Deadlines and negative float

So far the backward pass has started from the calculated finish, so the critical path always had zero float. Real projects have dates that matter: a contract completion, a shutdown window, a launch event.

Where negative float comes from

When a project has a deadline (in P6, a Must finish by date), the backward pass starts from the deadline instead of from the calculated finish. If the work fits, every activity gets positive float. If it doesn’t, the late dates come out earlier than the early dates, and total float goes negative.

Negative float is the number of days a path has to recover. It is a warning, not an error.

The fit-out, late

In the lab, the light fittings now take 5 days and the client has fixed Handover to the end of Day 14. The lighting path now needs 15 days:

ID	Activity	Total float
A1000	Start	−1
A1010	Strip out the old fit-out	−1
A1020	Build partition walls	0
A1030	Install ceilings	0
A1040	Order light fittings	−1
A1050	Install lighting	−1
A1060	Final clean	−1
A1070	Handover	−1

Every activity on the lighting path is one day short. The walls-and-ceilings path still fits the deadline exactly, so it shows zero. The table tells you where to look: the problem is on the path with −1, and fixing anything else won’t help.

Activity constraints can do the same thing. A finish no later than constraint on one activity sets a ceiling on its late finish, and if the logic can’t meet it, the float goes negative on the path leading to it.

Recovering

Sound options, roughly in order of preference:

1. Re-check the logic and durations on the negative path. Is every relationship real? Is every duration honest?
2. Crash: add resources to shorten an activity on the path.
3. Fast-track: overlap activities on the path, accepting the risk.
4. Change scope, or negotiate the date.

What not to do: delete relationships, or pin activities with constraints until the number turns positive. The float looks better, but the work hasn’t changed, and the schedule no longer tells the truth.

Your turn

In the lab, check which activities show −1 and why A1030 shows 0. Then recover the deadline without touching the logic or the late delivery: crash A1050 Install lighting until no activity has negative float.