Magnet trick for ceiling joists

[Wayners]

Double board for garage fire regs might account for thickness.
Not sure why it seemed like nothing there. Guess I've had similar in the past with confusion and a little frustrating.
Sounds like your sorted.

 

[conny]

Get a wire coat hanger and straighten it out. Drill a hole sufficiently wide enough to poke the coat hanger through. Moving around in a circular motion, poke the rod in until it hits a solid object. Mark the rod where it goes through the ceiling and then withdraw the rod. Lay it against the ceiling in the direction you hit something. By lining up the mark on the rod with the drill hole you will get the position of the joist. This point will be the face of the joist so you need to allow a few mm more to drill a small test hole. Once you have established that it is a joist, (and the direction it runs in), measure 400mm to the side and, in line with your test drilling, do another test drill. You should strike the next joist.
To judge the width of the joist you need to drill a few holes close together in a line across the joist. When you hit free air that is one edge. Work across the opposite way until you hit free air and that is the other edge. The distance between the two outer holes, (free air), is the approximate width of your joist. You can now calculate where the centre will be on subsequent joists.

 

[Deleted member 174758]

ah, hadn't come across metal framed ceilings before - are they commonly used in new builds in the UK?

Very. Much faster to install than timber cheaper and arguably less prone to fire damage. And I'm a joiner saying that

.... (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?

If you have living space above the garage I'd expect there to be some form of firebreak between the garage and the living space depending on the floor structure (Edit: you found double boarding, so that's the fire break - screws driven at an angle to hit the stud/res bar/top hat indicates that they didn't get the spacing quite right). If you are trying to drive a 5mm diameter woodscrew through a metal section it will not pierce the metal in most cases - 4mm on the other hand will and it will pull up - and your ceiling is possibly on 600mm centres and may be onto resilient bars, which are all of 20mm wide and can be difficult to find. Note that I don't drill - I just screw

You might want to note that you"ll additionally sometimes find thin metal joiner strips (about 100mm wide) underneath the joints of the top layer of double boarded walls, etc. This is used to tie sheet edges together where there is no stud beneath

 

[freddiemercurystwin]

Double board for garage fire regs might account for thickness.
Not sure why it seemed like nothing there. Guess I've had similar in the past with confusion and a little frustrating.
Sounds like your sorted.

You don't typically need 2 layers of plasterboard for a ceiling above an integral garage for a dwelling. One layer of ordinary wallboard will typically achieve 30mins.