Magnets

[GMM]

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

[Chris Hogg]

Two suggestions: the magnets from an old disc drive (very strong, if you get them stuck together, they're the devil to separate). Failing that, try these people:

They stock a huge range of shapes and sizes. I've used them and found them reliable.

[Tim Watts]

I've got some magnets - pretty sure it's these.

I used them to locate the nails in my ceiling to locate the joists. A metal detector failed miserably.

I typically used about 4 and stuck them to the nails along the same joist - then took a mean for the centreline.

[Andrew Gabriel]

I've used a teloscopic magnetic pick-up tool with plasterboard walls and ceilings, and that works well because the nail head will be only a couple of mm behind the surface.

The problem you will have with lath and plaster is that the nail heads are much smaller, and they could be an inch behind the surface. I suspect it's much less likely to work.

[Theo Markettos]

A while back I asked about a similar problem (behind tiles+plasterboard) and one of these:

did the trick. It even found the supports underneath 28mm kitchen worktop quite easily. It's quite neat in that it projects a arrow from an LED onto the surface, so you can an accurate position. Not the cheapest, but turned half a day of tapping and messing about with magnets and cameras on sticks (to 'see' the backside of the board) into 10 seconds work. It's way better than the 1980s inductive 'stud and cable finder' it replaced.

One caveat is that I don't know how well it would work on a non-flat surface, eg artex.

Theo

[meow2222]

hard drive voice coil magnet

NT

[fred]

I like the switchable 'deep' setting which increases depth to a claimed

50mm, ideal for lath & plaster.

Significantly cheaper on amazon I see, I wonder if Toolstation are going the way of Screwfix:

As I mostly use these things in poor lighting, I'm tempted to get the LED based e40 which seems to have the same features:

[Robin]

and if there's a room above there's also the risk of spurious results from the fewer (you hope!) but possibly much bigger old cut nails, hair-clips, razor blades, iron gas pipes etc under the floorboards :(

[GMM]

Interestingly though, even Zircon say this is not the thing for this situation and recommend a dedicated metal finder, so I'm reluctant to shell out on one.

[GMM]

It think you're right there: The plaster must be pretty deep as there's no noticable difference on tapping and the so-called super magnet gave me no results. As you say, in palsterboard the nails or screws are just beneath the skim and are a bigger target.

[GMM]

I take it this was in L&P. Interesting, as these don't look like the strongest magnets out there, but the site doesn't really give a specification. My current 'target' is a ceiling, well a few ceilings actually, so much the same situation. I was hoping to take a similar approach - mark a few positions along the joist and get an impression of where it runs.

Bit of a pity these things aren't really available except for online, as it's a pain waiting for a delivery then finding it doesn't work.

[GMM]

If only I could remember what happened to the old HDD I swapped out a couple of years ago. I'd guarantee I didn't bin it, but where it ended up is anyone's guess!

[Tim Watts]

No - Plasterboard. But with a 2nd skim coat.

Assuming the link is what I actually bought, mine are mentally powerful

- will hold about 50 sheets of paper to a fridge in one go.

[Adam Aglionby]

Neodymium Iron Boron , NeFB or neodymium magnets for short on your online retailer of choice, dinnae trap your fingers..

[Chris French]

In message , Phil L writes

I don't think it's to do with the cost of the filler.

I have indeed done this (though I prefer a drill in old L&P) though our studs are all over the place :-)

Fine if you are redecorating, or it's emulsion and you have some to touch up. Not so good on wallpaper.

[Andy Burns]

Using a single magnet salvaged from a hard disc, it will stick to the clout nails or screws in a PB ceiling, or to the head of a large screw in a stud (not yet boarded over), but it won't find the same screw "side on" in the stud

Two of the magnets stacked will find the screw side on, and will just about support themselves at a depth of 1cm, they struggle to find a nail end-on a similar distance though.

A stack of four will support themselves on a screw buried 2cm from the surface

If you're wanting to feel the 'fluence from nails at an inch or more I reckon you'd need a magnet equivalent to at least 1" cube, I don't know how hard disc magnets rate on the N35, N50 etc scale.

[Jonathan]

+1

Jonathan

[alan_m]

I have just tried this with a couple from old disks. They are very strong but detecting the nails in lath and plaster on a couple of my stud walls - nothing!

I've also just tried some very small (light) powerful magnets - same result.

I have ziron branded stud finder that works well at finding the edges of studs. An Aldi stud finder was only fit for the bin.

[Brian Gaff]

Well, it depends how big the nail is, or more to the point, the head of it is. The problem is that if it were side on, a nail would be very easy to detect with an inductive system, like most metal detectors use, but head on not so easy, as the distance of the detector to nail needed for a good detection will be very small.

Same I guess goes for magnets. You almost need a wide track, so you can do the search across and down and centre the search that way.

Brian

[Dave W]

Have you tried the treasure-hunters type of metal detector?