External render cracks & mould growth

[booky1]

Hi. About a year ago I finished the modifications/extensions to my house and had a local plasterer apply the external render. This involved him rendering onto old brick, new lightweight celcon blocks and new dense concrete blocks.

The render was formed from a sand/cement scratch coat with additives (I don't know which one) and a smooth top coat of white sand/ white cement with additives (same as scratch coat). There were no chippings used and no dyes. The walls were PVA'd prior to each coat being applied.

About 3 months ago I noticed some fine cracks in the bell cast area of one gable wall. Since then, the cracks have become larger (see pictures), more cracks have appeared higher up the wall and green mould has slowly spread around the area of the cracks.

It is only this wall that is affected. It is built of new dense concrete block and faces southwest (the direction of our local prevailing weather).

There is one other difference with this wall, although I don't know if it's relevant. Originally, I asked for the house to be wet dashed and this was the first wall the plasterers did. I didn't like it and asked them to change to a smooth render (not their fault so I naturally paid for the extra work). They said they would first try just taking off the top coat and apply a new finish on the old scratch coat. Unfortuneately this 'didn't go off properly' so I committed to taking it back to bare blocks and starting again.

I'm about to ask the plasterers back to have a look. If possible, I'd like to have some clue about the possible causes before we talk. Any advice would be appreciated.

Untitled

• booky1
• 9 Jan 2012
• 1

Untitled

• booky1
• 9 Jan 2012
• 1

 

[peaps]

Hi. About a year ago I finished the modifications/extensions to my house and had a local plasterer apply the external render. This involved him rendering onto old brick, new lightweight celcon blocks and new dense concrete blocks.

The render was formed from a sand/cement scratch coat with additives (I don't know which one) and a smooth top coat of white sand/ white cement with additives (same as scratch coat). There were no chippings used and no dyes. The walls were PVA'd prior to each coat being applied.

About 3 months ago I noticed some fine cracks in the bell cast area of one gable wall. Since then, the cracks have become larger (see pictures), more cracks have appeared higher up the wall and green mould has slowly spread around the area of the cracks.

It is only this wall that is affected. It is built of new dense concrete block and faces southwest (the direction of our local prevailing weather).

There is one other difference with this wall, although I don't know if it's relevant. Originally, I asked for the house to be wet dashed and this was the first wall the plasterers did. I didn't like it and asked them to change to a smooth render (not their fault so I naturally paid for the extra work). They said they would first try just taking off the top coat and apply a new finish on the old scratch coat. Unfortuneately this 'didn't go off properly' so I committed to taking it back to bare blocks and starting again.

I'm about to ask the plasterers back to have a look. If possible, I'd like to have some clue about the possible causes before we talk. Any advice would be appreciated.

Untitled

• booky1
• 9 Jan 2012
• 1

Untitled

• booky1
• 9 Jan 2012
• 1

pva applied to an external wall will fail.

 

[booky1]

Thanks for your response.

What you've said rings alarm bells. Iv'e recently cement rendered a section of internal wall that will form part of a shower encloser. When I researched how to do it, it became apparent that I should not use PVA because of the damp conditions and I ended up using an SBR compound. I should have realised then about the PVA that had been used outside.

Could you please explain what is likely to happen if the PVA bond fails i.e. would you expect the cracking shown in the photos or is something else more typical?

Am I right in thinking that the only reason the mould is there is because the wall is'nt drying out inbetween periods of rain and, if so, what is it that is making the wall retain the moisture for so long?

Why do you think I have'nt had the same problem anywhere else on the house?

Any thoughts would be appreciated. Thanks

 

[tim00]

From your pic of the gable there appear to be various shadow patches - this can sometimes indicate a "piecing -in" of the rendering ie. some areas were allowed to dry out before the next patch was applied: a gable such as yours can be one coated in 8hrs by a 2&1 gang, there should be no stopping and starting.
Now, i might be wrong in what i see in a pic. However, the corner arris also appears to have been patched-in. Was it re-done?

Were you given the option of a Tyrolean finish?

Background and initial treatment of background are the keys to rendering.
All backgrounds/structures move over time: for my money, lime in the top coats is the best bet for render elasticity, and breathability.

 

[roy c]

Those cracks look like the bond has failed (dried out to quickly) I dont think that your plasterer equalized the suction of the different back ground ie. brick thermalite and concrete block. Have you tried tapping the cracked area to see if there is a hollow sound? If the scratch coat failed then the cracks would show through the top coat ... What was the weather like when it was rendered? Hot,cold, frosty? With those cracks if we have a cold snap the frost will freeze any water behind those cracks and could "blow" the render off the wall!!!

 

[peaps]

Thanks for your response.

What you've said rings alarm bells. Iv'e recently cement rendered a section of internal wall that will form part of a shower encloser. When I researched how to do it, it became apparent that I should not use PVA because of the damp conditions and I ended up using an SBR compound. I should have realised then about the PVA that had been used outside.

Could you please explain what is likely to happen if the PVA bond fails i.e. would you expect the cracking shown in the photos or is something else more typical?

Am I right in thinking that the only reason the mould is there is because the wall is'nt drying out inbetween periods of rain and, if so, what is it that is making the wall retain the moisture for so long?

Why do you think I have'nt had the same problem anywhere else on the house?

Any thoughts would be appreciated. Thanks

If the PVA has failed the render will start to fall off. what direction does your wall face?

 

[tim00]

On consideration, i think that Roy and peaps have hit it on the head - utilise their advice.

 

[booky1]

Very sorry for the delay in replying but thank you for your responses.

The whole gable was rendered in one go, there was no patching in anywhere.
The wall faces southwest, the direction of our prevailing weather.
I was offered any option of finish. I chose a smooth finish using white sand & cement because I liked the look of it.
This wall was rendered in winter. The daytime temperature was around 4 or 5C but I'm not sure what it was during the night. The plasterer did have a problem with it 'not going off'. He had to set up some lights and keep coming back to check it had 'gone off' enough for him to carry out the final finishing (rubbing/smoothing with a sponge).

Iv'e spoken to him recently about the problem. To his credit he accepts that it's his problem. He does'nt know why this has happened. His initial suggestion is to wait until we have a dry spell and power wash off the green mould then give the whole wall a 'mortar wash' followed by a couple of applications of a clear water proofer.

I've asked him to take more time to think about it. I'm not comfortable not knowing what the underlying issues are.

 

[peaps]

Very sorry for the delay in replying but thank you for your responses.

The whole gable was rendered in one go, there was no patching in anywhere.
The wall faces southwest, the direction of our prevailing weather.
I was offered any option of finish. I chose a smooth finish using white sand & cement because I liked the look of it.
This wall was rendered in winter. The daytime temperature was around 4 or 5C but I'm not sure what it was during the night. The plasterer did have a problem with it 'not going off'. He had to set up some lights and keep coming back to check it had 'gone off' enough for him to carry out the final finishing (rubbing/smoothing with a sponge).

Iv'e spoken to him recently about the problem. To his credit he accepts that it's his problem. He does'nt know why this has happened. His initial suggestion is to wait until we have a dry spell and power wash off the green mould then give the whole wall a 'mortar wash' followed by a couple of applications of a clear water proofer.

I've asked him to take more time to think about it. I'm not comfortable not knowing what the underlying issues are.

I was asking because the green stuff looks like the start of moss, we get it all over the place here but not on south facing walls etc......

Maybe it has something to do with an addative.....? Not sure.

I don't like his solution to the problem and now we know it was done in the cold weather I would say the lot has failed and it needs to come off.

Summer time is the only way to go to get a good week to get it done without problems.

 

[booky1]

Thanks very much. I agree, anything other than starting again would just be hiding the problem.

I appreciate everyone's time.