Shearing layers refers to the rate of change of various layers in a built environment.
The work of Duffy and DEGW identified four shearing layers (Duffy, 1992):
- Shell – the traditional structure of the building that might last for 30-50 years.
- Services – cabling, plumbing, aircon that needs replacing every 15 years.
- Scenery – layout of partitions and dropped ceiling that last 5 years.
- Set – the layout of furniture that might change every few months, weeks, or even more frequently.
Steward Brand expanded this list:
- Site – This is the geographical setting, the urban location, and the legally defined lot, whose boundaries and context outlast generations of ephemeral buildings. “Site is eternal.” Duffy agrees.
- Structure – The foundation and load-bearing elements are perilous and expensive to change, so people don’t. These are the building. Structural life ranges from thirty to three hundred years (but few buildings make it past sixty for other reasons).
- Skin – Exterior surfaces now change every twenty years or so, to keep up with fashion or technology, or for wholesale repair. Recent focus on energy costs has led to re-engineered skins that are air-tight and better-insulated.
- Services – These are the working guts of a building: communications wiring, electrical wiring, plumbing, fire sprinkler systems, HVAC (heating, ventilating, and air conditioning), and moving parts like elevators and escalators. They wear out or obsolesce every seven to fifteen years. Many buildings are demolished early if their outdated systems are too deeply embedded to replace easily.
- Space Plan – The interior layout—where walls, ceilings, floors, and doors go. Turbulent commercial space can change every three years or so; exceptionally quiet homes might wait thirty years.
- Stuff – Chairs, desks, phones, pictures; kitchen appliances, lamps, hairbrushes; all the things that twitch around daily to monthly. Furniture is called mobile in Italian for good reason.
Software has shearing layers too:
- Site – foundational protocols (e.g. internet)
- Structure – infrastructure and architecture
- Skin – external interfaces such as APIs
- Services – implementation languages and libraries
- Space Plan – UX / wire-frame
- Stuff – UI that expresses the UX
- State – dynamic configuration of the software with use
Graffiticode inserts into the conventional tech stack a Services layer that exposes the domain semantics as task oriented languages. The developer or domain expert is able to express elements of the Services, Space Plan and Stuff that would normally have to be encoded in the implementation language and libraries of the system.