Thin Coat render problem

[Rich123]

I’ve had my extension rendered for around 2 years. One corner on the front has badly cracked around the corner bead and the render has lifted off by about 4 mm over an are around 200 mm .

The render is applied on to a render board which is a magply external BBA board. The thin coat is the Johnson high silicone 1.5 .

First issue , Johnson’s advise that the magply isn’t an approved base and therefore thats why the render has lifted, but it’s ok everywhere else . They approve the knauf external board , which is a similar product , cement based board. 
Second issue , when the board was installed the corner was not battened due to the detail at the corner with a post , therefore the board over sales the batten by about 200 mm , which seemed solid at the time. I’m thinking this may be the issue causing differentIal movement at the joint ?

3. So I can remove the corner panel and replace it with the approved knauf board in that location. I will then batten the corner and reinforce it to stabilise it, but the render will need to be repaired and re sprayed leaving a joint line . Luckily it’s beside a door opening so only about 800 mm either side of the corner, but I’m concerned at cracking between the new and existing render as I will need a joint vertically above the door head about 400 mm.

Any thoughts on the above would be appreciated

 

Edited February 11, 2023 by Rich123

[ProDave]

Some pictures may help.

 

Of course the manufacturer is going to wriggle out of any responsibility, that is what they do, they are never going to admit their product is not fit for purpose so they will blame wrong installation, poor installation etc.

 

Movement won't help, but did the joint really move or does that remain just a theoretical possibility?

 

What is the weather exposure at this point?

[Rich123]

Hi the movement is a theory , the weather exposure is a south corner so lots of sun. But the cracking is along the length of the render bead. Could be damp board or something else 

[Nickfromwales]

It doesn't look like the mesh is continuously evident, and possibly that the mesh didn't get wrapped around the corner ( in both directions ).

 

The mesh is obvious at the bottom of the pic, but then is not visible / evident from the middle and top ( you can see the broken fibres where the mesh defo was ).

 

What ar the manufacturers guidelines / installation practices regarding mesh at corners / where mesh stops and starts? eg overlap etc. 

 

Also consider bonding Conpacfoam or Marmox to the steel, and then bonding AND mechanically fixing it the the timbers. This will give a very solid substrate for the following layers of build up. The floating timbers will be the cause for sure, and there would have been nothing stopping them making a timber L section to cap the external angle and allow opposing fixings for the boards. Just laziness on the installers behalf here I'm afraid vs manufacturers responsibilities ( unless they show this as a permissible detail? ). 

[Nickfromwales]

[nod]

It’s a quick and easy repair on a small area 

As a contractor I use 1000s of meters of cement boards and there’s no difference between any of them Other than Knauf are 40% more expensive than what I usually use 

Over lockdown I was bringing them in from Poland without any issues 

It’s pretty poor Johnson’s are looking for lame excuses Rather than getting to the bottom of it 

 

I’ve stopped quoting for Webber Rend jobs Due to the reps all following the same line when there’s a batch match issue 

[ADLIan]

Problem is that magply is not a ‘cement’ board it is based on magnesium oxide. Problems were identified with these ‘MgO’ boards several years ago including not being compatible with some render systems. Several warranty providers, including LABC and NHBC, banned their use.

[Nickfromwales]

1 hour ago, nod said:

It’s a quick and easy repair on a small area 

This needs a remedy and then a repair? Two bits of board flapping about on that corner is terminally ill AFAIC.

[nod]

2 hours ago, Nickfromwales said:

This needs a remedy and then a repair? Two bits of board flapping about on that corner is terminally ill AFAIC.

Definitely 

[nod]

Just now, nod said:

Definitely 

I give a no quibble five year guarantee 

Any issues will surface within that time 

[joe90]

1 minute ago, nod said:

I give a no quibble five year guarantee 

Any issues will surface within that time 

It’s a shame you did not do @ProDave ,s for him 🙄

[nod]

30 minutes ago, joe90 said:

It’s a shame you did not do @ProDave ,s for him 🙄

Definitely 

[JoeBano]

That’s poor fixing on the concrete board. Should have been fixed on the corner 150mm spacing with stainless screws. Even a batten horizontally would have been better instead of vertical. I disagree with all the concrete board being the same, I’ve used many different brands some snap with a knife some you need a diamond disc some snap when you look at them. Just poor install not the products. Weber are snakes but still the best product on the market