Help identifying this plaster?

[moonhog]

Hi

I've just had a re-wire done, and I'm about to fill in the chases / tidy-up before skimming/fitting 7" skirting etc (I'm not going to plaster down to the floor).

I've removed the loose chunks of plaster, but in one corner only there is a soft, chalky layer under the plaster.

Anyone know what this is? Could it be some kind of plaster used in conjunction with an old damp treatment? Lime?

And how best to deal with it - would you scrape it all off before bonding?
Thanks.

 

[roy c]

It is the original plaster (sand and lime) and probably skimmed with Keenes cement or maybe sirapite(pronounced se-ra-fight).It was what they used years ago .I take it your house is quite old? Anyway I would tap around the plaster and see if you have any "Boxy"(Plaster blown away from the wall) areas' If so then get a plasterer in to see how much needs to come off. I would take the plaster off the bottom of the wall to a height of 18-20 inches then coat it with a 5 or 6 sand 1 lime 1 cement scratch coat and the same for float coat and then skim as normal.You will have to kill the suction on the brick wall (Your plasterer should know how to do that) before you put any muck on it and then it should all dry out equally.Dont use bonding (because it wont give you any damp protection if there is damp in the wall). But I would check that out anyway before you get it put back on .. Good luck.. i wouldn't worry about the soft white plaster just scrape it off before you get it plastered

 

[moonhog]

Cheers for that Roy. It's basically a Victorian terrace.
I've already knocked-off the boxy/hollow sounding plaster in the above photo, the rest seems sound. It's only in one corner of the room too, the other walls are generally fine, and where the plaster has blown elsewhere - there isn't any of the soft white stuff on the brickwork.
Sand/lime cement coat - is that the same as what every damp-proof company want to sell me, following the injection of a chemical DPC? The last guys prescribed "1:3 cement/sand rendering incorporating proofing solution". But they wanted to install the DPC above my suspended-timber floor...

I'd like to get a professional around, but I've just spent a fortune paying an Electrician to rewire the house. It also seems like I'm taking the p!ss asking a professional plasterer to come round just to sort out a small corner of a room.
What would be your verdict on Thistle Dri-coat? Apparently it contains cement and lime, and is supposed to "restrict the migration of hygroscopic salts through the plaster"? If I took off 18-20 inches as you suggested, could this be used instead of bonding to be on the safe side?

 

[moonhog]

I've also found this stuff from Tarmac called 'Limelite' Renovating (about £25 / 25kg bag in Travis Perkins) which says is designed to have exactly the same properties as the ratios that you mentioned above:

http://www.pozament.co.uk/products/limelite-renovating/

Like I said, I'd really liketo get a Pro in, but due to budget constraints I've got to do this one myself unfortunately. Might either of these products be suitable? I've only ever used bonding and multi-finish before.

 

[roy c]

If there is no damp evident then I would go for the mix ratios of what I have said. But if you have got damp then I would probably use the "Limelite". But if you do then do everything to the instructions. The damp-proofing game is big buisness, I was in it for a while using different chemicals for different sinarios but to get everything done properly by the book (To get the manufactors guarantee) wasn't a quick fix!! So it might be worth your while to get a reputable damp-proof firm round and pick their brains..This will give you an idea of what they think needs to be done.

 

[moonhog]

I've had a couple round (had to have PCA timber & damp survey for mortgage company). No major concerns, but they both suggested a chemical DPC in a couple of places, with the necessary hack-off & specialist replaster etc. for between £750 and £850 in the room in question.

I'm just loathed to go injecting a barrier in a wall above a suspended timber floor (I have a basement); the joists are in direct contact with brickwork and appear fine, showing no signs of damp/rot currently.

The mix you've said -
5-6 sand
1 lime - is hydrated lime OK to use (non-hydraulic?)
1 cement - normal Portland type?
Any other additives?

Suction on brick - is dampening with water likely to be sufficient, or would you suggest a bonding agent?
How long would you leave between float and then finish coat?

Thanks for your help Roy - appreciate this.

 

[roy c]

Yes hydrated lime and normal cement, i sometimes put a bit of plasticiser in but it should mix up nice anyway.I would brush some sbr and let it soak in and when you are ready to put your scratch on brush some more on and give it 5 -10 mins then lay on your scratch. You could just soak the wall with water but it can get a bit wet around the area you are doing I find sbr more controlable when you brush it on. Put your scratch on then give it a day then float it out and leave that till it drys out and then skim it. (We use sbr because all the materials are bought by the guy we work for but you could use pva to kill the suction or like i said you could spray water on till the water runs off..Your choice..

 

[vinn]

You seem to have damp brickwork and fungal signs on the skirting backing plate. Remove that wood and the wood plugs that go into the wall.
Was the back of the skirting fungal affected?
Whats happening on the outside of that corner?
Your joists will run into that wall. Lift the last board, and see if the joist ends in the wall are rotted?

 

[moonhog]

Hello Vinn - all removed. The skirting was mismatched (obviously been part-replaced at some point), but didn't seem too bad. I guess there has been a problem here before. That dark brick in the middle of photo is made from wood for some reason.

The wall with the plug socket is party wall, the one on the left is external front. Apart from the Sky cable coming in, there's only a concrete post fixed to the wall where the party fence is mounted.
Joists look OK - I've had a D&T guy look recently; he commented that the joists are saved by the fact they run left to right, rather than front to back.

 

[vinn]

Great. And 10% on the r/s 8" x 2" joist is very dry. So is the backing plate and plug. Maybe i was getting too emotional about any possible damp?
For some reason i thought that that was the ground floor.
I'd remove any leftover wood plugs in the external wall.

 

[roy c]

Don't you be getting emotional on us vinn, together we all will sort it out..

 

[moonhog]

For some reason i thought that that was the ground floor.

You were right - it is the ground floor, I have a cellar below. All wooden wedges/plugs and nails removed (I'm going to plug and screw some new batten in for the skirting).

Sand/lime/cement and SBR purchased this afternoon, so busy day tomorrow...cheers gents.