Freshly plastered ceilings have developed many cracks

[Wayners]

Cracks from heating coming on? When did they appear? I've had that a few times even though radiator was turned off... But it wasn't! Came on and ceiling dried too fast and cracks appeared the following day. Never seemed to happen in summer. It's the forced uneven drying from artificial heat causes cracks like that in new plaster.
If your going to get cracks from movement they happen very quickly after plaster is applied. If a ceiling don't crack in first 3 days it's unlikely to.
Won't be plaster too thin.
Tape works best when it's near the surface. Deeper the tape the less it works from my experience.
If no heating then that's flex from someone walking above. Have you tried that and watched from below for movement? I've had that a few times. Had to install cross timbers fixed to each joist.
If you called me back after I skimmed that I'd be all over it looking for why it happened.
Always a reason

 

[akist]

No heating in the house yet.

Weather was 20C-22C/day and 12C-13C/night in first week, rising to 19C-25C/day and 11C-16C/night the following week. However, 4th week in, dropped to 11C/day and 2C/night. But I think the cracks had already been.

Ceilings are meshed with steel beams, largest joist run is 3.7m. Absolutely rock steady.

Walking above yes and no. 70% of one ceiling has seen no one walking above.

My observation is that in no other room, except the two rooms with a thousand cracks, can I see see the outline of every single board. It is whitish colour whereas the rest of the board has a more orange-pink colour.

****************************

I have filled in 3 small sections in different places, and so far they are holding...

 

[robinbanks]

It will be nearly no effort at all to use a multi tool to cut out a small section of ceiling across a crack (use a magnet to locate any screws and avoid having them in your section) then you can look at it sideways on and see the depth of skim. If it ain't the manufacturer recommended 3mm I dare say you should be getting the plasterer back to throw another skim on. If it is, then perhaps something wrong with the plaster or the mix

At the end of the investigation, if ceilings are okay as you say then screwing the investigation piece back into place is a doddle. If you're skimming, no need to make good. If you aren't, then easifilling over it is quick and easy

 

[Wayners]

You can see outline of boards most of the time. It's where the thicker material is at joints when polishing off.
I don't know then why yours cracked. I thinking movement. If it's reskimmed and it's movement it will crack again though.
I often taped ceilings using fibre fuse tape the day before I skimmed. For years if I could I would tape before and see if it cracked over night before I skimmed.
This also stopped the cracks from shrinkage with dry cut board joints, although I always added PVA to joints as I fitted boards.

 

[sparkwright]

Exactly that. Oh the irony. I have been given 100% chance those ceilings will crack but they have not yet.

I think now, on the ceilings with a million cracks, the plaster is spread too thin, since I can see the outlines of every single board. Being it too thin it then cracked. Other ceilings you cannot see boards at all; probably a thicker coat, or maybe two coats instead of one.

From everything you have said, could it be that if you plasterboard over ply wood, then you SHOULDN'T tape plasterboard joins?

 

[akist]

From everything you have said, could it be that if you plasterboard over ply wood, then you SHOULDN'T tape plasterboard joins?

Ha ha, good idea. But why would tape cause a problem?

Not sure if I mentioned this, plywood is a prolific material in construction: over the rafters, under the rafters, dormer cheeks and fronts, membrane above and below joists, partition walls, tiled over vertically or horizontally, it is used everywhere.

To me it looks as if the typical minute vibrations of a timber joist floor have caused cracks on those two ceilings but not on the remaining ones. And the only difference I can imagine must be in the plaster, ie the thickness of it or the preparation how it was mixed maybe.

I have eliminated many other factors which could contribute:

weather conditions during plastering and weeks after
construction detail (down to screw size)
age of joists, plywood and plasterboards
amount of steel beams in ceilings
walking above
carrying and storing large weights above
north/south facing rooms

 

[sparkwright]

Just thinking out aloud, the plaster skim can't be that thin, as a very thin layer would almost certainly allow the tape to be just visible when dry.

Could the plasterboard joins be in the same places as the plywood joins?

Could the plywood have been very tightly jammed in, causing pressure on the joins?

Could the joists not have any noggins or herringbone struts between them, where there should have been?

 

[akist]

There are plenty of noggins and steel beams there is absolutely no movement anywhere.

Plasterboards are placed across the plywood and the joints do not coincide

The cracks have only appeared where plasterboards join and nowhere else. On a lot of plasterboards.

Will fill in all the cracks because the couple I already did seem to be holding.

 

[Koolpc]

I have seen it where all visible cracks were raked out, small holes made along the cracks, then expanding foam used.

Then, once dry, all filled and sanded.

Job looked like cracks had never occured.

 

[robinbanks]

Will fill in all the cracks because the couple I already did seem to be holding.

Does really feel like something funny about the mix or the way it dried. C'est la vie?

 

[BlueLoo]

No heating in the house yet.

Weather was 20C-22C/day and 12C-13C/night in first week, rising to 19C-25C/day and 11C-16C/night the following week. However, 4th week in, dropped to 11C/day and 2C/night. But I think the cracks had already been.

Ceilings are meshed with steel beams, largest joist run is 3.7m. Absolutely rock steady.

Walking above yes and no. 70% of one ceiling has seen no one walking above.

My observation is that in no other room, except the two rooms with a thousand cracks, can I see see the outline of every single board. It is whitish colour whereas the rest of the board has a more orange-pink colour.

****************************

I have filled in 3 small sections in different places, and so far they are holding...

Steel beams?

There's your answer.

 

[robinbanks]

There's your answer.

I thought the assertion was that steel beams are everywhere in reasonably equal measure but only some rooms have cracked. Plenty of steel in my timber frame but no cracks to speak of other than one room where the ceiling boards aren't mounted to the same structure and some differential settling has occurred

 

[BlueLoo]

I thought the assertion was that steel beams are everywhere in reasonably equal measure but only some rooms have cracked. Plenty of steel in my timber frame but no cracks to speak of other than one room where the ceiling boards aren't mounted to the same structure and some differential settling has occurred

Probably correct tbh. But.
Application is everything. Steel beams, plywood substrates, no tapping. The installation is all over the place.

Typical Self build tbh

 

[akist]

I thought the assertion was that steel beams are everywhere in reasonably equal measure but only some rooms have cracked.

Yes and only 2 rooms have developed the cracks. I am terrified now and keep checking everytime I visit for new cracks but none so far. Touch wood.

If it transpires that the cracks remain constant, then I will simply fill them in. The issue is that they are hairline almost, and impossible to fill in unless they get raked in a small, maybe 1-2mm, V shape. In which case you are causing a bit of damage. I think the paint itself should cover most.

 

[robinbanks]

Give it a few weeks of everything else turning into a sheetshow and you'll stop worrying about hairline cracks/seeing them.

Put it at the end of a long list of things to get round to when you're done with building mode and into maintenance mode!