Hard Logic vs Soft Logic
Last reviewed 16 September 20268 min read
Physics vs preference; why the distinction decides claims
🟩 In one line: Hard logic is a relationship the physical work forces (you can't pour the slab before the columns); soft logic is a relationship you chose for resources, access or preference (pour Zone A before Zone B). Both are legitimate in a baseline, but when a delay hits, soft logic is the first thing a reviewer asks you to change — and the first thing an analyst asks you to justify keeping.
👤 Who this is for: Juniors building logic; mid-level planners defending a critical path; seniors deciding whether a "delay" is really a sequencing choice. You should know relationship-types-explained and total-float.
First, let's be honest about why this page exists
Planners draw soft logic all the time and forget it's soft. Two years later the critical path runs through "Block C before Block D", the Employer delays Block C, and the contractor claims the whole project moved. The Engineer's reply: "Why didn't you just start Block D?" If the answer is "because the programme said so", the claim is in trouble.
The reverse also happens: a reviewer strips out soft logic to make float appear, and the contractor loses a legitimate resource constraint because nobody wrote down why the link was there. The distinction is not academic. It decides who owes whom time.
🟨 The standard — what "good" looks like
| Source | What it says (paraphrased) |
|---|---|
| SCL Protocol 2nd ed., Part B Guidance on programme preparation and on mitigation | Programme logic should reflect a realistic plan; when delay occurs, the Contractor is expected to take reasonable steps to mitigate, which includes re-sequencing where soft logic allows |
| AACE RP 29R-03, section on logic validation | Distinguishes physical (mandatory) logic from preferential/resource logic; analyst must identify which governs the delay |
| AACE RP 49R-06, Identifying the Critical Path | Preferential logic can create a critical path that isn't the true minimum path; analyst should test it |
| PMI Practice Standard for Scheduling, 3rd ed. | Mandatory dependencies (hard) vs discretionary dependencies (soft) — discretionary should be documented |
| GAO Schedule Assessment Guide, Best Practice 2 (sequencing) | Logic should reflect how the work must be done; "soft" or "preferred" logic should be identified and justified |
| DCMA 14-Point, Metrics 1–4 (logic, leads, lags, relationship types) | Doesn't distinguish hard/soft but flags the patterns soft logic often produces — SS/FF chains, lags used as sequencing |
| NEC3/NEC4 Cl 31.2 | Programme shows the order and timing of operations the Contractor plans — soft logic is expressly permitted as "how the Contractor plans" |
🟢 Rule to remember: every soft link in the baseline gets a reason in the Notebook. If you can't write the reason, it's a preference you'll be asked to abandon.
How it actually works
The three kinds of logic.
| Type | Also called | Driven by | Example | Can it be changed in a recovery? |
|---|---|---|---|---|
| Hard | Mandatory, physical | Physics, sequence of construction, safety | Formwork before rebar before pour; waterproofing before backfill; MEP first fix before wall closure | No |
| Soft — resource | Preferential, resource logic | Limited crews, one crane, one hoist | Level 5 pour after Level 4 pour (same formwork set); Zone B blockwork after Zone A (one crew) | Yes, at a cost — second crew, second set |
| Soft — preference | Preferential, discretionary | Access route, client preference, convenience | Finish car park before landscaping (to keep access); build east tower first | Yes, often at little cost |
Some add a fourth: external — driven by third parties (authority approval before energisation). Treat it as hard for the contractor's purposes, but note who owns the dependency.
How to tell them apart on a real link. Ask one question: if I had unlimited people, equipment and access, would this link still be needed? Yes → hard. No → soft.
Where soft logic belongs.
| Situation | Model it as |
|---|---|
| Single crew moving zone to zone | Soft FS links, Notebook "one crew, resource logic"; or resource-load and level |
| One tower crane serving two blocks | Soft links between crane-dependent activities, Notebook "TC1 shared" |
| Client wants Phase 1 handed over first | Soft links or a Finish On or Before milestone with reason |
| Access route through Zone A | Soft link with Notebook "site logistics — access via Zone A until road complete" |
P6 mechanics. P6 has no field for hard/soft. Options:
| Method | How | Pros / cons |
|---|---|---|
| Relationship comment | Activity Details > Relationships > Comments column (P6 R8.3 onward) | Best — lives on the link; exports in TASKPRED comments field |
| Activity Notebook topic "Logic Basis" | Admin > Admin Categories > Notebook Topics; note per activity | Good for chains; not per link |
| Activity code "Logic Type" on successor | Codes: HARD / SOFT-RES / SOFT-PREF | Filterable; per activity not per link |
| Layout filter on relationship comments | Filter successors where comment contains "SOFT" | Quick review before submission |
How the distinction plays in delay.
| Scenario | Hard logic on the path | Soft logic on the path |
|---|---|---|
| Employer delays a predecessor | Successor genuinely can't start; delay flows through | Engineer asks why you didn't re-sequence; you need the resource reason documented |
| Contractor asked to recover | Only crashing (more hours/resources) available | Re-sequencing available and cheaper — usually the first mitigation |
| Forensic analyst tests the as-planned critical path | Path stands | Analyst may remove soft links to find the "true" longest path; if the reason was documented, it stands as resource logic |
| Float ownership argument | Float is what it is | Employer argues soft logic hid float; contractor argues it's a genuine resource constraint |
📜 Mitigation and re-sequencing
Under SCL Protocol guidance and most standard forms, the Contractor must take reasonable steps to mitigate — not incur significant extra cost, but not sit still either. Re-sequencing through soft logic is the textbook "reasonable step". So:
- If the delayed activity is followed by soft preference logic, you are expected to re-sequence and the EOT is reduced accordingly.
- If it's soft resource logic, you're expected to consider re-sequencing; if that needs a second crew or second formwork set, that's acceleration, not mitigation — you can offer it but the cost sits with the Employer if the delay is theirs.
- If it's hard logic, there's nothing to re-sequence; the claim proceeds.
Document the reason at baseline stage and this conversation takes five minutes instead of five weeks.
🟥 Where people go wrong
- Drawing soft logic and calling it hard when challenged. "The slab can't start until the other block's slab is done" — physically it can; you only have one formwork set. Say so. The resource reason is legitimate; the fake physical reason gets exposed in the first site visit.
- Stripping all soft logic to please a reviewer. Now every zone starts on day one with unlimited crews, float appears everywhere, and the programme is a fantasy. Soft resource logic is real; keep it and label it.
- Not documenting the reason. Eighteen months on, nobody knows whether "Block C before D" was crane, crew or the client's preference. Now it's whichever side argues better.
- Using lags as soft logic. "FS +10 days" to hold a start back for crew availability. Reviewers see an unexplained lag; DCMA Metric 3 flags it. Use a link to the real driver, or a constraint with a reason (leads-and-lags).
- Confusing soft logic with a constraint. A Start On or After for access is a constraint with an external cause. A soft link is your choice. Don't put a constraint where a link to the access milestone would do.
- Building the tender programme on soft logic, then treating it as the contract sequence. The client accepted a plan, not a physical law. If you change the sequence for your own reasons, you don't get time for it.
⚖️ When you're challenged
"Why didn't you just start Zone B when Zone A was delayed?" "The link is resource logic — one blockwork crew, stated in the Notebook and the SBM at baseline. Starting Zone B meant a second crew, which we offered as acceleration on 14 March. Without that instruction, the delay to Zone A flows through."
"This link is soft. We're removing it and your float goes to +12." "It's soft resource logic, not preference — the formwork set is the driver. Remove the link and the programme assumes a second set we don't have and weren't priced for. If you want the float, instruct the second set."
"Your critical path is artificial — it's all preferential." "Every link on it is tagged. Six are physical, three are shared tower crane, one is the access route through Zone A. Each has a reason. Show me which one you think is optional and we'll test it."
"Can we re-sequence to recover?" "Yes — the soft links are filtered in this layout. Re-sequencing façade Zones B and C saves 5 days at no cost. Anything else needs extra resource."
📄 Related pages
- Relationship types explained — FS/SS/FF/SF mechanics
- Leads and lags — why lags are the wrong tool for soft sequencing
- Construction Sequencing and Method — where hard logic comes from
- Standard Sequences — reference hard logic by trade
- Mitigation and the duty to mitigate — what re-sequencing you owe
- Schedule Basis Memorandum — where resource logic reasons get recorded
- Critical path vs longest path — why preferential logic can hide the true path
- Building a Recovery Plan — soft logic as the first lever
✏️ Worked example
A hospital extension in Abu Dhabi. Employer delays access to the east wing by 30 days. Baseline critical path: east wing structure → east wing envelope → east wing MEP → commissioning. West wing was sequenced after east wing.
| Link on path | Type | Reason recorded | Reviewer's position | Outcome |
|---|---|---|---|---|
| East wing access → east wing piling | External / hard | Employer possession date | Accepted | Delay starts here |
| East wing piling → east wing raft | Hard | Physical | Accepted | Flows through |
| East wing raft → west wing raft | Soft — resource | "One piling rig, one raft crew — SBM 6.4" | Challenged: "start west first" | Contractor showed rig demobilisation cost; Engineer instructed re-sequence as acceleration, cost paid |
| East wing envelope → east wing MEP first fix | Hard | Weathertight before services | Accepted | Flows through |
| East wing MEP → west wing MEP | Soft — preference | Not recorded | Challenged | Contractor couldn't show a reason; re-sequenced at own cost; EOT reduced |
Result: 30 days of Employer delay, 22 days awarded. The 8-day reduction came entirely from the one soft link with no reason written down.
📖 References
- SCL Delay and Disruption Protocol, 2nd ed. (2017), Part B Guidance — programme preparation; mitigation
- AACE RP 29R-03, Forensic Schedule Analysis — logic validation
- AACE RP 49R-06, Identifying the Critical Path
- PMI, Practice Standard for Scheduling, 3rd ed. — dependency types
- GAO, Schedule Assessment Guide (GAO-16-89G), Best Practice 2
- NEC3 / NEC4 ECC Cl 31.2 (check the edition in your contract)
- Oracle, P6 Professional User Guide — Relationships, Notebook Topics
From the field
Notes, corrections and additions from people who've done this on a real job. Everything is checked before it appears here.
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