Blockwork line pattern on plaster

[sparkwright]

Also posted similar thread on building forum.

14 year old house, ordinary plaster on the walls (NOT plasterboard).

Walls emulsioned, not papered.

On the two exterior walls in a room, the cement joints of the thermalite blockwork are showing through the plaster.

Not sure what's causing this.

Walls are due to be emulsioned next week (not by me).

Should some kind of primer or something be used to prevent these dark lines showing through later in the future?

 

[elrobbo82]

what do you mean "showing through"? is it staining or do they need filling? do they look like dampness coming through? have they always been there?

 

[sparkwright]

I have a more detailed post in the Building section.

You can make out the mortar lines; they are dark but feint marks on the walls. I suppose it could be described as a stain, but it's not like a brown water stain.

The walls are dead smooth, and the plaster is the normal thickness.

The people have lived there a year, had noticed a few of these lines but hadn't noticed on the whole of two external walls in the room. Only today have we noticed it's the outline of the blockwork.

You can even work out where the electrical capping is, as there are no feint black lines showing in these areas.

It's all quite feint, but it does notice, and the people don't want it reappearing once their painter has re-done the walls.

They will be asking their painter tomorrow, but thought it best to seek futher advice in case he doesn't know what to do about it (if anything).

 

[elrobbo82]

without seeing the problem on site it can be difficult to advise. if the lines are very faint stains an option would be thinned oil based undercoat or maybe zinsser 123. if its damp rectification of the cause then possibly alkali resisting primer. as said hard to say without seeing.

hopefully the painter on site will be able to advise after a visual of the problem.

 

[sparkwright]

I don't know if stain is the right word, it's just like a feint dirty mark.

It was first noticed above the television set, and at first was believed to be some sort of heat mark.

When some rooms need repainting, sometimes you get a very 'natural' looking black mark forming in the corners. It's a bit like that, only all over two walls following the formation of the blockwork.

I wonder if the thermalite blocks are a lot warmer than the mortar.

It reminds me of what you sometimes see on new builds before painting has started, where the wall is drying out and you see the mortar joints or the adhesive dots through the drying plaster.

This is not a new build though, but clearly there is some kind of damp/insulation/ventilation problem here.

 

[Growler]

I've had this on a house and I tried all sorts of primers and couldn't figure it out.
The only thing that cured it (up to now at least) was to have cavity wall insulation.

 

[sparkwright]

I've had this on a house and I tried all sorts of primers and couldn't figure it out.
The only thing that cured it (up to now at least) was to have cavity wall insulation.

Yes, this is what I am fearing.

I should point out there are sheets of celotex in the cavity against the inner wall.

All the other rooms in the bungalow have just been repainted, and we haven't noticed any marks before or after, then again we weren't really looking.

 

[sparkwright]

Duplicate post.

 

[sparkwright]

I don't know if this room has ever been re-painted in 14 years.


I wonder (hoping) if this particular room was painted before the blockwork and the plaster had dried out fully, and it 'stained' the wall. I suppose a new wall does take a lot of drying out when you consider the amount water of that's used.

Or I could be totally wrong.

 

[dcdec]

As an alternative to cavity wall insulation you could line it with a thermal paper. Paint or primers will not get rid of this problem as its not a paint problem, its dust and general household dirt settling on the paint surface.

 

[^woody^]

This is a phenomenon known as "pattern staining" and relates to the different thermal properties of the two materials - the blocks and the mortar joins. You may also see something similar with lines of joists on ceilings.

The cause is mainly minute dust and dirt particles being attracted to that area to form the dark lines

Normally a thick plaster base coat is enough to prevent the slight difference in temperature causing the stain

Wall insulation may help, or a thick lining paper or over boarding

But otherwise, in the slow time this staining starts to develop, its normally time to redecorate anyway

 

[sparkwright]

Excellents coments everyone.

The last two comments were very helpful, as they showed me that the dirt was being drawn to the colder areas - which I think is the case.

It is more of a fine dirt on the surface rather than a stain coming through.

So it would seem the dirt is a gradual thing so, as you say, it perhaps won't be a problem for a while until the room needs painting again.

I think the other rooms were re-painted more recently;- this may explain why no one ever noticed any marks. These particular have just been re-painted again this month.

Great comments all.