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Scheduling

Schedule links and lags without the confusion

By Review.LivePublished 3 min readHow we write

In the update meeting, the drywall foreman points at level 4: the schedule holds taping until every board is hung, four working days after the tapers really start. On level 5, where hanging slipped three days, taping still shows its original finish, before the last board goes up.

A hand marks a printed building plan with a black pencil.
Photo: Pickawood on Unsplash

Use finish-to-start when the next activity needs the previous one finished; a university scheduling text notes it is the most common link. When the next can start once the first has a head start, use start-to-start with a lag, and add a finish-to-finish link so the follower cannot finish first. A lag is waiting time, not work. The schedule recalculates from these links whenever a date changes, so a wrong link hides a real overlap or lets work finish on paper before what it depends on.

The links in site terms

  • Finish-to-start: B starts when A finishes. Insulation inspected, then board.
  • Start-to-start: B starts a set time after A starts. Taping two days after hanging starts.
  • Finish-to-finish: B finishes a set time after A finishes. Taping a day after the last board.
  • Start-to-finish: B cannot finish until A starts. Rarely needed, and banned by some owners.
  1. Start with finish-to-start; change it only when the second crew really starts before the first finishes.
  2. Draw an overlap as a pair: start-to-start with the head start as its lag, finish-to-finish with the gap at the end. On its own, a start-to-start link leaves the first activity's finish driving nothing.
  3. Keep lags for waiting that needs no crew, such as curing. Inspections, reviews and deliveries go in as activities, each with someone responsible.
  4. Put each lag on the right calendar. Concrete cures on weekends too; Indiana's transportation department, for one, wants cure times, reviews and fabrication on calendars with no non-work days.
  5. Avoid leads and start-to-finish links. The Indiana and US Army Corps of Engineers specifications both prohibit them, and Indiana asks for the reason for every lag.
  6. Test before you issue: add three days to a leading activity in a copy, and check its followers move as the site would.

Common mistakes

  • Linking activities in the order the list shows them, not the order the work needs.
  • Parking an activity on a preferred date with a long lag.
  • Changing link types in an update without a note.

Checklist

Checking one activity's links

  • Finish-to-start unless the crews really overlap
  • Every start-to-start paired with a finish-to-finish
  • Lags only for waiting that needs no crew
  • Each lag on the right calendar, with a reason
  • No leads, no start-to-finish links
  • Tested: a slip moves the followers

Check your understanding

A schedule links "Submit steel shop drawings" to "Fabricate steel" finish-to-start with a 15-day lag. What would you change?

Show the answer
The lag stands in for the consultant's review, which is work by a named party. As a lag it has nobody responsible and no progress to record, so a late review shows only as late fabrication. As an activity, "Consultant review, 15 days", it can be updated, and a slip shows who held it.

Sources

  1. Project Management for Construction, Chapter 10: Fundamental Scheduling Procedures (10.6 Leads, Lags and Windows)Carnegie Mellon University · accessed
  2. Recurring Special Provision 108-C-215: Critical Path Method ScheduleIndiana Department of Transportation · Indiana, United States (one public owner's contract specification) · accessed
  3. UFGS 01 32 01.00 10 Project Schedule (3.3.11 Open Ended Logic; 3.3.16 Leads, Lags, and Start to Finish Relationships)U.S. Army Corps of Engineers, via the Whole Building Design Guide · United States federal construction contracts that use this guide specification · accessed

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