Will a new build ceiling be plasterboard?

Yes it will be plasterboard and yes fix it to joist. Unless it is extremely light in which case a plasterboard fixing might be sufficient 

I'd say it would be plasterboard. 

How heavy is the plant holder? 

Can you access the ceiling void via the floor from above? Would it be easier to move a wardrobe and go through the floor, add support noggins then hang the holder?

Do you have or can borrow an inspection camera?

It will be, and I'd have zero hesitation about using a plasterboard fixing. They're bloody strong. The expanding metal plug type ones are great and spread the load nicely. 

If you can buy/borrow a setting tool, even better. Obviously a just would be best, but like you say, it's a case of if the positioning is suitable.

Cheers all. I think I've found appropriate fittings on eBay so I'll get some of them.

Posted by: retrorick

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Would it be easier to move a wardrobe and go through the floor, add support noggins then hang the holder?

I love how this is described as 'easier' 🙂

You're overthinking it. Plasterboard is stronger than you think - even those metal screw-in anchors (eg Fischer Self-Drill Plasterboard Fixings Metal 35mm 100 Pack - Screwfix) will hold close to 30kg before pulling out.

Easier than repairing a fist sized hole in the ceiling

Posted by: Fursty Ferret

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even those metal screw-in anchors (eg Fischer Self-Drill Plasterboard Fixings Metal 35mm 100 Pack - Screwfix) will hold close to 30kg before pulling out.

Even on a straight pull? Saying that, we're probably talking 3-4kg max of plantpot

It will pull straight out using a plasterboard fixing.30kg is in a vertical wall and only if its put in perfectly. Iirc.  I have seen loads of plasterbord fixings fail it much lower loads

Spring toggles are what I’d use, plenty of different load ratings to choose from 

I've set up a test rig! 😉

Using some 9mm plasterboard with a thin skim on it. 

Some Aldi work zone plasterboard fixings. 

8mm hole. 

Dangling an 8kg weight between an improvised 400mm centered joists. Back to front so not the best rig as your plasterboard will be nailed/screwed to the joists rather than sat on top.

I’d not trust a plasterboard fixing on its own, whatever the type.  How many points of attachment are there and how much does the ‘thing’ weigh?

Plasterboard can hold about 30kg vertical weight spread evenly over 1m2.  You are putting a point load over a much smaller area, relying on a clean drill and no damage to the face board when fixing.   I’d want the weight into a larger pad behind the board to be sure. if I was doing for a customer, I’d hole saw where the fixing is and slide a baton behind the board, then screw the ‘hole’ back in and easifil/paint.

Retro Rick - now lift that weight up and drop it again.  This is what would happen if someone would be to pull on the fitting, drop the planter whilst filling etc.

Timco produce a flat bar plaster board fixing , as do Fischer. Both resembles small lolly pop sticks on a plastic cable tie.

You drill a small hole , insert the stick , pull on the cable tie and it squares it over the hole ready for fixing. 

Or use a grip it . TV clamp kit 

1cm drop coming up....

My shoelace attachment detached and 8kg of dirt* emptied itself over the settee and carpet after knocking a hot brew over my lap! 😉

A few sub 5cm drops pulled the fixing through. 

Found a metal self tapping fixing like mentioned above from screw fix. 

Same 8kg weight...

Didn't even survive 1 drop!

You might fare better with an elastic string. Doesn't need to be a really soft bouncy thing, but something to take the edge off the peak loading. Though it really doesn't sound like that good an idea to me anyway. It'll get a lot more wear and tear in use than a light fitting for example, and a plasterboard ceiling probably wasn't really built with such loads in mind.

Can you mount the plant hanger on a bigger circle cut out of 11mm mdf  which could be painted to match the ceiling and mounted with three or four fixings? If you were being posh you could rout it out with a rounded edge.

Or simply insist that the only place that the thing could be hung is off a joist.

Both options are better than being blamed for a hole in the ceiling when the thing pops after a vigorous watering.

Try three mount points connected with wire like an inverted pyramid.  a) it's no longer a direct vertical load and b) if one fails you've got two left.

Blazing Saddles has the correct approach, unless you can find a joist to secure it to.

If you're thinking of impact loads then you need to start moving away from a light inextensible cord, or just add a spring and a damper 😉

The tests are actually very interesting, although perhaps a bit redundant until you prove there isn't a joist in the desired location?

However fyi ceiling board will be 15mm if not 2x12.5mm, so you might have more strength than your test 

However fyi ceiling board will be 15mm if not 2x12.5mm, so you might have more strength than your test

I very rarely come across a house with 15mm ceiling boards, let alone 2x12.5mm, most are 9 or 12.5.  

I chose 9mm for my non scientific tests as I reckon the builders would have used the lighter (cheaper) variant of plasterboard for the ceilings. 

There was some stretch in the shoelace. 

I think finding a joist will be easy enough in the corner or around the edge of the room. 🤔

Fire wall is 2 x 12mm sheets with offset joints , but that's between garage and habitable rooms iirc .

Posted by: IHN

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Even on a straight pull? Saying that, we're probably talking 3-4kg max of plantpot

Yep, have a quick look on YouTube. No shortage of people comparing plasterboard fixings. I think historically an advert for some red wall plugs involved dangling a washing machine from the ceiling at one point.

Plasterboard will most likely be 12.5 with no skim as tape and fill is often the norm on sites. 
Wouldn’t use MDF either as that doesn’t hold screws very well. 
As most have said plasterboard fixings on they type of load will most likely fail and you wouldn’t want a child beneath it when it does. 
Either find a joist, which shouldn’t be too hard to do, look for nail pops or lines of screws can sometimes be seen with the filler. Also taping the ceiling to hear the change in noise gets you a good idea then use a small 2mm metal drill to confirm that way you only have a tiny hole to fill. Make sure and find both sides of the joist so you can get in the middle of it. 
Pre drill the holes too before putting the screws in as it helps stop splitting. 
Failing that it is a case of removing a section and getting some timber in ideally fixed to the joists if possible if not a big section to spread the load across the board but you are then relying on the screws holding the board to the joists and if it is a typical new build there won’t be the required amount just the absolute bare minimum and the shortest they could get away with. 

just put it in a joist? you can use a magnet to locate the screws holding the plasterboard up, mark a couple to work out where the line of the joist is.