new plaster cracking

[Lisa]

Please help me! I recently had some skimming done by an amateur diy bloke, and as the skim has dried it is cracking. These cracks start off as being like light pencil lines on the wall, but over a few weeks they grow in length and width. Any ideas about why this has happened? My house is over a hundred years old, but base plaster is no more than

10 years old i'd say and looked reasonably ok (to me - a complete know-nothing of diy!!) when the wallpaper came off. My amateur re-did one area believing he may have forgotten to bond it! But as other larger areas have begun to crack too, and his re-done bit has gone again too, he is less inclined to get involved! I guess you really do get what you pay for!!! But if anyone could tell me possible reasons for these cracks i would be very very grateful! Many thanks, Lisa x

[Andrew Gabriel]

As the house is over a hundred years old, it will have been built with lime mortar and lime plaster originally. Lime doesn't set hard like cement mortars and gypsom plasters used today. This means that lime houses routinely move around, and the lime is tollerant of this. However, modern gypsom plaster is hard and will not flex as the house moves, so you will get hairline cracks. Usually this is not enough to matter and the cracks are small enough to vanish when painted. The correct way to do this would have been to use lime plaster, but that's rarely done nowadays unless required for a listed building, and it takes weeks to set.

If the cracks are of any significant width, you may have some other problem.

[Lisa]

Thanks very much for your reply Andrew, sounds like you really know your plaster! The plaster that i found under the wallpaper, before the skim, didn't look as old as the house and i think it was probably done by the people who owned the house before me, within ten years. Judging by some of their other DIY i wouldn't think they had taken the trouble of using lime as the base plaster but it seems to be just the skim that is cracking. As the cracks are developing they look too deep to cover with paint alone. Any ideas how i can get the smooth walls i'm after Andrew? Thanks again, Lisa x

[Ken]

It all depends on what the backround was. I have skimmed over old limewash paint with great success, you just have to prepare it properly 1st. Normally, cracking like you describe is because the backround has'nt been prepared right. It sounds like you had a high suction surface and the way to do this is to apply a good coat of pva to the area, let it dry and then apply another coat and whilst it is still wet, apply the new plaster. But how to get over your problem now. You either scrape it off and start again or hang lining paper, filling the cracks 1st.

ken

[Rob graham]

If you scrape it off and replaster over expanded metal this should cure it.

Rob Graham

[Lisa]

Thank you so much for your advice :o) I love that i can get advice like this! Thanks for taking the trouble

- any offers to do it for me too? lol xx

[Ken]

'- any offers to do it for me too? lol xx'

Depends if you are 18 years old and blonde. Only joking.

ken

[Andrew Gabriel]

How wide are the cracks and how long ago was it plastered? You might have some other problem if they are really too wide to fill with paint.

[marble]

Sounds like it wasn't bonded. Generaly to do this kind of job the plasterer generously paints the area to be plasterd with PVA glue slightly diluted with water then he plasters that wall while its still wet and sticky.

The bit he re-did, he scrapped off the old skimming right? Which means you wont have too hard a job rip away all of his poor work. If he just plastered over the bit that he said he *may* have forgoton to bond then where is the sence in that?

[Lisa]

Ken, i'm blonde but unfortunately a couple of 18 year olds in one! ha ha

[Lisa]

Hi Andrew They start off looking like pencil lines, but it was done about 2 months ago now and some cracks are as wide as if a pencil was dragged through the plaster (best way i can think of describing it) and this is still growing. So paint would go over but you would still see the crack underneath it, and i don't know when it wll stop cracking. Thanks again, Lisa x

[Lisa]

Hi Marble, i think it points to the bonding too. But when he re-did the bit he "may" have forgotten to bond, he did bond the second time (after scraping the 1st skim from that section) and cracks are still appearing there 2nd time around. I'm wondering if he "may" have forgotten to bond the rest too, where cracks are at their worst. But he must also have done something wrong on the re-done bit? I am wondering if, as Kev said, he didn't follow the right process for bonding being a novice. Perhaps the dried coat of pva first would've made the difference - or perhaps he didn't use enough pva? or too much? Really appreciate your help! Lisa x

[Lisa]

Andrew - my pencil example may not be very good! I don't mean the entire thickness of the whole pencil, but a couple of mm into the sharpened end - so maybe 2 or 3mm wide. I'm not very good at this measurement thing - well i am blonde as we've already established :o) L x

[Andrew Gabriel]

If it was a bonding issue as others have suggested, plaster must be loose to have moved that much, so much so it would be falling off which you have not said is the case. If it was some other fault with the plaster mix or application, cracks would have formed within a couple of hours as the plaster set, not months afterwards.

So I'm left thinking it's either excessive movement in your walls, or the scratch (base) coat has lost key and come away from the wall, so it can move by itself. Does the plaster sound hollow if you tap it? Does it noticably move if you push on the plaster?

[marble]

Hi Lisa It should be very difficult/ impossible to scrape off a properly bonded re-skim ,and as you've said there are no cracks in the old base plaster, I think that settles what the problem is. The only thing you can do is scrape it off and get a better plasterer in. Either he could try the 2 coat PVA method Kev mentioned or consider using the more expensive bonding agents like Bond-It from British Gypsum or the Knauf equivelent from Wicks.

[nthng2snet]

What was the colour of the area that was platered over? if was pink I would say it was recent( within 10 years plaster). If it wa white-ish and had a thick grey powdery backing then I would say it was original Lath and plaster. What was the reason for plastering over? Was the background loose or flakey? Basically I would start from what the condtion of background surface was/is and move through to materials used and method of application. I have walls that have been rendered and platered 4 months ago and are showing new cracks evey day ( haven't been painted yet). I put this down to shrinkage nd the fact that there is no heating in the house and its often left open to drafts and cold weather. Thats my particular circumstance so you see every situation is different.