I've struggled to come up with the correct answer but I'll try it here:
The easiest answer is that I don't know how to do it. I'm winging it. But that response usually elicits a response of shock and fear and I can see them planning out their careful decline of my next party invitation as they wait for the house to crumble around me.
As I try to explain that I am able to figure things out many people wonder whether my engineering background makes it easier. Again the answer is more complex than a simple yes or no. The engineering background doesn't specifically help. I studied and practiced electrical distribution system engineering. Even the residential electrical system installation is only tangentially related. I have no structural, civil, or plumbing experience. But the same traits that made engineering come easily do help me figure out construction. I am logical, organized, and patient. The ability to confidently (read: without mistakes) do simple math does help.
But here is the secret. It is the secret to so many things. It is, honestly, the secret to my engineering career. Almost every bit of information you need is available if you know how to ask the question.
The trick is the lexicon.
In construction this consists of knowing what things are called and using the correct word. Joist and stud are common but the trick is to know: subfloor vs. underlayment, sheathing vs. siding, ledger vs. beam vs. header, post vs. column, sill plate and sole plate are the same thing, house wrap vs. felt, king stud vs jack stud vs cripple stud, blocking. I also spend my time being observant. I'm watching a house down the street get built (faster than my kitchen but they have 7 guys working). I ask questions and I try not to be afraid of sounding stupid. And I've learned a ton. The difference between laminate and 3-tab shingles, solid and engineered flooring, rough opening and all of the other measurement of windows, vertical and plumb and level.
In engineering, I learned to pick up on the important words. I would often get frustrated that anybody in the world could do my job if they knew how to listen for the right words. When I would need to do something I would listen for the right clue and I could go learn how to do what I needed. I don't believe that I was EVER required to invent from the ground up for the first time. The trick then is to know where to look to see how someone else solved the problem. Occasionally you are able to improve the method. Perhaps in my next career path I'll get a chance to create substantially more inventive solutions. I hope so.
I spent 5 years doing electrical engineering at the county. Now some will argue that this is not the most difficult engineering setting imaginable and they would be correct, but the theory extrapolates to almost any field. This is not to diminish my abilities as an engineer. I passed my schooling with flying colors and I am an excellent problem solver, extremely intuitive to knowing when my answer is close or not, trained in the subtleties of electrical systems along with the engineering accoutrements of economics, statistics, higher level math.
Anyway, the best answer to the initial question is that it all just makes sense. Water goes where the pipe is laid, poo goes down, walls work better when they are vertical and measure twice, cut once.
And in the end all that matters is: can you swing a hammer and actually hit the thing for which you are aiming?
