How to build a construction schedule around dependencies and lead times

A construction task can start only when both the predecessor work and the required material are ready. Set the start to the later of those two dates. This prevents a short installation task from appearing on time while the material needed for it is still in transit.

Construction schedule comparing 43 working days, 57 site days, and 14 waiting days.

Use the formula

Task start: later of predecessor finish or material arrival
Material wait: max(0, Material arrival - Predecessor finish)
Order-by date: Required-on-site date - Verified lead time

Worked example

In the verified bathroom schedule, the task sequence contains 43 crew working days but spans 57 site days. Fourteen site days are waiting rather than productive task duration.

Line	Result	Meaning
Crew working days	43 days	Task durations on working calendars
Site elapsed time	57 days	Start through finish on the site timeline
Waiting time	14 days	Delay caused by material readiness in the verified schedule
Shower pan order point	6 days before start	The lead-time control in the example

Repeatable method

List deliverables as tasks with clear completion conditions.
Give every dependent task one real predecessor or an explicit start constraint.
Assign durations using the crew calendar that will do the work.
Add required-on-site dates and verified supplier lead times.
Review the critical path and near-critical material constraints each week.

Questions and answers

What is the difference between working days and site days?: Working days measure task effort on the assigned calendar. Site days measure elapsed time from project start to finish, including weekends and waiting.
Does every material delay change the finish date?: No. A delay changes the finish only when it consumes available slack or affects the controlling path.
Should supplier lead time be a fixed default?: No. Verify it for the actual product and order date, then update the schedule when the supplier confirms a change.

Sources

Project Management Institute: From work breakdown to critical pathDependencies, durations, sequencing, and critical path development.
Project Management Institute: Schedule integrationSupply and calendar constraints in a realistic critical path.

This is general project planning information, not construction management, engineering, safety, contract, or legal advice.