Lags and leads

A lag is waiting time on a relationship. A lead is a negative lag: it lets the successor start before the relationship would normally allow.

Lags

Concrete has to cure before the forms come off. Nobody works during that time, so it isn’t an activity; it is a delay between two activities. In the lab:

• Pour slab finishes at the end of Day 5.
• Strip forms follows with FS+3.
• Days 6, 7 and 8 pass, and stripping starts on Day 9.

The lag sits on the relationship, so it moves with it. If the pour slips a day, stripping slips a day too.

Leads

A lead is a negative lag. Hang drywall follows Electrical rough-in with FS−2: drywall can start two days before rough-in finishes, because the electricians work room by room. Rough-in ends on Day 16, so drywall starts on Day 15 instead of Day 17.

Why to use them sparingly

Lags and leads are convenient, and that is the problem.

• A lag is invisible work. Nobody owns it and nobody can report progress on it. If curing actually needs five days, the schedule doesn’t show it until someone edits a number on a relationship.
• A lead hides an assumption. FS−2 says we will overlap these two activities by two days. What if rough-in is late in the rooms drywall needs first?

Schedule health checks reflect this. The DCMA 14-point assessment expects no leads at all, and lags on no more than 5% of relationships.

Two cleaner alternatives:

1. Make the wait an activity. Add Cure concrete (5 days) between pouring and stripping. It appears on the Gantt chart, it can be statused, and anyone reading the schedule sees it.
2. Replace a lead with an SS + FF pair. Drywall can start some days after rough-in starts and finish some days after it finishes. That states the overlap explicitly, at both ends.

One more catch: lags count working days on the lab’s Monday-to-Friday calendar. Concrete also cures at weekends. Real tools let you choose which calendar a lag uses; P6 has a project setting for it. A visible Cure activity on a seven-day calendar is another way to get it right.

Your turn

In the lab, the supplier says the slab needs 5 days to cure: change the lag and watch the finish move from Day 18 to Day 20. Then remove the lead from drywall (the finish moves again, to Day 22). Finally, replace the lag with a visible Cure concrete activity.