Plaster cracking on blockwork

[Richard C]

Hi all you plasterers.
Blocked up a cavity back door opening last September, it was into an internal porch but is now totally inside. I used Thermalite blocks both sides, tied into the existing brickwork both sides & between the cavity. I then plastered using Hardwall with a multi-finish skim. The blockwork was pre-treated with one sealing coat of PVA & then a wash coat immediately prior to applying the Hardwall. The original finish was a sand/cement render with finish skim over old fashioned Breeze blocks.

I am now slightly peeved to see hairline cracks appearing along the outline of the blockwork, what (if anything) did I do wrong? Through necessity I have taught myself to put the stuff on, learned how it behaves & can now achieve a very good finish, even on ceilings. All of the repair work & plasterboard skim work I have done in the rest of the house is OK, it’s only the stuff on the new blockwork openings that seems to be playing up. I have a large new wall to plaster & I don’t want the same thing happening again!

 

[Static]

Have you been doing any other works (ie, creating a new opening or digging a foundation) near this opening after its been closed up and rendered?

 

[Richard C]

A new conservatory went in adjacent to the old door last year but this was completed before I bricked it up. Also, the front wall of an adjacent, old extension has just been knocked out & is currently being rebuilt but the light cracks had already appeared in the plaster by then. On tapping it appears sound & is not detaching from the blocks.

 

[Richard C]

No more takers then!

 

[dougalhouse]

 

[jbonding]

did you use fibre tape where it met the original wall?

 

[Richard C]

did you use fibre tape where it met the original wall?

It's not cracking there, hairline cracks are tracing the outline of the blockwork underneath the plaster/

 

[Static]

How long did you let the blocked up opening dry before rendering it?

 

[jbonding]

did you use fibre tape where it met the original wall?

It's not cracking there, hairline cracks are tracing the outline of the blockwork underneath the plaster/

Each block or around where the doorway was?

 

[Richard C]

How long did you let the blocked up opening dry before rendering it?

The blocks were left unplastered for about 3 months while the rest of the kitchen was being finished off so everything should have settled down to the ambient moisture by the time it was plastered. The blocks were PVA sealed & then a bonding coat applied immediately before the Hardwall was applied flush with the surrounding plaster. A further bonding coat was then applied over the whole wall immediately before a Multifinish skim coat was applied over the new blockwork & the rest of the wall. The new skim finish over the rest of the wall is still fine with no signs of cracking, even a couple of cracks over another door that were raked out, repaired & reinforced with fibre tape show no signs of reappearing.

The original, surrounding wall had 12mm render/plaster finish & I’m now wondering if this was too thick for the Hardwall, in other smaller areas that needed repair I used a render base to match the original with a Multifinish skim & this hasn’t cracked.

 

[Richard C]

[/quote]

Each block or around where the doorway was?[/quote]

Around the outline of each block; no cracks around the outline of the dorrway yet.

 

[stanna]

Could be to strong a mortar mix for the thermalite.

This is an increasing problem that i was discussing with local BI last week.

People assume the more cement the better but as Thermalite are so soft they tend to expand with temp variations but the strong mix doesnt move with them.

The biggest cause of a strong mix in my opinion is half bag (or full bag as they are now) mixers. Think Thermalite recommend 5-6 sand to one cement but most labourers just chuck a 25 kg bag in and fill with sand giving more like 4 to 1.

Wouldnt worry about it falling down but cant see how you will stop the cracking unless you take off plaster and dot n dab pboard on.

 

[Richard C]

Hi Stanna, you’re a bit of a sleeper, first post in 2006 & then nothing till yesterday Welcome back but uf I can give you a little tip, look at the last post date before posting. That early thread of mine you resurrected dates back to June 2006 or was that the point ; a lot of plaster, render & boards gone under the bridge since then M8 & I'd only just progressed from skimming boards back then.

I agree with you about the probably cause being a strong mortar mix, the wall hasn’t fallen down, the cracks never got any worse & the plaster hasn't blown; a little Easyfill sorted them out.

 

[stanna]

Yep sorry about that.

Was looking at the mini thread list at bottom of pages.

Doh!