Above-Ceiling / Plenum Space
The single most common reason architects lose ceiling height is a plenum that was reserved too shallow for the ducts, sprinkler mains, cable trays and drainage that must cross it. These are concept-stage rules of thumb to protect your floor-to-floor — the real figure comes from a coordinated services section.
| Space / service intensity | Typical clear void | Drives the depth |
|---|---|---|
| Light services (lighting, small VRF refrigerant, sprinkler only) | 450–600 mm | Largest branch duct + sprinkler |
| Office with ducted AHU + sprinkler + cable trays | 600–900 mm | Main supply/return duct depth |
| Retail / mall / lab — large ducts, multiple services | 900–1,200 mm | Large ducts crossing + layering |
| Service crossing / duct-over-duct or below deep beams | +150–300 mm extra | Crossings and beam soffit |
At schematic stage, reserve the void BELOW the structural beam soffit, not below the slab. For a typical ducted office, holding ~600–900 mm clear under the beams keeps a usable ceiling; tight floor-to-floor projects need an early coordinated services section.
Verify — These are rules of thumb. Actual plenum depth depends on the largest duct, beam depth, service crossings, fall on drainage and required maintenance access — confirm with a coordinated MEP services section.
Want this coordinated against your structure?
Send us your schematic and we'll mark up a coordinated services section — so your ceiling height and shafts survive detailed design. The other clearance guides, checklists and specification libraries are on the architects' hub.