Lean Scheduling:
An Innovation in Construction Scheduling

By Benjamin B. Hyden, Guest Author and Bricker & Eckler Construction Fellow

Reprinted from October 2006 ohioconstructionlaw.com

Highlights: Following up on last month’s discussion of scheduling, this article examines a relatively new approach to proactive scheduling that goes a step further than the Critical Path Method. Lean Scheduling takes a leaf from auto manufacturing’s book and focuses on controlling the scheduling process through a master “pull” schedule, Look Ahead Schedules that change weekly, a Weekly Work Plan for each contractor, and a PPC report (summarizing the Percentage of Plan work Completed). While the new system is not without costs of its own, its advocates think that having a more reliable workflow and reducing waste make it worthwhile.

Scheduling Developments and Technology

Last month, Jennifer J. Chapman started our discussion on construction scheduling by describing a construction schedule, how it’s created, and how it should be used on a construction project. As she explained, the construction schedule is a powerful tool and important to the owner. Maintaining the schedule requires attention to detail and a concerted effort of all parties. Still, a schedule can be a difficult tool to use and follow.

As our technological competence increases, however, construction scheduling should become easier. The traditional Critical Path Method (known as “CPM”) is considered the gold standard for managing construction projects. The CPM was developed in the 1950s by a joint venture between two manufacturing companies who used it to manage their plant maintenance projects. The method must have looked very different before the days of the personal computer.

Today, producing a CPM schedule is simple: using the software, all a project manager has to do is input a task with its duration and link that task to other tasks that must be completed in order for the first task to begin. Once the project manager has input all of the tasks and durations, a computer program usually tracks the critical path.

Modern computer programs have become so powerful that contractors and owners can evaluate the schedule, document the history of the schedule, and even create hypothetical situations with just a couple of computer keystrokes. Yet, even with such powerful scheduling tools, many construction projects still aren’t completed on schedule.

In the late 1990s, a research organization called the Lean Construction Institute (or the “LCI”) was created to reform the management of production in construction. The LCI set out to apply principles pioneered in manufacturing to construction. Just as it did with the CPM over 40 years before, the construction industry again looked to the manufacturing industry for improved management practices. Specifically, the LCI studied a management technique known as “lean production.”

Lean Production

According to the Lean Construction Institute, the lean production practice originated by Toyota is applicable to construction. Lean production was created after Toyota engineers toured the production facilities of other automobile makers and decided that the normal mass production system produced too much waste.

At the time, it was the philosophy of the automobile manufacturers to keep every machine running in the plant at maximum production. This meant the manufacturers stockpiled raw materials and stockpiled intermediate inventories. The pressure created to keep the assembly line moving and every machine operating resulted in defects that were built into the car. Defects resulted in disruption of the production line or left completed cars with defective components.

With the goal of reducing waste, the Toyota engineers began looking for new ways to produce cars. The first goal was to remove rework due to errors. The outcome was a process that focused on the entire production system and not just the local concerns of keeping each machine running. At one point, the engineers required that all assembly line workers who received a defective part or a defectively assembled product stop the assembly line. (This was no longer necessary after control was perfected.)

Next, the engineers reduced inventory by creating a system that allowed the downstream demand to dictate the upstream production. In other words, instead of pushing large amounts of inventory through the machines, the upstream demand pulled the required inventory through the machines. The result was the production of only non-defective inventory, which was immediately pulled upstream into the next activity. This low inventory approach reduced the cost of changes and saved the cash required to buy inventory.

The goal of the system was to produce a reliable workflow, not to increase the speed of any one particular activity. A reliable workflow was created when every actor in the process received from the upstream activity a product that was not defective and met the customer’s specifications.

Applying the Principles to Lean Scheduling

By implementing the principles of lean production, the LCI has developed a Lean Scheduling technique for construction with a central principle of production “control.” According to the LCI, control is proactive. Rather than merely looking at planned construction and actual construction and identifying the variances, the scheduler causes the desired future events to occur as planned. The goal of a lean schedule is to cause the work on a construction project to occur as scheduled while meeting the owner’s specifications.

To understand how Lean Scheduling improves upon the CPM method, let’s contrast the two. How is a schedule created under the CPM method? First, the project is broken into pieces, and then the pieces are put together again in a logical sequence, estimating the time and resources required to complete each activity. Each piece or activity is further broken down until it is assigned to a responsible party, and the process continues at nearly every level of the project: The construction manager assigns activities to a contractor, a contractor assigns an activity to a subcontractor, and in the end the activity is assigned to a foreman.

During construction, the activities are monitored. If the activities along the critical path fall behind, the construction manager acts to reduce the duration of the offending activity or change the sequence of work. If these steps do not work, the construction manager will trade cost for schedule—paying to have the work accelerated—by working out the best sequence to make progress.

According to the LCI, the CPM’s attempt to manage the project activity-by-activity hides the waste caused when there is not a predictable release of work and arrival of needed resources between the activities. Where the CPM method may reward a contractor or subcontractor for finishing ahead of schedule and, therefore, shortening the critical path, the lean approach will punish the contractor or subcontractor because the activity was not completed when promised and, therefore, resources were wasted because the construction manager was forced to communicate with the subsequent contractor that work was available.

The Lean Schedule attempts to reduce variability in construction scheduling by managing the interaction between activities. This is accomplished by one simple principle: creating a reliable flow of work by matching labor to work. Work is planned in what is called a “closed loop” system. Work is not dictated by a master schedule, but is planned by the foreman or “last planner” who makes detailed short-term plans based on such considerations as available materials, weather forecasts, and whether the previous crew is reliable and can be trusted to be out of the way.

The reliable release of work should result in productivity gains and a significant reduction in construction site conflicts and wasted resources. Lean Scheduling attempts to minimize two types of “waste”: (1) wasted time, which is the estimated 20% to 30% idle time spent by workers on a construction project; and (2) wasted space and money caused by the need to stockpile material on the project site. Why is such stockpiling usually necessary? Because the contractor is waiting for an area to be released by another contractor, or because the material must be there to accommodate quick adjustments to the schedule when activities are rescheduled.

But just because the work is planned by the “last planner” at the lowest level of the project, that does not mean that the big picture is overlooked in Lean Scheduling. The Lean Schedule addresses construction inefficiencies by focusing on the productivity of the overall process, not just the individual tasks.

In addressing the overall process, Lean Scheduling maps the construction process, determines the most efficient flow of work, and establishes a “pull” system. The demands of the end user, the owner, pull the work through the schedule instead of letting the work be “pushed” by the presence of supplies on the site or the need to occupy idle workers.

Key Steps in Implementation

Milestone Dates. Like all other schedules, a Lean Schedule is based on milestone dates dictated by the owner in the construction contract. Milestone dates are specific to each project. An owner who is constructing an elementary school or an office may have only two milestone dates: a “dried-in” date and a completion date. A more complicated project, like a laboratory or a phased project, may have several milestone dates.

Planning a Pull Schedule. Like a CPM schedule, a Lean Schedule requires the creation of a master schedule or several master schedules, depending on the complexity of the project and the number of milestones. The master schedule is created by looking at what the completed project should be, then working backwards, identifying each preceding step.

The downstream or later activities determine what the earlier or upstream ones will be and when they should take place. For example, the first date on the schedule is the completion date; then, working backwards, there will be the certificate of occupancy, then the completion of punch lists, all the way back to the groundbreaking. In theory, activities are pulled through the project instead of pushed through. When this schedule is set, the workflow is established.

“Look Ahead” Schedules. The next step in the Lean process occurs weekly at the project meetings. The Look Ahead Schedule is a look at the next six weeks of the construction project. One week rolls off the pull schedule and onto the Look Ahead Schedule every week. The Look Ahead Schedule is updated weekly and discussed at the weekly planning meeting.

The Look Ahead Schedule consists of a spreadsheet with a list of activities in columns, rows above the activities with the dates beginning each of the next six weeks, and lists of each possible constraint on the work. During the Look Ahead Schedule, a constraint analysis is performed on each future activity. In scheduling terms, a “constraint” is anything that restricts the project team from performing an action. Typical construction constraints include safety, contract, change order, submittal, request for information, materials, labor, equipment, prerequisite work and space. The six-week Look Ahead will also explain the constraint and, most important, assign the constraint to a responsible person who is required to take action to remove it.

The six-week Look Ahead is a valuable planning tool. The construction manager is able to track constraints on activities for six weeks before the activity is scheduled to occur. Each week the construction superintendents and foremen review the Look Ahead Schedule to assure that when work becomes due, the necessary materials are at hand and all pre-work is completed. By monitoring the approaching work and allowing adequate time to be sure everything is in place, the Look Ahead Schedule creates reliable workflow.

The six-week Look Ahead Schedule also allows an owner the same opportunity to remove any constraints it has on the contractors’ work. For example, when the contractor is waiting for the owner’s engineer or architect to resolve a request for information, it is a constraint on the contractor’s activity. The six-week Look Ahead gives all parties ample time to review upcoming work, to request information, and to obtain responses to those requests.

Additionally, the Look Ahead Schedule helps to identify defective work. Defective work is considered a constraint and can prevent any subsequent work from proceeding until it is corrected. For example, installing drywall and insulation in areas where water infiltration is a problem would be tracked as a constraint on that work.

Weekly Work Plan. The next step in Lean Scheduling is the creation of a Weekly Work Plan. As the activities are freed from constraints, they fall onto the contractors’ Weekly Work Plan. How often is a Weekly Work Plan created? Weekly, of course. The project superintendent or foreman is given a clear path to perform the activity on his Weekly Work Plan. The theory behind the Weekly Work Plan is that after tasks are scrubbed of constraints by the Look Ahead Schedule, the contractors should be able to commit to a full week of work without the fear of waste they might have with an unreliable workflow.

When the superintendents discuss the weekly planning, they are expected to commit to five days worth of work. The superintendents and foremen will schedule the appropriate materials, manpower and any other resources without fear of variances in the workflow. In other words, the superintendent can match his labor to the available work. The commitment is made to the construction manager and the other subcontractors at the weekly meeting.

Percentage of Plan Work Completed (PPC). The last step in a Lean Schedule is to review the work completed. This means the contractors review their Weekly Work Plan from the last completed week. They track the work that was completed as scheduled as well as the work that was not completed as scheduled. A contractor is given a pat on the back for completing his work as promised. A contractor is criticized for not completing the work as promised regardless of whether the work was completed early or late. This practice allows the construction manager to address contractors and problems causing work to be completed late and to discourage contractors from padding durations of activities, making the schedule unreliable and hard to predict.

Knowledge is power. The construction manager who knows what is preventing the activities from occurring in a reliable flow can take appropriate actions to create a reliable flow in the future.

Some Pros & Cons

The Lean Scheduling technique has many qualities that would be beneficial to a project owner. The Look Ahead Schedule gives a forecast of the project. This lets the owner know when critical decisions must be made to avoid an interruption in the work. This helps the owner manage its consultants who are resolving RFIs or making color selections. Since it is updated weekly, the Look Ahead Schedule creates a paper trail which can be reviewed if required to determine causes of delays.

With the Weekly Work Plan, contractors can commit resources to a project with some confidence that there will be no unforeseen constraints to prevent the work from being performed.

Of course, there are drawbacks to Lean Scheduling. As you can tell from the description of each task, Lean Scheduling requires a commitment from the party responsible for implementing it. The Lean Schedule requires a substantial time commitment for the weekly planning and preparation.

The construction manager must review and update the six-week Look Ahead, meet with each contractor and prepare the Weekly Work Plan, and still keep track of the work as it is being completed in order to complete the PPC. Next, the construction manager must lead the meeting with the contractors where the schedules are discussed and coordinated. Finally, the construction manager must distribute meeting records so that everyone is on the same page. Further, Lean Scheduling requires a commitment from each contractor on the project. It takes time and money to have the project manager and foreman attend the planning meetings, especially the meeting where the pull schedule is developed, as that one could last several hours.

Just as in lean production, Lean Scheduling focuses on the entire construction process. The goal is to have a project that flows smoothly from beginning to end and that eliminates uncertainty and waste created by an unreliable workflow. There are contractors in Ohio who use Lean Scheduling on every project. Is it the wave of the future? It will be interesting to see.

Anyone desiring more information in order to get started on Lean Scheduling should check out the website of the Lean Construction Institute.