Prescriptive vs. Performance Specifications in Construction
Today’s Ideas & Insights is about two different ways to write specifications: prescriptive and performance. I’ll tell you about them, why they are different, and what you should consider before choosing them.
The first one, which we are very used to using in the design-bid-build world, is what we call prescriptive specifications. If you were to look at a standard highway construction contract, you would see the owner sometimes even telling the contractor what pieces of equipment to use and how to use them.
Prescriptive specifications are recipes: do this, then do this, then do this, etc. If you do all those things, you know we will accept whatever the results are. That can be a successful way to specify work on a design-bid-build project. It has been used for centuries. But it doesn’t fit very well within the design-build process, where you are not entirely certain exactly what you’ll be getting.
The kinds of specifications that might fit a little bit better with design-build projects are what we would call performance specifications. These specifications aren’t recipes. They don’t tell the contractor how to do the work. What they tell the contractor is what we want.
For example, a prescriptive specification would tell the contractor exactly what size motor we want to drive the vacuum cleaner. In contrast, a performance specification would tell them what kind of suction we want at the other end. Then, it’s the design-builder’s decision as to how to size the motor to meet our performance objectives.
The Challenge of Writing Performance Specifications
In order to draft an appropriate performance specification, you have to know exactly what it is that you want at the other end. I went through an exercise not too long ago where I had to develop a performance specification that would have ended up with the contractor constructing a sidewalk that was acceptable to the agency. It sounds simple, right? Well, let me tell you that it is not an easy thing to do.
You think of a sidewalk as a relatively straightforward thing to build. But if you really drill down and start thinking about how you would write it as performance spec (as opposed to a prescriptive spec) it becomes much more challenging.
Just how much latitude are you going to give the design-builder to solve problems? If you are not comfortable with just telling them how much suction you want from your vacuum and leaving it up to them, then you are going to be writing more prescriptive specs.
You are going to have to anticipate what the contractor might be delivering to you or you are going to need a huge raft of specifications available so that you have a specification ready regardless of what the contractor proposes.
If you want more input in the design, how do you limit the contractor’s latitude? Maybe there are differences in how much latitude you are going to give the contractor depending upon what it is we are talking about. In other words, maybe you will give the contractor an awful lot of latitude when it comes to how they design a lunchroom, but you might want more control as to exactly how they lay out a lab. Therefore, there could be different levels of latitude you’re willing to give a contractor.
All these things need to factor into your decision whether to use prescriptive or performance specifications.
Scott Lowe is a Principal of TRAUNER and is an expert in the areas of critical path method scheduling, construction claim preparation and evaluation, and specification writing. He can be reached at email@example.com
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