Scheduling basics: the CPM approach

We've covered work load management, estimating, inventory control and labor. The next basic is job scheduling.

So, you were either invited to bid a job, awarded a street bid job through the Blue Book or Dodge Reports, or negotiated a project. You did a takeoff, produced a solid estimate and made a winning proposal. Your work schedule is clear with no conflicts. You've signed a contract and put a pre-lien on the job. Your supplier has your material on pallets ready to go. You've got a crew available to start, and the project is ready to break ground.

For the sake of discussion, we are assuming that the scheduling concerns we have relate to a single project. In the real world there are many scheduling variables, not the least of which have to do with overlapping projects as discussed in workload management. Although an infinite number of scenarios can and do arise, the principles of proper scheduling remain the same.

Almost every general contractor utilizes some sort of computerized scheduling software. Most, if not all, are based upon systems developed in the 1950s — the U.S. Navy created Program Evaluation and Review Technique (PERT) and DuPont created Critical Path Method (CPM), which is sometimes called Critical Path Analysis. CPM is a mathematically based algorithm for scheduling a set of project activities. In the construction industry, CPM is used to create a two dimensional model of a project from start to finish, which contains a list of all activities required to complete the project, the time (duration) that each activity will take to complete and the dependencies between the activities.

CPM calculates the longest path of planned activities to the end of the project, and the earliest and latest that each activity can start and finish without making the project longer, as defined on Wikipedia.com. This process determines which activities are “critical” (i.e., on the longest path) and which have “total float” (i.e., can be delayed without making the project longer).

Where and, more important, when your work is scheduled can mean the difference between a smooth progression or a jerky stop-and-start project. When reviewing the CPM graph, take special note of the overlaps with the other trades. Make sure that you consider the float built in to the critical path. Most general contractors will ask you how long you expect to take for each phase of your work on a project, but some will simply plug in the number of days that they think are right. If you allow them to do that with no input, you are at their mercy when it comes to crunch time — you've got a week's worth of work left to set fixtures for a restaurant that is opening tomorrow.

If you use your labor estimate as a guideline, you will be much more likely to have time to complete your work without crashing the critical path. Notice I said to use the estimate as a guideline instead of using your actual man/hour calculations. When it comes to critical path management, there is enough float time in the phases to absorb extra days for a given trade's work. Take all you can get, just in case.

Take into consideration how other trades will affect your schedule as well. At any given time, there will be plumbing, HVAC, electrical, fire protection and special systems, just to name a few, all being installed at roughly the same time. In most cases, you can't expect a clear avenue to install your work without allowing for delay, or at least deviation, from the schedule. Keeping a close eye on the progress of the other trades is one key to staying on schedule and out of the general's crosshairs.

When the contractor is trying to keep to the critical path, you will be urged, coaxed, cajoled and threatened to maintain a presence on a project even if it's not ready for your trade. You will be asked to jump through hoops, figuratively, and to hurry up and wait, literally.

You know your business. You know that manning a job too early or at the wrong time will cost you in wasted labor, duplication of work and, in some cases, extra charges from a sub-subcontractor or supplier. If you are being forced to man a job that's not ready or begin your work when other trades prevent the efficient installation of yours, ask for a remobilization change order or some other consideration. That will at least inform your client, in a businesslike way, that their schedule is having a negative impact on you.

Taking the time to analyze the CPM schedule and address any issues up front makes you proactive instead of reactive, and can help keep your bottom line intact.

The Brooklyn, N.Y.-born author is a retired third generation master plumber. He founded Sunflower Plumbing & Heating in Shirley, N.Y., in 1975 and A Professional Commercial Plumbing Inc. in Phoenix in 1980. He holds residential, commercial, industrial and solar plumbing licenses and is certified in welding, clean rooms, polypropylene gas fusion and medical gas piping. He can be reached at [email protected].