Stud finders are another option we tend to use as well.

If you are doing remodeling/remediation, some pre-design destructive observation is necessary. Since the finishes will likely be compromised in the remediation that is not a big deal.

Density based stud finders work well. Metal detecting stud finders are not as reliable.

Mike McCann, PE, SE (WA)

Thanks Ron and Mike. I should also put these projects into perspective. I'm referring to relative small remodels for single family residences. For non-resoidential projects we always do some selective demo. For the single family residential projects we often get push back due to

1) other engineers in the area make do without it.
2) often construction is 6+ months away and the homes are being lived in.
3) often times a contractor is not yet involved with the project.
4) pricing is not finalized and occasionally the proposed work does not actually get built.

In a perfect world we would always have the areas opened up. However it's usually a pretty big uphill battle. For larger scale residential projects we require it. It's the small ones I'm talking about. Usually only an issue in 2 story construction and we have access to attic and crawlspace.

Does everyone just require everything be opened up or what do you do in these situations?

Look for subtle clues such as where are HVAC outlets are compared to the location of the HVAC unit. If the outlets are in the ceiling of the first story or the floor of the second story, the ductwork almost certainly runs parallel to the second story floor joists.

Usually, layout of the second story floor joists will match the first story layout.

Interior stairs are likely parallel to floor joist direction.

In one historic home I investigated the floor joists were so long and "springy" that it was fairly easy to see, by inspection, which way the deflection curve ran.

This may be obvious, but I'll mention it anyway, make a sketch of the house floor plan, including any crawlspace supports. The floor joists probably run in the shortest direction. Lumber can only be so long.

The above are not sure fire answers, but pretty good clues.

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Stud finders have not been reliable for me due to blocking etc. I usually look for clues such as hardwood floor direction, reasonable span limits, locations of beams in the basement and crawlspace etc. when demo is not an option. Using these methods, I get it right about 90% of the time. No matter what you do, you never get the entire picture until FULL demo has occurred.

I've used thermal cameras to find studs at walls where I could get a large thermal difference between the two faces of the wall. For most interior walls the studs will not show up well unless you can drastically change the temperature (open a window in winter or something like that).

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

Depending on the flooring, whack it with a soft mallet and pretend you're a whale.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.