Why do they want to convert their old floor plans into a 3D model?

They discovered some sagging and structural issues in their home, and spoke to a structural engineer who said their home needs some reinforcement done. He asked for a CAD version of their home in 2D and 3D with cross sections. I can help them with the CAD but am not too sure about reading their plans accurately.

No stuctural engineer worth hiring ”needs” CAD for this. He just needs to do a bit of crawling around, IF he has the experience in the field needed for this. Probably a basic framing contractor could do the same thing, and cheaper. Unless you have weird snow loads or earthquake issues, or some mind of cantilever?

I never trusted a drawing in my life, no matter who produced it or how it was produced. Ask the engineer why they need the drawings and if they can do without it. It may be that it will save them time in them producing drawing to explain what needs to be done.

Thanks all, I agree it seems odd that the engineer would need all of this done first. Anyway, I have the 2D already done (dwg file) and will send that over to him.

But does the cross section math make sense to anyone? Now I just really want to figure it out for the sake of learning.

Zero clue.I can say this is WAY over my excellent pay grade...

Are you using original drawings that were used to build the house? If so, there are often differences between what is drawn and what was built. Will suggest that you measure the house yourself to get a more accurate representation of what is actually there.

The drawings may not not be what is there as drawings and the home are never exact. Always a reasonable facsimile not exact. I would find another engineer or most likely a competent contractor before the engineer, you nay not even need the engineer.

Ask the opinion of the person you have chosen to perform the work. They may have a viable option for you.

It is odd that the structural engineer is asking an average homeowner to create CAD drawings. At a minimum, I would not want the structural engineer to rely on amateur drawings to make structural recommendations.

The fact that you have turned to an anonymous Internet forum for help is concerning. Who is the builder/GC that will actually perform the remedial work? Why aren't they involved?

When I needed structural work at my house as part of a large addition/renovation, my “team” included my excellent GC who interacted with the structural engineer. Everyone came to the jobsite to see it in person. My town permit process required building plans which included the structural drawings to be submitted with the engineer’s seal.

Agree with all the comments above. My question was just focused on that 6" discrepency. It's not my home and I don't know who their builder is, all I know is that I have a design background and can easily put together CAD drawings, so my parents asked me to help them out rather than hire a draftsman. Everything was going along great until that plate height threw me off, and I thought to ask if maybe I'm just reading it wrong or if the plans are wrong.

An engineer asked for you to do this , very odd indeed/

Well, he didn't ask me to do this, he asked my parents (the homewoners) to have a digital CAD version of the plans made by a draftsman or architect so they can work off of that. Since I work related to this field I am helping them draft it per their request. However the math didn't add up on this cross section and I posted here to see if anyone can explain it to me, or more likely, confirm that the plans are incorrect. Thanks everyone for the feedback. I'm going to just assume the plans are incorrect and will confirm over the holidays when I visit them and can measure it myself.

Yes, the math is off. While you're there, crawl in the attic too as that will verify a lot for whatever is going to be remodeled. Many pictures are invaluable for you later. Make copies of the original cdocs to mark up the asbuilt, saves lots of time.

Thank you @3onthetree that helps so much!