A shop drawing came back with a stamp on it. The field does not match. Somebody is about to eat the cost of that and it is worth knowing in advance who.
These are two different documents doing two different jobs, and mixing them up is one of the more expensive mistakes available to a subcontractor. This is part of how to read HVAC drawings on a commercial job.
What is a construction drawing?
The construction drawings, also called the contract drawings or the design drawings, are what the design team issues for the project. On a commercial job that package is drawings plus specifications.
They describe what the finished building has to be. They do not describe how your shop is going to build it, and they are not trying to.
What is a shop drawing?
A shop drawing is the subcontractor’s interpretation of the engineer’s design.
That word interpretation is doing real work in that sentence. The shop drawing is how you communicate back to the design team that you understand the intent, and that you can manufacture or buy material that meets it. It is a conversation, in drawing form.
It is also the document your shop actually builds from. The construction drawings tell you a duct is 20 by 12 and runs down a corridor. The shop drawing is what turns that into pieces with lengths on them.
What is the difference?
| Construction drawings | Shop drawings | |
|---|---|---|
| Who makes them | The design team | The subcontractor |
| What they describe | What the building has to be | What you are going to build and buy |
| What they are for | Defining the work | Producing the material |
| Which one governs | These do | These do not |
The contract drawings govern in design. Shop drawings are presented as a communication to the design team, and they are what the material gets produced from for field install. Those are two different kinds of authority and only one of them settles an argument.
Why does a sheet metal contractor need both?
You cannot produce a shop drawing without first understanding what is being asked. The shop drawing is interpretive by definition, and you cannot interpret a document you have not read.
That is the whole reason reading the set comes first. Every hour somebody spends in the construction drawings before the shop drawings get started is an hour that does not get spent rebuilding duct later.
What happens when they disagree?
This is the part worth knowing before it happens to you.
Your contract sets an order of precedence. It says which document wins when two of them say different things, and it gets agreed before anybody picks up a tool. Go find out what yours says. It is one of the few pieces of paperwork that saves you money on a bad day.
On every job I have worked, the contract documents governed, and a shop drawing coming back with a stamp on it did not change that. Which means when the shop drawing turns out to be wrong, the correction lands on the subcontractor. That is a hard sentence and I am not going to soften it.
Reviewed, not approved
Learn this phrase. It shows up on returned submittals and people misread it constantly.
A stamp that says reviewed is not somebody else taking responsibility for your work.
The design team reviewing your submittal means they looked at it against the design intent. It does not mean they checked your dimensions, and it does not transfer the consequences of a wrong one onto them.
If you read a review stamp as a guarantee, the first time that costs you will be memorable.
When you did not follow the drawings, and when the drawings were wrong
These are two different situations with two different outcomes, and the difference is worth being precise about.
- You did not follow the drawings. You fix it.
- You followed the drawings and the design was wrong. That is a clarification from the engineer of record, and typically a change order.
That second one is why an RFI exists. It goes to the architect through the general contractor, it documents that you asked, and the answer becomes part of the contract documents. A verbal answer in a hallway is worth nothing six months later. An RFI answer is worth what the job pays.
Raise it the moment you find it. I say that as somebody who now sits on the receiving end of those questions. The people looking ahead are the ones I want on a project, and the discrepancy that surfaces in week two costs a fraction of the one that surfaces in week nine.
One more thing that shows up in this fight
Offsets and special pieces around work another trade already installed do not show up on the construction drawings at all. Nobody drew them because nobody could have. They get built because the conduit bank was already there.
That is not a drawing error and it is not a change order. That is coordination, and it is most of the job. Who owns the duct design when it is wrong goes further into where that line sits.
The short version
- Construction drawings define the work. Shop drawings produce the material.
- The contract drawings govern. Your contract’s order of precedence is where that is written down.
- Reviewed is not approved, and neither one moves responsibility off you.
- Did not follow the drawings means you fix it. Followed them and the design was wrong means an RFI and usually a change order.
- Ask early. The cheap version of every one of these problems is the one found in week two.