Introduction
Dependency types in construction scheduling are the backbone of modern project management. Without clearly defined logical relationships between activities, even well-funded and well-resourced projects drift into chaos: crews arrive before prerequisites are complete, equipment sits idle, costs escalate, and schedules collapse. This is where dependency types (FS, SS, FF, SF) become critical.
In construction projects—whether it’s a residential building, highway infrastructure, industrial plant, or high-rise commercial complex—every activity depends on another. Excavation depends on site clearance, concreting depends on formwork, finishing depends on curing, and commissioning depends on installation. These relationships are not random; they follow structured logic known as construction dependency types.
Understanding these relationships allows project managers, planners, engineers, and contractors to build reliable project schedules, control critical paths, reduce delays, and optimize resources. Poor dependency planning is one of the top reasons projects exceed time and cost targets. Studies show that poorly sequenced schedules can increase project durations by 15–25% due to idle time, rework, and conflicts.
This comprehensive guide explains Dependency Types (FS, SS, FF, SF) in practical construction terms. It goes beyond theory into real-world applications, worked examples, formulas, case studies, tools, mistakes, and implementation checklists. By the end, project teams will know exactly how to structure dependencies to build realistic, risk-resilient construction schedules.
Table of Contents
Why Dependency Types Matter in Construction
Schedule Reliability
Correct dependency logic ensures that activities occur in the right sequence. Incorrect logic creates false float, unrealistic timelines, and misleading critical paths.
Cost Control
Idle labor, idle equipment, and rework directly increase costs. Poor sequencing can increase project costs by 8–12% through inefficiencies alone.
Risk Reduction
Dependencies reduce risks such as:
- Safety hazards from overlapping incompatible works
- Structural risks from premature loading
- Quality failures due to improper curing or sequencing
Contractual Compliance
Most contracts require logic-linked schedules (Primavera P6/MS Project). Poor dependency modeling can lead to claim rejections and delay disputes.
Fundamental Concepts of Construction Dependencies
Activity Relationships
Dependencies define how one activity relates to another:
- Logical sequence
- Physical constraints
- Resource constraints
- Safety constraints
Predecessor and Successor
- Predecessor: Activity that must occur first
- Successor: Activity that follows
Network Logic
Dependencies form a network diagram that determines:
- Critical path
- Float values
- Schedule sensitivity
Formula (Critical Path Basis)
Project Duration:
D=max(∑tcritical)
Where:
- tcritical = duration of activities on critical path
Overview of the Four Dependency Types (FS, SS, FF, SF)
| Type | Full Name | Meaning | Construction Example |
|---|---|---|---|
| FS | Finish-to-Start | Successor starts after predecessor finishes | Brickwork after concrete curing |
| SS | Start-to-Start | Successor starts after predecessor starts | Electrical rough-in after wall framing starts |
| FF | Finish-to-Finish | Successor finishes after predecessor finishes | Testing finishes after installation finishes |
| SF | Start-to-Finish | Successor finishes after predecessor starts | Shift handover operations |
Finish-to-Start (FS) Dependency Explained
Definition
Successor cannot start until predecessor is complete.
Construction Examples
- Excavation → Foundation concreting
- Formwork removal → Finishing works
Worked Example (Calculation)
If excavation = 5 days, foundation concreting = 3 days:
Start(Foundation) = Finish(Excavation)
Total duration = 5 + 3 = 8 days
Checklist
- Verify physical necessity
- Confirm safety clearance
- Validate material readiness
Start-to-Start (SS) Dependency Explained
Definition
Successor starts after predecessor starts.
Example
Wall framing (Day 1 start) → Electrical rough-in (Day 2 start)
Practical Use
Used to compress schedules without compromising logic.
Finish-to-Finish (FF) Dependency Explained
Definition
Successor finishes after predecessor finishes.
Example
System testing finishes after installation finishes.
Start-to-Finish (SF) Dependency Explained
Definition
Successor finishes after predecessor starts.
Example
Old system shutdown completes after new system starts.
Lag and Lead Time in Dependencies
Formula
Startsuccessor=Finishpredecessor+Lag
Example
Concrete curing lag = 7 days before formwork removal.
Practical Scheduling Methodologies Using Dependencies
Step-by-Step Process
- Define WBS
- Identify physical constraints
- Assign dependency type
- Add lag/lead
- Validate critical path
Resource Optimization Checklist
- Labor leveling
- Equipment balancing
- Material delivery sync
Advanced Applications in Large-Scale Projects
Mega Infrastructure Projects
- Parallel sequencing using SS
- Commissioning using FF
Risk-Based Scheduling
Dependencies linked to risk buffers.
Tools & Software for Managing Dependencies
- Primavera P6
- Microsoft Project
- Oracle Aconex
- BIM 4D (Navisworks)
Common Mistakes and Solutions
- Overusing FS → Use SS where possible
- Ignoring lags → Add curing/setup lags
- Wrong logic links → Validate physically
- No risk buffers → Add contingency logic
- Software default logic → Manual verification
Detailed Case Studies
Case Study 1: Residential High-Rise Project
SS dependencies reduced project duration by 18%.
Case Study 2: Highway Project
FF logic improved commissioning coordination.
Frequently Asked Questions (FAQ)
Q1: Which dependency is most common?
FS is most common in construction.
Q2: When to use SS?
For parallel activities.
Q3: Are SF dependencies practical?
Rare but useful in operations.
Q4: Do dependencies affect claims?
Yes, incorrect logic weakens claims.
Q5: Can dependencies reduce cost?
Yes, through reduced idle time.
Q6: Which tools support dependencies?
Primavera, MS Project, BIM 4D.
Conclusion
Dependency Types in Construction Scheduling (FS, SS, FF, SF) are not just planning theory—they are operational control systems for real projects. When properly applied, they improve time performance, reduce costs, enhance safety, and strengthen contractual compliance. Projects that apply optimized dependency logic consistently achieve 10–25% schedule efficiency improvements.
Construction teams that master dependency modeling build resilient schedules that adapt to risk, resource constraints, and real-world site conditions. Proper dependency logic transforms scheduling from a reporting tool into a decision-making system.
Related Articles (Famcod)
- Critical Path Method (CPM) Guide for Construction 2026
- Dependency Types in Construction Scheduling (FS, SS, FF, SF) Explained: Complete Guide 2026
- Float Calculation Methods: Total Float vs Free Float in Construction Scheduling
- Schedule Compression Techniques in Construction: Crashing vs Fast-Tracking Explained
- Resource Allocation Strategies in Planning for Construction Projects
Recommended Resources
“Construction Scheduling: Principles and Practices” – Practical scheduling guide
[Amazon Affiliate Link]
“Project Management for Construction” – Comprehensive reference
[Amazon Affiliate Link]
“Critical Path Method in Construction”
[Amazon Affiliate Link]
Coursera: “Construction Project Management”
[Coursera Affiliate Link]
Coursera: “Planning and Scheduling Using Primavera”
[Coursera Affiliate Link]
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