The figure is not a measurement, it's a back-of-the-envelope estimate.

Using the same criteria, a 15-storey building with 60 flats concentrates 840 daily flushes. Over 16 hours, the average rises to 52 per hour. In other words, practically one flush per minute on the ground floor. The calculation also leaves out extended households —when more people live together than planned— which would only push the number higher.

How many flushes can a drain stack take before it clogs?

Nobody has run drainage calculation software on the matter, and that is the first caveat. A 500-millimetre pipe, half a metre in diameter, is proposed as an indicative figure, with no engineering verification to back it up. The reasonable question is what flow a standard drain stack can tolerate when dozens of flats discharge at once.

Building upwards as the housing fix

Some argue that building upwards is the way to end the housing crisis. Drainage rarely enters that equation. The objection is that land is saved, yes, but the infrastructure —drain stacks, sewers, treatment plants— is sized for a specific population volume, and that volume grows more slowly than the floors.

When the drain stack saturates and the network can't cope

The scenario that emerges is not a pretty one. Blockages, communities that don't pay for maintenance, and drains ending up discharging onto the façade, with the classic cry of Agua va ("water coming!") as the only waste management. Added to the list is coexistence: loud music until two in the morning.

And one step further: the question of when wastewater treatment stations, designed for a number of inhabitants that doesn't keep pace with construction, will start to overflow.

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Given all this, the sensible thing is for any serious debate about building height to end up factoring drainage into the design. Sensible and, judging by how housing is discussed, rather unlikely. The drain stack will remain the last thing anyone looks at.