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How We Fixed -19 Days of Negative Float in Primavera P6: A Construction Case Study
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At 7:40 on a Monday morning, our monthly schedule update for a hospital ward block finished calculating and the Total Float column on the fit-out path read -19 days. Four weeks earlier it had been -6. Three months earlier it had been +14. Nobody on the project — including me, the project controls lead — had raised a formal alarm until the number turned red.
This article is the honest post-mortem of that schedule: how the float eroded, why our early warnings failed, exactly which P6 mechanics were involved, and how we recovered 18 working days without adding a single crew. If you learn negative float from a story rather than a settings menu, this one is for you. Names and dates are altered; the mechanics are exactly as they happened.
Figure 0 — A Primavera P6 schedule with the negative-float anatomy annotated: data date, contract finish, driving activities, finish constraint, and the executive dashboard KPIs (SPI, CPI, schedule status, delay indicator) that first alerted the client.
The project, and the slow leak
The job was a four-storey ward block: structure complete, envelope closing out, and a heavily serviced fit-out ahead of a fixed handover to the health board. The completion date carried liquidated damages and — more importantly for the client — a patient decant plan booked eight months in advance. The Must Finish By date in P6 was genuinely immovable.
Here is the float on the driving path across six monthly updates. Notice that the story is not the month the float went negative. The story is the three months of steady erosion beforehand that we reported as line items and never as a trend.
Float erosion on the ward-block fit-out — six monthly updates
+25d
Baseline
+21d
Upd 1
+14d
Upd 2
+8d
Upd 3
+2d
Upd 4
6d
Upd 5
19d
Upd 6
Total float (days) on the fit-out driving path, six consecutive monthly updates. Green = healthy; amber = watch; red = negative.
Figure 1 — Total float on the fit-out driving path across six updates. The slope was visible from Update 2; the escalation happened at Update 5.
What we reported vs. what we should have reported
| Update | Total Float | What we reported | What we should have reported |
|---|---|---|---|
| Baseline | +25d | Comfortable buffer on fit-out | Buffer assumes level-by-level sequential fit-out — fragile assumption |
| Upd 1 | +21d | Minor slippage on envelope | Envelope slippage consumes fit-out float 1:1 — start tracking burn rate |
| Upd 2 | +14d | Drywall procurement delay noted | Float burn rate now 5–7d/month; at this rate, negative by Update 5 |
| Upd 3 | +8d | 'Recoverable' slippage on Level 2 | Recovery plan required NOW, while options are cheap |
| Upd 4 | +2d | Watching brief | Two days is zero days. Escalate. |
| Upd 5 | -6d | Escalated to project manager | Three months late |
| Upd 6 | -19d | Recovery workshop convened |
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📌 The first lesson. Negative float is a lagging indicator. Float burn rate is the leading one. From that project onward, every dashboard I build carries a float-erosion trend chart, and any path burning more than 20% of its float in a single period triggers a review — while it is still positive.
The forensic walk-through: three causes stacked on top of each other
When we finally sat down with the schedule, the -19 days was not one problem. It was three, compounding. This layering is typical — real schedules rarely fail from a single cause, which is why quick fixes so often disappoint.
Layer 1: Out-of-sequence progress strangled by Retained Logic (7 of the 19 days)
The mechanical contractor had jumped ahead: ductwork on Level 3 started while Level 2 ceilings — its predecessor in our network — were still open. Sensible on site, chaos in the model. Our scheduling option was Retained Logic, so P6 held the remaining Level 3 duct duration hostage until the Level 2 predecessor finished, pushing the forecast right and dragging float down. The work was actually ahead; the model said we were behind. Seven days of our negative float were fiction created by logic that no longer matched the build sequence.
Should We Have Flipped To Progress Override? Here Is The Comparison We Worked Through, Because The Difference Matters More Than Most Planners Realise
| Scheduling option | What P6 does with out-of-sequence work | Effect on float | When to use it |
|---|---|---|---|
| Retained Logic (default) | Holds remaining duration until the predecessor finishes, even if the successor has actually started. | Pushes dates right; float looks worse than reality. | Almost always. Then repair the logic to match the real sequence. |
| Progress Override | Ignores the incomplete predecessor for the started successor and continues. | Cosmetically improves float; hides broken logic. | Rare — and never as a permanent setting on a contract programme. |
| Actual Dates | Recalculates float using actual dates for started activities. | Similar to Retained Logic but honours actuals more strictly. | Sensible in forensic analysis; be explicit about it. |
We kept Retained Logic and spent half a day correcting relationships to the as-built sequence — converting the level-to-level Finish-to-Start relationships into Start-to-Start with lags that reflected the crews' actual leapfrog pattern. Seven days of negative float evaporated, honestly.
Layer 2: A milestone constraint nobody remembered adding (4 of the 19 days)
Buried on an interim milestone — "Level 1 Ready for Medical Equipment" — sat a Finish On or Before constraint from a superseded equipment delivery plan. The vendor had long since re-booked, but the constraint stayed, quietly compressing the backward pass on everything feeding that milestone. Four days of negative float were the ghost of a delivery date that no longer existed. We removed it, documented why in the update narrative, and added a standing agenda item: review every constraint, every update, against its current justification.
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Layer 3: Genuine delay (8 of the 19 days)
After the fictional float was cleared, eight days of real negative float remained: drywall procurement plus a rework issue on Level 2 had truly pushed the forecast past the immovable handover. This is the important discipline — separating artificial negative float from honest negative float — because the responses are completely different. You repair the first in the model; you recover the second on the site.
The recovery: 18 days without adding a crew
The knee-jerk suggestion at the recovery workshop was overtime and a second drywall crew. We modelled it first in a copy of the schedule — always model before you commit — and the crashing option bought only six days at significant cost, because the fit-out was sequence-bound, not resource-bound. The winning move was structural: abandoning the floor-by-floor sequence in favour of zoned, overlapped trades.
Sequential vs. zoned & overlapped fit-out
Before — Sequential fit-out
Level 1
Level 2
Level 3
Finish-to-Start between levels. Level 3 finishes 19 days past handover.
After — Zoned & overlapped
L1 Zone A
L1 Zone B
L2 Zone A
L2 Zone B
L3 Zone A
L3 Zone B
SS + lag between trades within each zone; FF between predecessor trades. Handover recovered with +4 days float.
Figure 2 — The re-sequenced fit-out. Splitting each level into zones let MEP second fix start 12 days into drywall instead of waiting for full-floor completion.
In P6 terms, the change was: split the level activities into zone activities, replace Finish-to-Start relationships with Start-to-Start plus lag between trades within each level, and keep Finish-to-Finish ties so no trade could finish ahead of its predecessor trade. The reschedule showed the handover recovered with four days of positive float restored. The site team validated the zone boundaries before we published anything — a re-sequence the foremen have not agreed to is a work of fiction with nice formatting.
Expert tip. When you present recovery options, present at least three and price all of them: do nothing (accept delay + LDs), crash (cost of acceleration), and re-sequence (cost of disruption/supervision). Management chooses faster and defends the choice better when they can see what was rejected and why.
What we changed permanently after this project
- Float trend charts on every report. One number tells you where you are; the trend tells you where you are going. The trend is the report.
- A constraint register. Every constraint in the schedule is listed with its justification, its source document, and a review date. Orphan constraints get deleted on sight.
- Out-of-sequence review as a standing update step. The Schedule Log's out-of
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