Project Management guide · Scheduling and time
Hammock Activity: Definition, Example and When to Use It
A hammock activity is a summary activity that spans a group of other activities and takes its duration from them. It starts when the first linked activity starts and finishes when the last one finishes, so it stretches or shrinks like a hammock hung between two points.
Key takeaways
- A hammock spans a group of activities and takes its duration from them.
- Link it SS from the first activity and FF from the last.
- Use it for level-of-effort support and time-based costs.
- It should follow the network, never drive it.
How a hammock activity works
- It is linked start-to-start to the first activity in the group and finish-to-finish to the last one.
- Its duration is calculated, not estimated: it equals the time between those two points.
- If the activities in between slip, the hammock automatically lengthens.
- It often carries resources or costs that run for the whole period, such as site supervision, equipment hire or project management.
Worked example
Using our standard network (A to F, critical path A-B-D-F, 14 days; see critical path method), the team adds a hammock called Site supervision linked SS to A and FF to F.
| Scenario | A starts | F finishes | Hammock duration | Supervision cost at $400 a day |
|---|---|---|---|---|
| Plan | Day 0 | Day 14 | 14 days | $5,600 |
| D slips 2 days | Day 0 | Day 16 | 16 days | $6,400 |
| D crashed by 2 days | Day 0 | Day 12 | 12 days | $4,800 |
Because the hammock follows the network, the cost of time-dependent support updates itself whenever the schedule changes.
Hammock vs summary task
| Hammock activity | Summary task | |
|---|---|---|
| Based on | Dependency links to chosen activities | The outline (WBS) hierarchy |
| Activities covered | Can span activities from different branches | Only its own child tasks |
| Can hold resources | Yes, commonly | Usually not recommended |
| Typical use | Level of effort support across a period | Grouping and rolling up a phase |
When to use one
- Level-of-effort work such as project management, quality assurance or site security.
- Equipment or facility hire that lasts as long as a set of activities.
- Summarising a cross-phase stretch for senior reports.
- Tracking time-related costs in earned value management.
How to create one
- Identify the first and last activities the hammock should span.
- Create a new activity with no fixed duration.
- Link it start-to-start from the first activity and finish-to-finish from the last.
- Assign the resources or costs that apply for the whole period.
- Check that it recalculates when you change durations upstream. Some tools have a dedicated hammock or level-of-effort activity type.
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Frequently asked questions
What is a hammock activity?
A summary activity whose duration is set by the start of one activity and the finish of another, so it changes automatically when they move.
Why is it called a hammock activity?
Because it hangs between two points in the schedule and stretches or shrinks as they move.
How is the duration of a hammock activity calculated?
It equals the finish of the last linked activity minus the start of the first linked activity.
Is a hammock activity on the critical path?
It is not usually treated as critical because it does not drive any other work, even though it spans critical activities.
What is the difference between a hammock activity and a summary task?
A summary task groups its child tasks in the WBS; a hammock is linked by dependencies and can span any activities.
Is a level-of-effort activity the same as a hammock activity?
Not exactly. A level-of-effort activity is support work, such as project management, that has no specific output and lasts as long as the work it supports. Hammock activities are often used to model it.
Can a hammock activity have resources?
Yes. Assigning time-based resources and costs to a hammock is one of its main uses.
Does Microsoft Project support hammock tasks?
It can be imitated by linking a task's start and finish dates to other tasks. Some other scheduling tools have a built-in hammock type.
Should a hammock activity drive other activities?
No. It should only follow other activities, otherwise you can create circular logic.
How does a hammock activity help with cost control?
Costs that depend on time update when the schedule moves, improving cost control and forecasts.