Magnets

[GMM]

I reckon you must be right there: Certainly I've yet to feel even the slightest grab from any of the magnets I've tried so far. Of course, it's not possible to be definitive about the depth of the plaster but if tapping detects nothing, it must be fairly thick.

There seems to be a bit of smoke and mirrors about the way magnets are marketed though. While I'm sure the idea of 'Xkg pull strength' is useful for people wanting to stick a magnet to a steel plate and hang something on it, it's not very informative for a purpose like this. I'm not sure what would be a good measure, but it seems that it would be tricky to detect a weak pull on a heavy magnet.

If I could be sure it would do the job, I would be more than happy to buy something even a bit expensive, but the more I look into the idea of finding studs this way, the less likely it seems to be a good plan, despite being recommended by many sources online.

[GMM]

That had occurred to me, but when I tried looking them up it wasn't clear what might work. I guess pin point detectors would be the thing for the job, but their sensitivities weren't clear when I looked. I guess my prejudice is that if they did the job, someone would market them for this pupose. Should take another look though.

[alan_m]

Having removed the plaster/laths on a wall recently the row of nails often wasn't down the centre of the stud but offset to one side - especially where the wood wasn't of the best quality.

[Roger Mills]

Indeed. Don't forget that magnetic force follows the inverse square law

- so if you double the separation, you reduce the force to 25%. The XKg pull is doubtless when the magnet is actually in contact with a large lump of steel. Move it away a bit and replace the large lump with a small nail head, and you'll get peanuts!

[polygonum]

Might be difficult to achieve, but I can't help wondering if the appropriate part of a hard disk drive to use might not be the head. They are extraordinarily sensitive. Anyone fancy working out how to make a head traverse a wall, which sounds to be fairly uneven, and collect all the "data" it can read?

[Tim Streater]

Actually inverse cube, AIUI, because of the effect of the other pole.

[F Murtz]

I use a thing like this for finding nails before using woodwork machinery.

[Martin Brown]

They are quite likely to work. You may actually want to wrap them in a bit of soft plastic to avoid them damaging things when they grab onto steel. They are brittle. The larger magnets can give you a serious nip.

I'd guess that something around 5mm diameter and 10mm long would do. It depends how deeply the studs a buried but I'd expect them to have more than enough grab to be self supporting at almost any size.

[Richard Tobin]

Maybe you could use a big magnet to find it...

-- Richard

[GMM]

Just resurrecting this thread for anyone who's interested.

In the end, I gave up with magnets as nothing seemed to give me anything useful in terms of finding joists and, to push the job ahead, I took a punt on a 'Zircon Metalliscanner M40', which arrived today.

By the time I got home it was a bit late to do much in anger today, but I shoved a battery into it and ran it over the sloping ceiling on the top floor (lath and plaster just the same as the one I really want to work on but much more accessable). Obviously, I can't be certain until I do a job with it, but it does pick up very clear signals that run in vertical lines pretty much where I would expect the rafters to run (in terms of the horizontal spacing), so I conclude it's picking up the nails that secure the laths to the rafters.

A bit of a specialist tool but it looks like 33 quid (including the P&P) well spent as it's unblocked this job and I'll find other uses for it (overboarding some ceilings is on the horizon). Ordered Sunday, arrived today, from Conrad electronic (no connection blah blah - in fact I'd never even heard of them before I searched and found this price, below the going rate by a bit).

[Rob]

What you really want is a thermal imaging camera

[GMM]

I certainly do. Although what it would give me in this context, I've no idea...

[meow2222]

:) It would work a treat if you set fire to the floor

NT

[GMM]

Then I'd know which way was down......