A brilliant superintendent told me that 25 years ago we didn’t have crash-landing projects like we do today, and now they’re all over the place. A lot of people blame supply chain interruptions. I will grant that there’s a labor shortage and that COVID-19 put us through the wringer. But before COVID, I was already ordering unitized curtain wall in design development. We’ve always had these problems. The real issue is where we put our buffers.
Short answer: Where do you place a buffer? Not inside your activities. When owners disallow float and contingency, teams pad every activity, which starts successors late, breaks alignment with procurement, and kills takt rhythm. Instead, target each activity at its most realistic date, put a little buffer at the end of phases, most of it at the end of the schedule, and a material inventory buffer at the end of every supply chain.
What Book Explains Where Buffers Belong?
There’s a killer book you’ve got to read called Critical Chain by Eli Goldratt, the same person who wrote The Goal, the famous book everyone in construction reads about flow. It talks about delays, missing materials, and how to overcome the problems, and it gave me insight into exactly what is happening in our industry. One of its answers is moving buffers out of the individual activities and to the end.
How Did Owners Create the Padding Problem?
Fair warning, there’s some CPM bashing here. My problem with CPM is how it’s used and how owners beat us over the head with it. Contracts say the contractor can’t own the float and can’t get a time extension unless it’s on the critical path. The schedulers, lawyers, and owners who wrote that thought they were doing good by controlling contractors, but they pulled a system lever that counteracted the intended effect.
In Peter Senge’s The Fifth Discipline, this system archetype is called “Fixes That Fail.” A problem symptom cries out for a resolution, a fix is quickly put in, but the unintended consequences of the fix make the problem worse. Here, trade partners and general contractors start hiding float. Owners know that any time between the contractor’s planned finish and the contract completion date is float, so they disallow it. So people start to pad their activities. And schedule contingency is in the CPM books and manuals, by the way, everybody.
To be clear, a needed buffer at the end of an activity or takt time, maybe a couple of hours or more to do the work right and QC it, is good. Padding activities past that needed buffer time is a garbage idea. See Who Owns The Float In A Construction Schedule?.
Why Does Padding Break the Supply Chain?
In design, there aren’t a bunch of supply chains feeding an activity. You just go design it. In construction, the material is on an independent supply chain, the labor is a separate contractor, the information is in a separate submittal, and then there’s the equipment and the layout. All of it has to hit at the right moment. I’ve likened it to building a railway with the bridge builders three miles out ahead, so the railway can cross the bridge when it gets there.
If you pad a bunch of activities and they finish sooner, your successor activities start late, and the supply chain isn’t targeting those earlier dates. One superintendent I love is super paranoid and pads all of his activities. The director of construction got an email saying, hey, we need the roof trusses a month early. A month early? The activities were padded so much that the team had to knee-jerk move material procurement up. Bro, stop padding.
So padding does three things. It starts activities late for no reason, throwing schedule time away. It detaches realistic procurement dates. And it gives you an unrealistic schedule people don’t trust, with no takt rhythm and no predictability.
Where Should the Buffers Go Instead?
The buffers padded into the middle of the schedule should move out. A small portion goes to the end of the phases. Most of it goes to the end of the schedule. And every value stream, every supply chain, every lead time for every material should have a material inventory buffer at the end. If there is variation in that supply chain, whether it’s curtain wall or precast, you don’t order it to arrive just in time on the exact date. You bring it to a laydown yard with a buffer, say two weeks. Then target the activity at its most accurate, most likely, most responsible date.
| Padded activities | Buffers in the right place | |
|---|---|---|
| Activity dates | Padded past the needed time. | The most realistic, most likely date. |
| When work finishes early | Successors still start late; time is thrown away. | Successors start as soon as possible. |
| Procurement | Targets a later date with no inventory buffer, so crews wait on materials. | Ordered sooner, with a material inventory buffer at the end of the supply chain. |
| When a delay hits | Nothing left to absorb it; the schedule spirals late. | The delay eats into the end buffers. |
| Takt rhythm | Lost. | Predictable. |
Now, when you have a delay, you eat into the end buffers, and activities start as soon as possible. Because the buffer has been transferred to material procurement, you have a buffer of materials and you’re not waiting on them, so your activities become more predictable. You have buffers on both sides of the accurate activity projection, and that protects it. More in How Buffers Work in Takt Planning.
I keep railing on procurement tracking systems, too. The general contractor should be doing it, and the software doesn’t do it right. See The Procurement Log. You Are Not Doing This Right.
So here is the challenge. Look at your current schedule and find the padded activities. Pull them back to their most realistic durations, move the time to the end of the phase and the schedule, and add a material inventory buffer to every long-lead item.
Bottom line: Stop padding the intermediate activities, and put your buffers at the end of the schedule and the end of every supply chain.
If your project needs superintendent coaching, project support, or leadership development, Elevate Construction can help your field teams stabilize, schedule, and flow. Want your team trained on this? See our Last Planner® System and Takt training.
On we go.
FAQ
Where should buffers go in a construction schedule?
Not padded into the intermediate activities. Put a little buffer at the end of each phase, most of it at the end of the schedule, and a material inventory buffer at the end of every supply chain lead time. The activities themselves should be the most realistic dates you can target.
Why is padding activities a bad idea?
Padded activities make successor activities start late for no reason, detach the schedule from realistic procurement dates, and destroy takt rhythm and predictability. When work finishes early, the materials aren’t there.
What is a material inventory buffer?
If a supply chain has variation, you don’t order material to arrive at the exact date. You bring it to a laydown yard with a buffer, say two weeks, so the activity can start as soon as possible without waiting on materials.
People Also Read:
Buffers Are NOT Shared Float
Goldratt’s Rules of Flow
A Stable Procurement
What Is The Biggest Delay In Construction? Material Procurement!
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