Mysterious stain on exterior wall render.

[NowWhyisThat?]

Hello all. Wonder if anyone can advise me on this. The stain in the below pics has been resident on the exterior of my kitchen extension’s rendering for the past 16 years! Ever since the extension was built. Has never gotten bigger. Has never gotten smaller. Gets a bit darker in hue when the weather is wet, as does the rest of the render coating, but that’s about all. As you can see it’s “crowned” by a thin margin of efflorescence, which continually reappears when the weather turns warmer and drier. No signs at all of any interior damp. As you can see it is well below the DPC.
Can anybody tell me what has most likely caused this? And- most importantly- what is the best way for me to eradicate it? The only objection I have to it is that it’s a bit of an eye-sore.
Would much appreciate someone solving this mystery for me.

 

[JohnD]

the downpipe is leaking in the ground, probably a broken salt-glazed clay gully.

the previous builders hid it under the paving hoping nobody would notice until after the cheque had cleared.

when warm weather comes, take up the slabs round it and dig it out.

It is very common.

IME in old houses they are always cracked and broken, and have been since 1940.

If you prefer, you can leave it unfixed, and it will carry on turning the soil under the wall to mud and washing it away, until the wall or the paving falls into it.

 

[NowWhyisThat?]

Thanks JohnD. The downpipe voids into a soakaway system which was installed at the time of the extension build. The “pit” for that is quite a way down the garden. Wouldn’t the staining be creeping sponge-like all over the wall render if what you say is correct? Is there anything else it could be?

 

[^woody^]

You have the same stains adjacent to the steps and the paving further along.

Although the drain may be contribbuting, the pattern does not suggest that in itself.

There may well be detached render (tap it for hollow sound) or perhaps some different or damaged bricks behind, leading to water retention.

You wont know or stop it without hacking the render off to check.

 

[NowWhyisThat?]

Thanks Woody. That kinda makes more sense. The one thing that has always had me scratching my head about it is that the stain never seems to spread beyond the parameters as depicted in the photos. The stain doesn’t even appear to extend right the way down to ground level. The staining you point out adjacent to the steps is in fact a patching job I did around 5 years ago when the render did indeed “blow” and needed replacing. That’s all quite sound (although not a great colour-match!). And the stains further along at ground level that you pointed out are rainwater stains which are unavoidable. I just can’t figure out the stain which has driven me to make the post about it. Beneath the render is block-work up to the point of the dpc and then brick facing thereafter upward. Could it possibly be that the builders left something within the “bowels” of the exterior elevation which has forever afterwards been leaning against the inside face of the wall at precisely that location which is causing moisture to leech through? A mound of soil perchance or some such-like? The floor of the extension is a concrete floor.

 

[^woody^]

The white salts indicate moisture being held and drying out - which brings the salt.

The defined salt line between dry and damp is typically a change of materials or a demarcation between detached and attached render, or maybe something in the render mix or the way it was trowelled. So it's not random.

You do appear to have some staining (salts?) around the side by the third step too.

 

[NowWhyisThat?]

Thanks again Woody. Is it the sort of issue that any builder worth his salt (no pun intended) should be able to accurately diagnose and fully rectify once he’s had a thorough look at it?

And one last question - would it be a good idea to apply a thick “belt” of new well waterproofed render along the entire lower base of the wall to a height of around 12-14 inches above ground level to repel rainwater splashback?

 

[JohnD]

I appreciate you want to hear an answer that has an easy solution

but

you have a wet patch exactly next to the rainwater downpipe

and other wet marks nearby.

at the base of the wall and by the steps.

all close to the downpipe

it gets worse in rain and better in dry weather.

Go on, tell me.

Where would you guess the water is coming from?

 

[NowWhyisThat?]

Thanks JohnD. Could you then explain why the entire area around that downpipe isn’t water stained and never has been? And that the footprint of that stain hasn’t altered in 16 years? Not looking for an easy solution. Just a proper idea of what’s causing it without any smart Alec attitude.

 

[JohnD]

water stains are often round or mountain shaped, specially on a plastered wall where absorbency is regular, but not always.

they are generally worst close to their source.

If the ground under the paving is wet, it would be consistent with the wet mark at the base of the wall.

Wet marks on a brick wall end at the point where evaporation from the surface equals absorbtion of water from the source. In clean bricks it is often only a few courses. Render will make it higher. Round your steps, the wall is not exposed to the air until the point where is rises above step level, so the wall is wet near the steps.

there may be differences in individual bricks behind the render, or the render may not be equal in texture or adhesion, either will lead to patchiness of damp.

if the space behind the wall has earth or rubble in it, that is probably also damp and passing into the wall.

No smart alec required.

 

[^woody^]

Is it the sort of issue that any builder worth his salt (no pun intended) should be able to accurately diagnose and fully rectify once he’s had a thorough look at it?

No. Builders build, they are not specialist in diagnosing things. Of course there may be builders who have extra or specialist knowledge, but your chances of randomly finding one is low. And beware the ones who "saw something similar on my last job" or "know of a friend of a friend who had the exact same problem". As a rule avoid asking people to diagnose problems for which they offer to do the work they may recommend.

The thing with any damp, damp stains, or defects generally is that there is a process of elimination which leads to a most likely cause, rather than the cause. So you can be in a situation where the actual cause might be one of several things, and there is some investigation or trial and error involved. The trick is to keep this to a minimum

For any non-standard diagnosis, a Building Surveyor should be able to diagnose and recommend suitable remedial action. But again there are building surveyors and there are numpty surveyors.

Perhaps getting several opinions can narrow down a common theme, but it can also confuse if everyone gives a different opinion.

That render should not be touching the paving, so you have some capillarity and water uptake going on for a start. But is that a problem? That's down to you. I don't think doing anything to the render is worth it ("a thick 'belt' of new well waterproofed render"), as splash back is not the problem - although it may be occurring.

Likewise the drain may be leaking so should that be CCTV surveyed to eliminate it? That's a £200 check to eliminate something. Again is that worth it?

If there is a mound up against the wall in the sub-floor void, how can you access it to check. Take up floors?

Otherwise it's a case of removing that render and seeing what's behind it. Again that's a few hundred pounds worth of investigation and trial and error.

Getting a surveyor out for a look had a cost, and that's before any investigation work or remedial work.

Basically that is potentially not an easy thing to diagnose - although it may be easy to remedy once the cause is know. So the issue is, how much of a problem is it, and how much do you want to throw at it?

 

[NowWhyisThat?]

Many thanks Woody for your common sense replies. Much appreciated.