Magnet trick for ceiling joists

[stubious]

I'm trying to find the ceiling joists in my garage (which is integral, sitting below our living room). And after failed stud finder attempts, have gone with the strong magnet approach. Which immediately seemed to work...but then I drill up and get through the plasterboard and then...nothing. Picture below as the example (and post picture, I tried drilling at 90 degrees in case I was unlucky and nails on different studs lined up to run to trick me...and still no luck.

Where I have tried to drill a bigger hole to get a look / put a camera up, I run into the hard insulation. It seems it is only one layer of plasterboard, with insulation above (which, based on a separate thread, I was a bit surprised it wasn't two layers given it is a relatively recent build.

I don't have any hair to pull out...suggestions on what silly mistake I'm making or should do differently?

And separately, I thought I could see a plasterboard seam and tried there, no luck, and what looked like nail pops and tried there, also no luck. I'm confused!

If it helps, 2016 new build...

 

[foxhole]

You should find stud at wall edge about 50mm in, then at 400mm centres from that.If there is a pendant light fitting it will normally be fixed to a joist.

 

[stubious]

50mm from the plasterboard? Or 50mm from the breezeblock and I need to adjust for dot & dab and plasterboard?

 

[Wayners]

Drill where the magnet is.

Joists may go other way to your holes.
If your going to drill start in centre of magnet in a cross shape.

I think your missing joists or not going deep enough

 

[stubious]

thanks - I'm definitely getting through plasterboard and into insulation, I must be just missing it. Doesn't sound like there is any likely reason for magnets to find something that isn't attached to a stud? Will have another go tonight!

 

[Deleted member 174758]

Metal top hat section (as used for ceilings) is quite narrow. If the have used metal C-section you might only be looking at 20 to 25mm width. And as others have said, the studs might be running the other way

 

[stubious]

ah, hadn't come across metal framed ceilings before - are they commonly used in new builds in the UK? (this is a integral garage sitting below a living room). And if that is the case, is it likely that the joists are above the insulation, with the metal framing below it? If so, it could explain why I'm unable to hit a joist - and given how thin these top hat sections are, I guess I could be drilling through them without noticing, when expecting solid wood?

 

[RandomGrinch]

I'm definitely getting through plasterboard and into insulation,

Where I have tried to drill a bigger hole to get a look / put a camera up, I run into the hard insulation. It seems it is only one layer of plasterboard, with insulation above

It sounds like it could be insulated plasterboard (or the insulation boards are fixed below the joists)?, and you could need really long screws!

 

[stubious]

It sounds like it could be insulated plasterboard (or the insulation boards are fixed below the joists), and you could need really long screws!

Interesting, another potential scenario to consider - not quite sure how to diagnose it though!

 

[RandomGrinch]

Interesting, another potential scenario to consider - not quite sure how to diagnose it though!

As above...

Drill where the magnet is.

Find if there is a screw there...
If there is, undo it and see how long it is

 

[foxhole]

Interesting, another potential scenario to consider - not quite sure how to diagnose it though!

You pic seem to show a line/ joint across the photo , unless it’s just heavy paint .

 

[stubious]

You pic seem to show a line/ joint across the photo , unless it’s just heavy paint .

Exactly my thought as well, but I've tried drilling into those lines and am not hitting any wood.
So either
1. JobAndKnock is onto something and I'm drilling through the metal top hat and just not feeling much of a difference vs the plasterboard
2. RandomGrinch is onto something and I'm not hitting the joists because I need to drill all the way through the insulated plasterboard (trying to search to figure out how deep that is likely to be)
3. I'm wildly unlucky and just missing the joists every which way in my drill attempts

I'm going to try to uncover a screw, remove it and see how long it is, which should rule in/out (2). I'll also go back to a few areas and make sure I haven't left enough room in my test drills to be hiding a stud and rule out (3). And then I may just need to take a 2"x2" square out on one of the nailpops or seams to get better visibility!

 

[freddiemercurystwin]

Or just lift the carpet in the room above and make yourself say a 10cm2 inspection hole, stick the camera down.

 

[stubious]

Or just lift the carpet in the room above and make yourself say a 10cm2 inspection hole, stick the camera down.

wood floor above and trying to get up a board is above my skill level!

 

[stubious]

Ok, update - finally found a joist! mixture of issues, in part because to hit wood, I've got to drill 50mm into the ceiling. Previously, I was drilling until I felt free air, and pushing the drill bit up further and feeling nothing. That bit was probably 40-45mm. I swapped to a longer bit ( after removing a screw I found, which was inserted at an angle to get to the joist, and for what it is worth was a 75mm long screw), I hit free air, but then pushing up further into the ceiling, I finally hit wood.

Now, maybe I'm just way off base, but I expected the inside of the plasterboard to be snug up against the joist, so that if my drilbit got through plasterboard and then found free air (and had at least a few mm of space) then I was in the wrong spot.

Is my understanding wrong here? Free air was a bit odd to find and ultimately, it seems that is what led to me not hitting the joist.