# Forum Home Renovation Plastering  Cornice Cracks

## rapa

Hi All, 
I was wondering if someone could help me out with my current issue. 
My house which is approx 4-5 years old looks like its cornices are coming away from the walls and ceilings in multiple spots. 
What can i use to fix this up as they really look terrible.. Do i use no more gaps etc? 
Any help is greatly appreciated. 
Cheers 
Paul.

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## CPE W&C

Poor workmanship is the main contributor. The 'tradesman' hasnt back filled any mitres by the looks of things, and this is crucial to stop mitres cracking and looking ugly. Youre not alone though, as ive noticed 80% of so called tradesman in Perth arent filling their mitres as they install it. Basically just about every new home will look like this soon except for the ones where the installer had a little pride. The cracks away from the mitres look like a bit of movement, as its not very common for that to happen. Could be lack of adhesive too. Picture 5 tells the story... 
I have done a couple of repairs like this on ornamental cornice, and to do it properly its time consuming and a pita. I got into the roof space and 'notched' holes in the plaster covering the cornice every 800ish and then dropped scrim(hemp/casting plaster) into them hooking it over the top plate to the other side. After that is done, straight stop(fill) the problem areas and redo all the cracked mitres. Alternatively, you can just try and direct fill where it needs and then run a thin bead of no more gaps. Try and push as much cornice cement into the externals as you can though... 
After rereading, someone else might be able to explain it better  :Confused:

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## ChrisZ

Couldn't you just push cornice cement into the gaps from below and wipe off with a damp cloth? Or will this not get enough in and craking will occur again?

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## CPE W&C

> Couldn't you just push cornice cement into the gaps from below and wipe off with a damp cloth? Or will this not get enough in and craking will occur again?

  That was the last bit of my post...  
To be honest, if it was my house id pull it all down and redo in a slightly larger cornice. Getting into the roof space to scrim would only be for a nice ornamental cornice you wanted to keep... Every mitre in my place has cracked for the same reasons. When it was built in the early 90's, the plasterer didnt fill one mitre and EVERY single one has either a hair line crack or a crack you could fit a $1 coin into... Of course movement comes into it, and thats why filling mitres during installation is vitally important

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## ChrisZ

> filling mitres during installation is vitally important

  How much cornice adhesive should you use? Do you fill the corner completely? If so I had better do that on my remaining rooms - I think I just buttered the cornice and then when the cornice was in place pushed a bit of cornice adhesive into the mitre to dress it!

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## CPE W&C

> How much cornice adhesive should you use? Do you fill the corner completely? If so I had better do that on my remaining rooms - I think I just buttered the cornice and then when the cornice was in place pushed a bit of cornice adhesive into the mitre to dress it!

  I always put a fair bit there, usually enough so its attached to both cornice and ceiling. When the next length of cornice is butted in, the filled mitre should spread on the backside of both lengths of cornice. Having a nice fill of adhesive behind the mitre obviously strengthens the join and makes it less prone to cracking...

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## ChrisZ

Thanks CPE. Will pay attention to do that next time.

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## rapa

Just a quick update - 
I decided to try filling one of the simple cracks (like pic 4) with top coat - waited for it to dry, gave it a quick sand and painted over the top.  Cant tell its there unless you know its there. Passes the bosses inspection so look like i have a bit of work to fill the rest of them up.  Will let you know how i go with the ones where the corners have moved. 
Cheers 
Paul.

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## CPE W&C

> Just a quick update - 
> I decided to try filling one of the simple cracks (like pic 4) with top coat - waited for it to dry, gave it a quick sand and painted over the top.  Cant tell its there unless you know its there. Passes the bosses inspection so look like i have a bit of work to fill the rest of them up.  Will let you know how i go with the ones where the corners have moved. 
> Cheers 
> Paul.

  Topcoat isnt a very good solution for this job. Use cornice adhesive and topcoat over that on externals.

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## Rod Dyson

> Poor workmanship is the main contributor. The 'tradesman' hasnt back filled any mitres by the looks of things, and this is crucial to stop mitres cracking and looking ugly. Youre not alone though, as ive noticed 80% of so called tradesman in Perth arent filling their mitres as they install it. Basically just about every new home will look like this soon except for the ones where the installer had a little pride. The cracks away from the mitres look like a bit of movement, as its not very common for that to happen. Could be lack of adhesive too. Picture 5 tells the story... 
> I have done a couple of repairs like this on ornamental cornice, and to do it properly its time consuming and a pita. I got into the roof space and 'notched' holes in the plaster covering the cornice every 800ish and then dropped scrim(hemp/casting plaster) into them hooking it over the top plate to the other side. After that is done, straight stop(fill) the problem areas and redo all the cracked mitres. Alternatively, you can just try and direct fill where it needs and then run a thin bead of no more gaps. Try and push as much cornice cement into the externals as you can though... 
> After rereading, someone else might be able to explain it better

  Sorry I disagree with this post CPE.  I have attended to many situations exactly like this and almost all cases were due to either subsidence or lifiting of the concrete slab in conjuction with a trussed roof.   when the drough broke here in Melbourne the clay soil expanded causing lift around the perimeter of the slab.   
What actually happens is that the outer walls lift slightly also lifiting the trusses in the center of the house.  The ceiling is attached to the trusses so something has to give.  In many cases if you look carefully along the cornice line you can see the paint line.  At first look it appears that the slab has subsided but usually it is the other way.  I have solved many arguments between contractor and builder/owner by pointing this out. 
The fix i use is to fill the cracks along the wall/ceiling and internal mitres with no more gaps and put screws in close to the external mitres and re do the mitre with cornice cement. 
As a rule when this occurs on one of our projects we do the repair at no cost and the builder re-paints at his expence.  There is no point in arguing about it just get it done.  It only takes an hour or two to repair a house of cornice.

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## CPE W&C

> Sorry I disagree with this post CPE.  I have attended to many situations exactly like this and almost all cases were due to either subsidence or lifiting of the concrete slab in conjuction with a trussed roof.   when the drough broke here in Melbourne the clay soil expanded causing lift around the perimeter of the slab.   
> What actually happens is that the outer walls lift slightly also lifiting the trusses in the center of the house.  The ceiling is attached to the trusses so something has to give.  In many cases if you look carefully along the cornice line you can see the paint line.  At first look it appears that the slab has subsided but usually it is the other way.  I have solved many arguments between contractor and builder/owner by pointing this out. 
> The fix i use is to fill the cracks along the wall/ceiling and internal mitres with no more gaps and put screws in close to the external mitres and re do the mitre with cornice cement. 
> As a rule when this occurs on one of our projects we do the repair at no cost and the builder re-paints at his expence.  There is no point in arguing about it just get it done.  It only takes an hour or two to repair a house of cornice.

  I must just be lucky, as ive never had to go back and repair cove cornice on a job ive done, except for the odd hairline crack in a join mitre. Maybe its because i use excessive cornice cement and make sure all mitres are back filled, again with a good amount of adhesive. I would be stunned if cornice i installed ended up looking like that. Picture 5, the external has definitely lifted, but on the same token hasnt even got a lick of cornice cement in it to hold the mitre steady. Its also why im an advocate of battening ceilings instead of direct fix to trusses.

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## Rod Dyson

Actually for the first time I had a look at the photos enlarged and you can definitely see the paint line on the wall.  This confirms exactly what I have posted above, either the interior walls have dropped or the external walls have lifted.  
The drop is nearly 10mm by the look of the pic.  No amount of cornice adhesive or battens would have prevent these cracks.  All you can do is repair them and hope that any movement has stopped. 
Take it from me I have seen to many jobs where this has occured,  if you or I had put up this cornice it would look the same.

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## CPE W&C

> Actually for the first time I had a look at the photos enlarged and you can definitely see the paint line on the wall.  This confirms exactly what I have posted above, either the interior walls have dropped or the external walls have lifted.  
> The drop is nearly 10mm by the look of the pic.  No amount of cornice adhesive or battens would have prevent these cracks.  All you can do is repair them and hope that any movement has stopped. 
> Take it from me I have seen to many jobs where this has occured,  if you or I had put up this cornice it would look the same.

  Not doubting you Rod :Wink:  Your reasoning is sound in my opinion, but ive never been called back for anything like it. Movement is nobodies friend in the building game.

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## Rod Dyson

> Not doubting you Rod Your reasoning is sound in my opinion, but ive never been called back for anything like it. Movement is nobodies friend in the building game.

  Too true

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## Eddy1962

To me it sounds like for one if your house is built on a slab when it was under construction the plasterer used citric acid in the plaster mix when putting up the cornice, or if your house is on stumps the your house may have moved a little. I would scrape out the cornice cement and just fill in the gaps, its a pane but nothing to get to worried about. {Have the same problem

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## Rod Dyson

> To me it sounds like for one if your house is built on a slab when it was under construction the plasterer used citric acid in the plaster mix when putting up the cornice, or if your house is on stumps the your house may have moved a little. I would scrape out the cornice cement and just fill in the gaps, its a pane but nothing to get to worried about. {Have the same problem

  Eddy this has nothing to do with citric acid or any other contractor error.  It is 100% caused by either subsidence  or up lift.  The most likely is uplift due to swelling of a clay soil at the perimeter as it has got wet post drought.  This is happening throughout the western suburbs of Melbourne. 
Look closely at the pics you can see a definite paint line where the ceiling and wall has parted company.  This is not caused by anything else.  The distance between the bottom of the truss and the top of the top plate has changed.  Something has to give it is always the cornice.   
Sometimes it is the top member that holds to the ceiling and other times it is the wall that holds.  It is common to see cracks both top and bottom at different intervals along the cornice.  I have even seen the cornice hold the ceiling and wall causing the ceiling sheet to pop from its fixings.

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## rapa

> Eddy this has nothing to do with citric acid or any other contractor error.  It is 100% caused by either subsidence  or up lift.  The most likely is uplift due to swelling of a clay soil at the perimeter as it has got wet post drought.  This is happening throughout the western suburbs of Melbourne. 
> Look closely at the pics you can see a definite paint line where the ceiling and wall has parted company.  This is not caused by anything else.  The distance between the bottom of the truss and the top of the top plate has changed.  Something has to give it is always the cornice.   
> Sometimes it is the top member that holds to the ceiling and other times it is the wall that holds.  It is common to see cracks both top and bottom at different intervals along the cornice.  I have even seen the cornice hold the ceiling and wall causing the ceiling sheet to pop from its fixings.

  Rod - you have got it in one - i am in the western suburbs of Melbourne,   i am also getting the same movement where the plaster and skirting  boards meet, but rarely on the same spots where i have the cornice issue - it seems to be either one or the other. 
This only seems to affect walls in the middle of the house for some reason. I originally thought this was due to us walking up and down the middle off the roof space installing downlights etc..

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## Rod Dyson

> Rod - you have got it in one - i am in the western suburbs of Melbourne,   i am also getting the same movement where the plaster and skirting  boards meet, but rarely on the same spots where i have the cornice issue - it seems to be either one or the other. 
> This only seems to affect walls in the middle of the house for some reason. I originally thought this was due to us walking up and down the middle off the roof space installing downlights etc..

  The reason it is only the middle walls is that the trusses and walls lift on the external walls creating no pressure on the cornice.  The internal walls are fixed to the slab and will stay put but the trusses lift pulling apart the cornice on the middle walls. 
Cheers rod

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## Batpig

Dear Guys, 
Sorry to chime in late on this one, but it has just occurred to me that where CWE lives (ie. over in Perth), the typically-sandy soils are enviably "stable" in nature (ie. not subject to much at all in the way of shrink and swell due to seasonal moisture change...) as opposed to us poor blighters over here on the east coast, where the foundation soils can shrink and swell substantially between the wet and dry seasons. Adelaide is the worst for it, but Brisbane for one (especially out towards Ipswich...) isn't far behind. Going by what Rod has said, it sound's like Melbourne could be pretty bad too. 
So CWE is most probably spot-on with what he's said, _in the context of the Perth market_ (and other stable-soil areas), but Rod's logic makes a heck of a lot of sense on reactive-clay sites - given the modern "truss" method of roof-construction for houses. For certain geotechnical reasons, the problem would be most prevalent in houses built on reactive clay sites during recent drought conditions. 
Best Wishes,
Batpig.

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