Still trying to find a way to locate joists/rafters behind thick lath and plaster. Tapping etc fails to show a significant pattern, so it seems to come down to finding the nails holding the laths, using either a metal detecting stud finder or the right magnet. (Or drilling a lot of holes, which I'd rather avoid.)
Given that it seems difficult to identify a detector that is recommended as reliable, even at quite substantial prices, I thought I would give magnets another try. I previously tried with a so-called supermagnet that my son bought from Maplin and found nothing. It doesn't have any obvious specification but it does stick well to any steel etc surface.
So I need to get a better magnet. I suspect the only practical way is online and there are plenty of sources about. The question is how to specify the best one for this sort of task.
The spec seems to be all about the pull and clearly higher numbers stick better, but it seems to me they will also be heavier, so the pull toward well-buried nail may be less easy to feel. Perhaps there is a 'sweet spot' or is it just a matter of getting the strongest one available? It would be good to hear from someone who has solved this before.
Cheers