Shearing Layers

Shearing layers

Shearing layers is a way of thinking about how different parts of a system change at different speeds.

The idea comes from architect Frank Duffy and was later developed by Stewart Brand in How Buildings Learn: What Happens After They’re Built (1994). It was originally used to describe buildings, but it has since been adopted by technology vendors and architects as a useful way of thinking about organisational and technical systems.

Duffy described four layers in a building:

1. The shell is the core structure of the building. It might last 30 to 50 years.
2. The services are things like cabling, plumbing, heating, ventilation, and air conditioning. These tend to change every 15 years or so.
3. The scenery is the internal layout, including partitions, ceilings, and other elements that shape how the space is used. These might change every five years.
4. The set is the furniture and day-to-day arrangement of the space. This can change every few months, weeks, or even more frequently.

The underlying insight is that buildings are not single, fixed things. They are made up of parts that move at different speeds. Some parts are slow and durable. Others are fast and temporary.

Brand connected this to ideas from ecology and systems theory, where different processes operate at different timescales. In nature, fast-moving processes and slow-moving processes often interact only loosely. Brand applied that intuition to buildings. He argued that adaptable buildings allow these layers to move independently. Faster layers, such as services or furniture, should not be trapped by slower layers, such as the structural shell.

This matters because adaptability depends on allowing the layers to “slip” past each other. A building becomes harder to change when the fast-moving layers are too tightly bound to the slow-moving ones.

The same principle applies to technology and organisations. Some things should change slowly: core architecture, governance, identity, data models. Other things need to change quickly: interfaces, workflows, experiments, campaigns, local team practices. Problems arise when we treat all of these layers as if they should move at the same pace.

This leads to the design principle known as pace-layering, which Brand later described in The Clock of the Long Now (1999). The point is not simply to identify the layers. It is to design the relationships between them so that the system can keep adapting without having to rebuild everything at once.