Damp wall from blocked hopper - solution?

[dizz]

Wall has penetrating damp from hopper which has been blocked for a few weeks. Very wet on inside of wall. Wall was plastered. The blockage was fixed and I used a dehumidifier to remove all the damp from the wall. On touch it felt very dry, and could then take a coat of paint. I though that was that until a couple of days of rain. Now the bottom of the wall is damp again.

Question
If there is no "new" water ingress from the blocked hopper, can the wall retain the damp simply from the previous damp (i.e. does the plaster now contain something that soaks up water vapour from wet conditions and thus the damp appears again, even though the original problem was sorted?)

 

[Nige F]

Yup - Hygroscopic salts

 

[dizz]

Thank you - we are a top floor flat, and the damp was penetrating from the hopper - how do the salts get in in the first place?

 

[Nige F]

they are in the sand/cement - just dry the room and then use a sealer paint for the walls - ask in the decorating forum

 

[dizz]

OK - I think there was too much damp re-appearing to apply a damp seal paint, so now I have hacked back the plaster to the brick.

I guess, it's now apply a layer of mortar followed by some plaster. I have skimmed plastered before, but never taken wall back to brick.

Advice please on :

1. correct mortar mix
2. Whether I should use a waterproofer in the mix and if so which
3. how long to wait until applying the plaster

Thank you.

 

[r896neo]

5:1

Yes do use waterproofer, any brand will do but make sure not to then add too much extra plasticiser because they usually come with it in already. Waterproofer is not normally required inside but it would probably help stop salts coming through.

Apply skim the next day. If you wait longer the walls will need damped down thorougly to stop the skim drying too quick.

 

[dizz]

1. sharp or building sand, or mixture of both?

2. Do I need to score the first layer, before applying second?

Thank you