Hammock Activity

Definition

A Hammock Activity (also called a hammock task, summary span, or LOE — Level of Effort — activity in some tools) is a special scheduling element whose start and finish dates, and therefore duration, are derived from other activities rather than estimated independently. It "hangs" between two anchor points, like a hammock between two trees: as the trees move, so does the hammock.

Hammocks are used to model time-related but non-driving work — site management, project controls staffing, security, scaffolding rental, software platform licensing — where the cost or resource demand exists for as long as a specified span of other work exists, but does not itself drive the schedule.

History

Hammock activities entered mainstream scheduling tools in the 1980s, when EPC contractors needed to load indirect costs (site overhead, project staff, equipment rental) onto activities that mirrored execution windows without distorting the critical path. The AACE International Recommended Practices and the U.S. ANSI/EIA-748 EVM standard formalised the treatment of LOE activities and their relationship to discrete work.

Principles

• Derived, not estimated. A hammock's duration is calculated from its endpoints; it is not an input.
• Not on the critical path. A hammock cannot drive successors; its purpose is to follow, not to lead.
• Cost and resource loading only. Use a hammock to load time-related costs and resources, not to model discrete work.
• One hammock per logical span. Multiple overlapping hammocks for the same cost create double counting.
• Earn proportionally to time. In earned-value terms, hammock activities are LOE and earn against schedule, not against discrete progress.

Typical Uses

• Site management and supervision staff during a construction phase.
• Project controls, planning, and document control during a major package execution.
• Tower-crane rental, scaffolding hire, dewatering, temporary power.
• Software-licence costs that exist for as long as an enabling platform is in use.
• Insurance, bonds, and other time-based contractual costs.

Real-World Construction Example

On a hospital project the site-supervision team — project director, construction manager, two QA/QC engineers, an HSE officer, and four foremen — cost approximately USD 240,000 per month. The team modelled this as a hammock activity spanning from the first foundation excavation to the issue of the practical-completion certificate. As the foundation work slipped 6 weeks due to a design change, the hammock automatically extended, and the cost forecast updated to reflect six additional weeks of supervision overhead — USD 360,000 — without any manual re-keying. The hammock pattern prevented the classic indirect-cost surprise where direct costs are updated but time-related overheads are forgotten.

Real-World IT / Agile Example

A cloud-migration programme used a hammock to model the cost of the source-environment data-centre lease, which had to remain active until the last application had migrated. As migration waves slipped, the hammock extended, capturing roughly USD 90,000 per additional month of lease. Without that linkage, the financial model would have under-stated migration costs by USD 1.1 million across the programme's slip.

Project Controls Perspective

Controls teams treat hammocks with two specific cautions. First, they are LOE for earned-value purposes — they earn equal to plan as time passes, regardless of actual physical progress, which inflates EV during slow periods. ANSI/EIA-748 recommends LOE earn-value content stay below 15% of total project value; above that, performance metrics become noisy. Second, hammocks can mask cost overruns: a hammock that extends silently with the schedule looks like neutral re-baselining when in fact it is real overrun. A discipline of variance reporting at the hammock level catches this.

Common Mistakes

• Allowing a hammock to drive successors — it shouldn't, by definition.
• Loading hammocks with too much project value — EV metrics become meaningless.
• Creating multiple hammocks covering the same span, double-counting the overhead.
• Treating hammock extension as costless re-baselining instead of real overrun.
• Using hammocks to model discrete work — they hide accountability.
• Forgetting hammocks in time-impact analysis, missing indirect-cost exposure of delays.

Expert Tips

• Tag every hammock as LOE. Most tools allow it; the tag flags the activity for filtered earned-value reporting.
• Anchor hammocks to milestones, not activities. Milestones are stable reference points; activities can be deleted or re-numbered.
• Run a hammock-extension report monthly. Any hammock that has grown by more than 5% since baseline is a real overrun, not a re-baseline.
• Keep total LOE under 15% of project value. Beyond that, EV metrics lose meaning.
• Use hammocks in time-impact analysis. When modelling a delay claim, hammock extension is part of the legitimate indirect-cost calculation.

Key Takeaways

• A hammock derives its duration from other activities; it does not drive the schedule.
• Hammocks are for time-related costs and resources, not discrete work.
• LOE earn-value treatment means hammocks earn against time, not progress.
• Keep LOE under 15% of project value, or EV metrics will be noisy.
• Silent hammock extension is real overrun, not re-baselining — report variance accordingly.

Related Concepts

Hammock activities relate to CPM, Earned Value, Time-Impact Analysis, Cost Control, and Baseline Schedule. Hammock-modelling templates for indirect costs are at PMMilestone.org.