DIY question – cutting into plasterboard ceiling

I need to cut an access point in the plasterboard garage ceiling to identify the run of some hot water pipes prior to a heat pump install. Will need to be fairly big (30cm x 30cm maybe). Am I better off trying to cut between two ceiling joists and then batten the piece to refit it in the hole, or include a joist in section and replace the plasterboard with a new piece?

I don’t think I can get away with just a tiny hole as it’s full of insulation. Thanks!

if you have to across joist so you can screw up, but if its only little you can add some battens around/across your hole to screw the piece back into

The likely hood of being able to cut a 300mm hole in between joists is slim.

Joists should be set out at 400mm centres so you don’t have much to play with.

Just cut a hole where you neee/want to

I too would cut where I needed, rather than thread the needle between joist.
I mean, you are going to cut in and discover exactly where you need something to be is where the ceiling roses, a couple of pipe junctions and a couple of noggins are anyway – such is The Law when it comes to any DIY.

If your garage ceiling isn’t painted then you should be able to work out where the edge of the sheets are.

Subsequently there will be joists/noggins there to support this, don’t know if that makes it any easier for you.

If you’re going to poke your head through it, you don’t want a joist in the way. Aim for the gap.

You can fix battens to the plasterboard around the opening (on the top of the ceiling) using one of the ‘stick like —‘ etc. builder’s adhesives and when they’re stuck, glue the piece removed back to them.

Or…

Endoscope.

If it’s a big hole like the 300×300 you mention I would usually get two lengths of wood Say 500 long and fix them over the hole by screwing them through the plasterboard to hold them in place then fixing the bit you just cut out to the wood I’d that makes sense

**Edit**  too slow at replying but same as above

Hot water pipes? Borrow/rent an IR camera make small hole* to confirm.

*In thebplasterbpard but not I repeat NOT the pipe.

Top tip for cutting pieces out of plasterboard – make the cuts at an angle.  Then you don’t need a batten behind it to re-fit – just a dab of polyfilla around the edge and push it in until it’s flush. You can’t push too far.

Yeah, endoscope, before cutting big ‘ole.

Really handy to have sometimes. Great for diy surgery on yourself too.

Hot water pipes? Borrow/rent an IR camera make small hole* to confirm.

Thanks! I have an endoscope camera already but it’s pretty useless for this sort of thing as the void is filled with insulation. The only solution seems to be to cut a hole and stuff my arm inside. To my annoyance I’ve cut three holes now and still not found the bloody pipe.

If you’re going to have pipe work up there, would you consider installing a hatch?

Either way, i’d go for the a gap in-between a joists.

Maybe a nail/pipe finder like this to work out where the joists are;

https://tinyurl.com/24umt3yd

sandwicheater beat me to it, might as well make a permeant access hole. I’d use something like this to cut the plasterboard.

https://www.screwfix.com/p/forge-steel-7tpi-wood-drywall-pvc-jab-saw-6-3-160mm-/210ky

Stop cutting when it gets hard, that’s the joist.