Lime Plaster Cracks

[Alex F]

During the month of January I had my internal granite walls plastered in lime plaster. In order to achieve the straight walls that I requested the plaster is particularly thick in places. As much as 70 mm in some areas places.

I have not lime washed the walls as I wanted a natural finish with the crushed slate that is mixed in to the plaster. I am pleased with the finish but I have got several areas with cracks that are up to 2mm thick. The appearance of the cracks is not such as problems as it looks more or less in charater but I am concerned at them worsening. On tapping with my knuckle there is a definite hollow sound compared to the rest of the wall.

These cracks have not tended to appear where the plaster was particularly thick as one might expect. In one instance they have appears where there was an electrical conduit although thes was cover will steel mesh attached to the wall.

Is this normal? I imagine that normally the plaster is lime washed and superficial cracks can be filled somehow. But these cracks are not so superficial.

I wondered whether the plastered did the job too quickly as they seems to start and finish in less than 2 weeks doing 3 rooms of about 5m sq. I had thought you were suppose to wait longer between layers.

I not sure if one just accepts this as part of the course with lime plaster or whether I should be calling them in. I do recall them saying before hand that there would be cracks.

Most grateful for your advice.

Alex

 

[Gervase]

If the cracks are anywhere other than over the wiring conduits, I'm sorry to say that it sounds as though your plasters have done a majorly duff job. The only way to fill in hollows to get a plumb and square finish is to do it in layers no more than 10mm thick and to make sure each layer is firmly knocked down, scratched and wetted before applying the next - otherwise cracks are inevitable.
The hollow sound is also ominous - that's a classic sign that the plaster has 'blown' and is not adhering either to the stone or the scratch coat underneath.
If, however, the hollow sound is just over the wiring conduits, that's not such a major worry. It is a pain, though and indicates that not enough care was taken with the covering of the conduits. Whatever your plasterers say, cracking is certainly not 'normal' with lime plaster.

 

[Alex F]

Hi Gervase

The answers is, no not at all the hollow cracks are only where there are wiring conduits underneath. Only one.

So presumably if one was going to repair the areas it appears to have blown one would have to knock out the areas of plaster back to at least the scratch coat and redo area which would then presumably be a noticeable new patch of plaster standing out from the rest of the wall. Not to to mention the mess. Sounds like a nightmare.

Why do you think this happened?

Thinking back I think the reason they may have said that cracks are inevitable is because of the movement of woodword such as osk window boards which could be a little more excusable. But clearly this apparent blown plaster is another thing.

I will try and take a photo tonight.

Alex

 

[BMcD]

What Gervase is saying is perfectly true but...

Alex F wrote:

During the month of January I had my internal granite walls plastered in lime plaster.

This concerns me a little as it suggests heating may have been used and dried the plaster too quickly giving rise to a weak set due to insufficient carbonation of the lime. If you could let us know it would help.

 

[Gervase]

... it suggests heating may have been used and dried the plaster too quickly giving rise to a weak set due to insufficient carbonation of the lime...

Good point. Did the plasterers ensure that each coat of plaster was kept damp at least for the day after they put it up - did they spray it or hang damp sacking in front of it? Too rapid drying can cause cracking - think dried up river beds!

 

[Alex F]

They definitely did not heat the place so as to dry it out quicker. I do recall them endeavouring to keep the walls damp over the subsequent days by wetting the floors and perhaps other means but they did not hang any damp cloths.

No heating what so ever has been used in the house during or since completion of the works. The main cracks arose or certainly worsened more than 6 weeks after the plastering was completed. This was at a time when I think some unusually warm March arrived. The walls being very thick granite, the rooms don't tend to ever get hot on a warm day but there may have been a 10 degree change in temperature.

Here are some photos. You have to look quite closely to see the full extent of the cracks


[Open Image] [Hosted by PhotoGalaxy.com]


[Open Image] [Hosted by PhotoGalaxy.com]



[Open Image] [Hosted by PhotoGalaxy.com]

 

[BMcD]

To me they look like shrinkage cracks which can be caused when the plaster has not been floated/scoured enough as this compacts and consolidates the sand grains within the plaster and prevents cracks or the backing plaster has not had time to dry and shrink back, as it does, before applying the finish or both.

 

[Gervase]

Aye, bloody big shrinkage cracks - and to be that size it looks a cert that the plaster has been applied too thickly as well. If the plaster's blown as well (sounding hollow when tapped elsewhere other than over the wiring conduits) you need to get the plasterers back in to put it right. A pain, I know, but you've paid good money and deserve a proper job.

 

[Nemesis]

I'm a spot puzzled about mixing in slate - why?

 

[Gervase]

It's quite common in Wales - at last I've seen it in north Pembs and up near Blaenau; it's not a decorative thing per se, just that crushed slate was the easiest aggregate bulker to hand. You can sometimes also see plaster with lumps of crushed coal and slag in it in old mining areas. It would always have been limewashed or distempered, though.

 

[Alex F]

This is quite depressing.

I still not sure how one puts it right. Would one basically have to skim the whole wall or could a patch be done? Presumably if one attempted to patch up the blown area the patch area would be visible. Redoing the entire walls means skirtings would have to be removed and window architrives and carpet, all of which is new.... nightmare.

Do you think the cracks will worsen if were not addressed?

Thank you for your advice anyway.

(correction to previous post it's three rooms of 5m X 5m not 5 m sq.)

Alex

 

[BMcD]

I would have thought that the blown areas could be hacked out and patched up, then (since you're not lime washing and any patching would be visible) the rest of the wall should be scarified and skimmed with finish ( a couple of mm thick ) so skirtings and architraves etc. could be left on and carpets protected.

 

[Gervase]

Agreed - the walls can be patched and reskimmed with everything in situ. Plenty of dustsheets required, naturally, but not an impossible task.