AEO Answer · MEP

How Are Mechanical Shafts and Chases Designed in Tall Buildings?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of building MEP systems: ductwork with diffusers, an electrical panel with conduit, and piping runs.

The concise answer

Mechanical shafts and chases are vertical enclosures sized to carry all duct, pipe, and conduit risers with working clearance, then fire-rated to match the floor assembly. Good shaft design balances system space against rentable area, provides access for installation and maintenance, and details every penetration with rated dampers and firestopping. The reason shafts deserve design attention is leverage: a shaft decision made in design development affects every floor of the building, every trade's installation sequence, and decades of maintenance. It's one of the few places where an hour of engineering in design saves weeks of field labor and a lifetime of service headaches.

What the design has to resolve

Shaft sizing starts with a riser diagram at real scale: every duct riser with its insulation, every pipe riser with room for thermal expansion, every conduit and tray run, plus the working clearance installers need to actually connect things. I want to see this layout before the floor plans are finalized, because shaft locations and sizes are nearly impossible to change once structure and architecture are set. Fire protection is the second design driver. Shafts are vertical openings in the floor assembly, which makes them smoke and fire pathways — the code treats them accordingly, with rating requirements, damper requirements at penetrations, and continuity details at the top and bottom. The MEP engineer and the fire protection engineer have to agree on every penetration detail, and the commissioning agent should verify dampers and firestopping before the shaft gets closed up.

Shaft design rules I enforce

Shafts are unforgiving of late changes, so the design has to be complete and honest. These are my non-negotiables.

Size from real layouts: draw every riser at installation scale with insulation, hangers, and valve clearance
Design for the installer and the maintainer: if a human can't reach it, it can't be serviced
Rate and protect every penetration: fire dampers for ducts, listed firestopping for pipe and conduit
Coordinate shaft walls with structure: shaft enclosures, supports, and seismic bracing need structural backing
Leave growth space: buildings change use over decades — a shaft with no spare capacity has no future

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?Smoke control systems explained Fire alarm notification design Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What's the difference between a shaft and a chase?

A shaft is typically a full-height rated enclosure — often with its own walls and fire rating — that runs the height of the building. A chase is usually a smaller, sometimes unrated enclosure or framed cavity serving a few floors. The distinction matters for fire code: shafts have specific rating, damper, and continuity requirements that chases may not, depending on height and occupancy.

How big should a mechanical shaft be?

Big enough for the actual risers plus installation and maintenance clearance — which I determine by laying out the duct, pipe, and conduit at real size with insulation, hangers, and valve access. There's no rule of thumb I trust; an undersized shaft discovered during construction means rerouting systems through occupied space or stealing area from the floor plan. Size it from the systems, in design, with the trades' input.

Do shafts need fire dampers at every floor?

Penetrations of a rated shaft generally require protection — fire dampers for duct penetrations and firestopping for pipe and conduit — per the building code's requirements for the shaft's rating. The exact requirements depend on the occupancy, the shaft rating, and whether the penetrating system is continuous. This is detailed in the fire protection design and verified during commissioning.

How do you maintain systems inside a shaft?

By designing access in from the start: access panels or doors at each floor for valves, dampers, and cleanouts, and enough working room inside the shaft for a technician. Systems buried in an inaccessible shaft become replace-instead-of-repair problems. I flag any shaft layout where I can't point to how a valve gets serviced.

Ready to discuss your engineering scope?

Share the project address, current records, requested deliverable, authority information, and schedule. Apex Grid confirms professional responsibility, availability, and scope before work begins.

Request a Project Review