Personal blog. I write about what I learn as I am learning it

Project Crashing: How Many Days Can You Actually Buy?

Project crashing is what you do when the date is wrong and the work is not going to shrink on its own: you pick tasks and do them a different, faster, more expensive way. Rent the spray rig. Order the wall panels prefabricated instead of stick-framing them. Hand the drywall to a subcontractor. The version in the textbooks is a tidy table of crash cost per day, and in twenty years of doing this for a living I have never once had a job hand me that table.

So I ran it properly instead of describing it. Same 21-task house renovation I have been using all series, three general crew, six alternatives a real builder could actually buy, and a solver that has to choose which ones are worth it. Here is what crashing bought, change by change, with nothing rounded in my favour.

Step chart: with no changes the renovation finishes 2 December; subcontracting drywall alone pulls it to 25 November, adding prefab framing to 19 November, and adding a roofing subcontractor to 17 November. — Every step down costs one more argument with a subcontractor. The first one buys the most days.

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A Critical Path Method Template That Does Both Passes

Most of what gets offered as a critical path method template is a Gantt chart with a column called Start that you fill in yourself. That is not the critical path method. The whole point of CPM is that you never type a date — you type durations and predecessors, and the dates are computed. If the template makes you enter the start, it has quietly handed the work back to you and kept the chart.

So this is what an honest one has to do, which half of it a spreadsheet handles well, which half it fights you on, and the file I ended up with. Numbers throughout come from the same 21-task renovation plan the template ships with.

Table of early start, early finish, late start, late finish and total float for 21 tasks, with the zero-float critical path rows shaded. — What the method actually produces. Four dates and a float per task; the shaded rows are the critical path.

Free Float vs Total Float, and the Third Kind

Free float vs total float sounds like the kind of distinction that exists to be asked about in an exam. It is not. They answer two genuinely different questions, they disagree on real plans, and the one your software prints by default is the one that will get you a phone call from a subcontractor.

I ran both numbers over a 21-task house renovation — the sample plan I use for all of this — and then a third number that no CPM tool prints, because it needs information the critical path method throws away. Here is all three, on the same plan.

Bar chart comparing total float, free float and crew-constrained float for the eleven tasks where the three measures disagree. — Eleven of the twenty-one tasks. Three measures of slack, and they rarely agree.

What a CPM Schedule Is, and How to Build One

A CPM schedule is not a chart. It is the output of an arithmetic procedure you could run on the back of an envelope: two passes over a list of tasks and the links between them. The bars you end up looking at are just a drawing of the answer. I think most of the confusion around the term starts there — people are shown the picture and told it is the method.

So here is the method itself, worked end to end on a real plan: a 21-task house renovation, five trades, no invented numbers. Every figure below came out of the same file, and you can run it yourself in a minute.

Gantt chart of a 21-task renovation showing critical tasks in orange and tasks with slack in blue, with daily resource load rows underneath. — The drawing at the end of the arithmetic. Orange bars have no room to move; blue ones do.

A Construction Schedule Template That Counts Crew

Download any free construction schedule template and you get the same thing: a task list, durations, start and finish dates, and a bar chart. What you almost never get is a column for how many people each task needs, and a row underneath that adds them up. Which is odd, because on a real job the binding constraint is rarely the calendar. It is that there are three electricians.

I built one with that row in it. It is free, it is a normal Excel file, and this is what the extra column changes.