Coordination Drawings and Who Gets the Ceiling Space
Why this matters
The design drawings put every trade in the same corridor and say almost nothing about height. The mechanical plan shows the duct, the plumbing plan shows the sanitary main, the electrical plan shows the cable tray, and each sheet is correct on its own. Stack them and two of them occupy the same air. The first crew to hang its work wins the space, and the crew that comes second either reroutes on its own hours or asks the first to come down. Coordination drawings exist so that argument happens on paper, before anyone drills an anchor.
What a coordination drawing is, and what it is not
A coordination drawing is a layout prepared by the trades, not the designer, that overlays every system in a congested area at real size and real elevation: structure, sprinkler, plumbing, duct, hydronic pipe, conduit, cable tray and the ceiling with its fixtures. On larger jobs it is a shared 3D model with clash detection; on a small tenant improvement it may be plans and two or three cut sections drawn by hand. Commonly the mechanical (sheet metal) contractor draws the base because duct is the biggest thing in the ceiling, and each trade adds its layer and signs off.
It is not a design document. It shows how the installers intend to build what the design drawings already require. If coordinating reveals that the design cannot fit, the answer is an RFI to the designer, not a coordination drawing that quietly changes a duct size. See related: The Paper That Changes a Job: RFI to Change Order.
Where it is required, the requirement usually lives in Division 01 of the specification (commonly section 01 31 13, Project Coordination), which states who prepares the drawings, at what scale, which areas, and that no trade installs in a coordinated area before sign-off. On a job with no such section, the general contractor may still run the process, and a shop that is not invited should ask. See related: The MasterFormat Divisions a Service Trade Lives In.
The order of priority, and the reason behind each place in it
There is no single code that ranks the trades. The order below is the common convention, and a project's coordination spec can set a different one. What makes it hold is that each system is ranked by how little it can bend.
- Gravity drainage first. A sanitary or storm line must fall continuously at no less than the adopted plumbing code's minimum slope (IPC Table 704.1 in the model code, as adopted), so its elevation at every point is fixed by its two ends. It cannot go up and over anything without a trap problem or a vent problem. Everything else has to work around it.
- Large duct second. A 20 by 12 duct is a big rigid rectangle, and changing its shape to dodge something changes its pressure drop, which is the engineer's call. Offsets in duct cost fittings, static pressure and space.
- Sprinkler mains and large pressure pipe next. They can offset up and down because the water is pushed, but sprinkler pipe carries its own hanging, slope-to-drain and head-spacing rules under the fire sprinkler standard the jurisdiction has adopted, and heads must land where the reflected ceiling plan puts them. See related: Reflected Ceiling Plans and What Lives Above the Grid.
- Cable tray and large conduit banks after that. Tray is straight and needs space above it to lay cable in, but it can usually jog sideways.
- Small, flexible runs last. Individual conduit, small copper, control wiring and flex duct can bend around almost anything, so they take what is left.
The fixtures in the ceiling sit outside this ranking. A recessed light, a diffuser or an access panel occupies a fixed depth above the ceiling plane at a fixed spot on the ceiling grid, and that zone is reserved before anyone else is placed.
The cross-section that shows the fight
This is a corridor on the first floor of a two-story building, cut across its width. The elevations are above finished floor (AFF).
============ BOTTOM OF STRUCTURE 144 IN AFF ===========
LANE 1 (WALL SIDE) LANE 2 (CENTER)
SPRINKLER MAIN 4 IN SANITARY 4 IN, CROWN
138 TO 144 141 AT UPSTREAM END,
FALLS 1/8 IN PER FT
CABLE TRAY 12 X 4
120 TO 124 +-------------------+
| SUPPLY DUCT 20X12 |
| WRAPPED, 15 TALL |
| 116 TO 131 |
+-------------------+
LIGHT ZONE 108 TO 116 LIGHT ZONE 108 TO 116
------------- CEILING 108 IN AFF ---------------------
The design drawings put the duct and the sanitary main in the same corridor and gave neither an elevation. The plenum is 144 minus 108, or 36 in. The cross-section is where you find out that 36 in is not enough for both of them in lane 2 for the whole run.
Worked example: does the sanitary main clear the duct?
The fixed claims, from the ceiling up.
- Ceiling at 108 in AFF. The fixture schedule gives the recessed light housing a depth that, with a working allowance, the coordinator books as 8 in. Light zone: 108 to 116 in.
- Supply duct 20 by 12 with 1.5 in of insulation wrap on every face: 12 plus 1.5 plus 1.5 is 15 in tall. Hung directly above the light zone it occupies 116 to 131 in. Add 1 in for the hanger strap and the clearance a pipe hanger needs to pass over: nothing else may sit below 132 in in lane 2.
- Sanitary main, 4 in nominal, outside diameter about 4.5 in. It serves second-floor restrooms and leaves the upstream end with its crown at 141 in, 3 in under the structure for hangers and couplings. Its bottom at the upstream end is 141 minus 4.5, or 136.5 in.
The slope. The run is 48 ft long at 1/8 in per foot, the minimum the adopted plumbing code allows for this size in this jurisdiction. Fall over the run: 48 times 0.125 is 6.0 in. So the bottom of the pipe at the downstream end is 136.5 minus 6.0, or 130.5 in.
Where it crashes. The pipe bottom reaches the 132 in limit when 136.5 minus 0.125 times the distance equals 132, which is a distance of 4.5 divided by 0.125, or 36 ft. The main clears the duct for the first 36 ft and occupies the duct's space for the last 48 minus 36, or 12 ft, ending 132 minus 130.5, or 1.5 in, into the duct zone.
What cannot move. The sanitary cannot run flatter, because 1/8 in per foot is already the code minimum for that pipe. It cannot start higher, because its crown is already as close to the structure as the hangers allow. Its downstream end is set by where the stack or the building drain picks it up. Gravity drainage is first in the ranking for exactly this reason: nothing about its geometry is negotiable.
The three ways out, and who decides.
- Move the duct sideways. If the corridor is wide enough to give the duct its own lane, nothing changes shape and the fix is a coordination decision the trades can make and sign.
- Drop the main into lane 1. Possible only if the sprinkler main and the cable tray can give up width, which they often can because they are lower in the ranking.
- Flatten the duct. Changing to a 24 by 10 duct with the same wrap gives 10 plus 3, or 13 in, so it occupies 116 to 129 and the hanger limit becomes 130 in. The pipe's lowest point, 130.5 in, then clears by 0.5 in. That is a duct size change, which changes velocity and pressure drop, so it goes to the mechanical engineer as an RFI. Half an inch is also a margin that a sagging hanger or a coupling hub can eat, which the coordinator should say in the same RFI.
What changes the answer. If the run were 32 ft instead of 48, the fall would be 32 times 0.125, or 4.0 in, the downstream bottom would be 132.5 in, and the main would clear the 132 in limit along its whole length with 0.5 in to spare at the far end. Length is what turned this from a fit into a crash, which is why the coordinator checks the low end of every sloped line, not the middle.
What it costs to install before you sign off
Suppose the sheet metal crew is first on site, sees an empty corridor and hangs the 20 by 12 duct in lane 2 at the lowest height that clears the light zone, which is where the cross-section put it anyway. Two weeks later the plumber arrives. The last 12 ft of the main cannot be installed at legal slope. The plumber does not have the option of flattening it, so the choice is between dropping the duct, rerouting the main, or stopping.
The duct crew lowering 12 ft of duct is not on the table, because it is already as low as the lights allow. The real fix is moving the duct sideways, which means taking down hangers, re-anchoring, possibly new fittings at each end of the shift, and re-insulating the joints, by a crew that has moved on to another floor. On a small job that is two people for most of a day, plus the plumber's crew waiting. Who pays is a contract question and commonly lands on the trade that installed in a coordinated area without signing off, which is why a coordination spec says so in writing. The cheap version of this mistake is a sign-off sheet with one more signature on it.
The failure is not always the duct crew's. A plumber who runs the main first, at a nice steep 1/4 in per foot because steeper drains better, takes 12 in of fall across 48 ft where the design needed 6, and creates a crash nobody else could have predicted. Steeper is legal; it is not free.
What a shop signs when it signs a coordination drawing
A signature on a coordination drawing commonly means: our system fits as shown at these elevations, we will build it that way, and we will not move it without telling the other signers. It does not mean the system meets the design, the code or the manufacturer's clearances; those are checked separately, and a coordinator is not a reviewer. Whether a signature shifts the cost of a later conflict is set by the contract, and that is a question for the shop's own attorney.
Before signing, a shop checks three things on its own layer:
- Every elevation it needs is printed, at both ends of every sloped line and at every crossing, and each matches the shop's own calculation, not the coordinator's estimate.
- Its supports are drawn. Trapezes, seismic bracing where required, and hanger rods take space that a single-line layer hides.
- Its access is drawn. Valves, cleanouts, fire dampers, volume dampers, junction boxes and filter access need a clear path to an access panel or a lay-in tile, and a coordinated ceiling where the cleanout ends up behind the duct has coordinated the shop into a code problem.
When the ceiling is already full: renovation work
In occupied or existing buildings there is no clean plenum. Abandoned conduit, old duct and unlabeled pipe sit where the new work is drawn. Coordination there starts with a field survey: lift tiles, measure to the structure and to the bottom of what is already there, photograph, and draw the existing as its own layer before anyone adds new work to it. See related: How to Field-Verify Existing Conditions Before You Order. Nothing existing is removed because it looks dead until its owner trade proves it is, and any above-ceiling work over occupied space carries the facility's own permit and ceiling-access rules.
How to verify the coordination actually happened
- Every sloped line on the coordination drawing carries an elevation at both ends and at every crossing, and the difference between the two ends divided by the length is at or above the adopted minimum for its size.
- Every crossing has a stated vertical clearance, and none is a bare zero.
- Every trade with work in the area has signed, and the sign-off date is earlier than the first anchor.
- Any size or route that differs from the design drawings has an RFI number next to it, or the coordination drawing is wrong.
References
- See related: Multi-Trade Remodel Coordination Reference
- See related: Reflected Ceiling Plans and What Lives Above the Grid; The Paper That Changes a Job: RFI to Change Order
- See related: How to Work Out Drain Slope from Invert Elevations; How to Field-Verify Existing Conditions Before You Order
- Project Division 01 specification, section 01 31 13 Project Coordination where included in the project manual
- The plumbing code minimum slope table in the edition your jurisdiction has adopted (IPC Table 704.1 in the model code)