Floor-to-Floor Cost

Floor-to-floor height is made up of the structure, the space needed for building services and the desired clear height inside. Changing one layer can affect the others, along with the building’s total height and the number of floors allowed on a site.

A real section through one floor-to-floor

Section through one floor · 400 ft of perimeter · 12 floors · $110 per SF of facade

Assumptions, not a structural or code design. Floor-to-floor = finish floor 1.5 in + slab + plenum + ceiling assembly 2 in + clear height. Plenum = structure below the slab + services below it. A duct hung under the structure sits 2 in below it, with 1 in of insulation on each side and 3 in of clearance above the ceiling grid. A duct may pass through a beam web opening only if its insulated height is at most half the beam depth (an assumed limit); the plenum is then the beam depth plus 3 in. A 4 in sprinkler main rides in the bay and never sets the depth. Sections are drawn schematically. Facade cost assumes a 400 ft perimeter (120 × 80 ft), 12 floors and $110 per SF of facade, and ignores everything else that scales with height. Slab dead load assumes normal-weight concrete at 150 lb/ft³; a composite deck's 1.5 in flutes count as half concrete, plus about 3 psf of deck. The reference is the tight PT slab.

Contractors and trade partners can help test whether a proposed section is buildable and what it may cost. But the general contractor does not set the height alone. The architect, structural and mechanical engineers, owner and code constraints all shape the decision.

One floor, drawn as a section

The drawing cuts through one floor-to-floor, from the finished floor of one level to the finished floor of the next. From the top it shows the slab, the beam, the insulated supply duct, the sprinkler main, the ceiling grid hung on wires from the slab above, the finished ceiling line, the clear height and the finish floor. The dimension chain on the left adds them up. Everything between the finished ceiling and the slab above is the plenum: the structure under the slab plus the services under that structure. The clear height is mostly empty air, so the drawing shows it at 40% scale, with a break.

Each example changes one thing. The assumptions belong to the study: a 2 in ceiling assembly, a 1.5 in finish floor, 1 in of insulation on each side of the duct, 2 in between the structure and the duct, and 3 in between the lowest service and the ceiling grid. They are not a design for any building.

Tight PT slab. A 7 in post-tensioned slab has no beam below it, so the 10 in duct hangs directly under the slab. The plenum is 17 in. With a 108 in clear height, the floor-to-floor is 135.5 in, or 11.29 ft. The other examples are measured against this one.

Steel beam with the duct below. A 24 in steel beam under 6 in of deck and topping sends the duct underneath it. The structure takes 24 in and the services another 17 in, so the plenum is 41 in and the floor-to-floor is 158.5 in, or 13.21 ft. That is 23 in more than the PT slab.

Duct through the beam. Cut a web opening and the same 10 in duct passes through the beam instead of under it. The services shrink to the 3 in of clearance below the beam, the plenum falls to 27 in and the floor-to-floor to 144.5 in, or 12.04 ft. The drawing allows an opening only when the insulated duct is no more than half the beam depth. That limit is an assumption of the drawing; the structural engineer sets the real one.

A higher ceiling. Raise the clear height from 108 in to 120 in and nothing else moves. The floor-to-floor goes up by the same 12 in, to 156.5 in, or 13.04 ft. Clear height adds to floor-to-floor inch for inch.

A bigger duct. Swap the 10 in duct for a 16 in duct and its 18 in of insulated height no longer fits through the 24 in beam. It drops below the beam, the services grow to 23 in, the plenum becomes 47 in and the floor-to-floor 164.5 in, or 13.71 ft. A duct 6 in bigger costs 20 in of height.

A tall retail podium floor. A 144 in clear height, a 12 in slab, a 30 in concrete beam and a 20 in duct hung below it give a plenum of 57 in and a floor-to-floor of 216.5 in, or 18.04 ft. The clear height is still the biggest layer, but the plenum alone is more than three times the 17 in of the PT slab.

What an inch costs. The study assumes a 120 by 80 ft footprint, 12 floors and $110 per SF of facade. One extra inch of floor-to-floor is 33.3 SF of facade a floor, $3,667 a floor and $44,000 across all 12 floors. Against the PT slab, the steel beam with the duct below adds 23 in, about $1,012,000 of facade. Threading the duct through the beam cuts that to 9 in, or $396,000. The podium floor adds 81 in, or $3,564,000. These figures count facade only. Structure, shafts, risers, elevators and everything else that grows with height come on top.

The useful move is to coordinate section and systems early. A lower floor-to-floor dimension may reduce overall height, but could make routing or maintenance harder. A larger dimension has its own cost and envelope implications. Project-specific sections are more reliable than a universal minimum. The section is a shared design decision, and the drawings should make the consequences visible to the full team.