Chemical DPC and re-plastering

[BarkingMad]

Hey all

Been lurking for a while picking up some useful advice and just registered.

Bought a house which was in need of a fair bit of work but didn't have much money so have been slowly doing what I can myself (hence the username because I must be Barking Mad to have taken this on!). Before I bought the house the previous owner had a chemical DPC installed along one wall of the house which had been suffering from rising damp. This was injected from the outside only as it's a solid wall but the plaster on the inside remains and will need to be removed and replastered.

I contacted the company who did the DPC and they advise that the bottom metre of plaster needs to come off and be replastered with a 3:1 sand/cement mix with a salt retardand additive. Now my understanding is that where plaster has suffered from damp in the past it will continue to attract moisture from the atmosphere and therefore needs to be removed and re-done. But why with sand and cement when the DPC was injected from one side only? Forgive me being sceptical but is it because it's this mix with additive, rather than the DPC, that wil stop damp showing on the inside

I've used bonding a few times in the past and didn't make too bad a job of it so am going to have a crack at this. I've used sand and cement too in the past but not for plastering or rendering, only as a mortar for brickwork. What worries me is that sand and cement mix is so much less workable than bonding and I'm unsure of a few things.

Will I need to PVA the wall just like if I was using bonding?
I assume I'll need two coats as it will be about 15 - 20mm thick in total so half on first coat, wait to harden a bit, scratch and apply second coat?
How much sand and cement will I need for about 10m square worth of render?
I can get the additive from the company who installed the DPC and although I figure I could get it cheaper elsewhere it at least means there is less argument in the case of the DPC failing.

Any tips or advice gratefully recieved. Thanks.

EDIT: Just found the render estimator in the building section. According to that I need about 3 bags of cement and 12 bags of sand. Sound right?

The other thing I was thinking is that if I'm not supposed to PVA the wall then do I just wet it down before rendering?

 

[jbonding]

forget rising damp it doesnt exist. Sand and cement with a waterproofer, forget paying more of the dpc company because they wont guarantee any work they havent done themselves. Pva is not required just a splash of water to kill any suction.

"But why with sand and cement when the DPC was injected from one side only? Forgive me being sceptical but is it because it's this mix with additive, rather than the DPC, that wil stop damp showing on the inside"
sounds like a wrong diagnosis in the first place.

 

[gregers]

forget rising damp it doesnt exist
???????

yes it does

as long as the dpc has been installed correctly and not done by a bunch of muppets.it should last.
they done the chemical dpc from 1 side,they drilled and injected the first 3.5 inches of wall then using a 12mm drill for clearance then redrilled the bricks to a depth of approx 8 inches or they done it with a gravity feed system,probably because the previous owner did not have enough money to do the job properly,ie by removing the internal plaster where required,if they had then the dpc company would of drilled and injected from both sides of the wall, they replaster to a minimum of 1m or more if there is an indication of high readings on the damp meter,they use the additive because of efflorescent salts that migrate to the surface of the plaster when a dpc has been installed,DO NOT USE BONDING this will suck moisture out of the atmosphere(dont know about hardwall),get hold of a damp meter and check the wall yourself,you could install a new dpc yourself,hire the kit from a hire shop,then replaster but make sure you cut the plaster back from the dpc line to eliminate bridging.

 

[jbonding]

chemical injection doesnt work either and neither does the damp meter you speak of. If the original owners didnt have enough money to do it properly why would any firm do half a job, oh i know, an excuse.
"make sure you cut the plaster back from the dpc line to eliminate bridging." now this could be the problem

 

[gregers]

chemical dpc doesnt work
and how do you come to this conclusion?

 

[jbonding]

chemical dpc doesnt work
and how do you come to this conclusion?

How could it ever work, its a con i was doin it 10 years ago and it doesnt make sense. drill some holes inject with fluid hey presto damp gone. i dont think so. If damp could rise that high in the first place how will injecting bricks with fluid stop it? if it does work what is the point of hacking off a metre up and floating with sand and cement with a waterproofer in it if the injecting is doing the job? Usually mis diognosis and dont get me started on damp meters

 

[BarkingMad]

Thanks people. I dodn't mean to start a debate though.

Basically, the plaster on the wall in question needs replacing anyway because it's all bubbled and blistered at the bottom so I'm going to have to do it regardless. I may as well follow the Dpc firms guidelines too and if it doesn't work then hey ho, win some lose some.

Have I used the calculator correctly and are 3 bags of cement and 12 bags of sand sufficient for 10 metre square?

Thanks again.

 

[gregers]

chemical dpc doesnt work
and how do you come to this conclusion?

How could it ever work, its a con i was doin it 10 years ago and it doesnt make sense. drill some holes inject with fluid hey presto damp gone. i dont think so. If damp could rise that high in the first place how will injecting bricks with fluid stop it? if it does work what is the point of hacking off a metre up and floating with sand and cement with a waterproofer in it if the injecting is doing the job? Usually mis diognosis and dont get me started on damp meters

so the thousands of houses getting bought and sold each year that surveyors think might have damp are all wrong aswell,so we will have to start building houses without a dpc in them then because as far as you are concerned RISING DAMP DOES NOT EXIST,id like the resident b/suryeors and inspectors on this forum to put there two peneth worth into that debate,me ill just bow down to your superior knowledge over it.