dirty water marks on new rendering

[Daniel3103]

Hello,

I am posting here to get advice about dirty water marks that are appearing on my new rendering when it rains.

Recently, I had my boundary walls and the front elevation of my bungalow rendered. I was happy with the job but, every time it rains, the water makes vertical marks on the rendering when drips run down against the walls. Those marks start about underneath the joints between the coping stones, and they can go all the way down to the ground - as you can see from the photo.

It is worth noting, however, that the problem doesn’t happen under every joint and that there is sometimes a slight offset between the joint and the vertical line underneath. Also, on the other side of the wall - not shown on the photo -, the water makes a vertical line that is close to the middle between two joints.

All the marks disappear in a few days after a rainy day, and they come back at the next rain.

I thought the problem was happening because the joints between the coping stones were not waterproof, so I coated those joints with a waterproof layer, but that didn’t change anything.

So, my questions are:

-Do these dripping marks indicate a problem that is more serious than just the cosmetic issue?

-Why is water dripping from specific points at the top of the walls, not from everywhere along the walls?

-If I need to do something about this problem, what can I do without causing any collateral damage? 

As you can see on the second photo, the same dripping problem is also happening on my bungalow’s front wall, below my windowsill which is cracked and hasn’t been painted yet.

[ProDave]

What type of render?

 

That air brick needs cleaning off and the holes unblocking.

[Lofty718]

The coping stones should have a big enough overhang to stop that from happening

[Mattg4321]

And drip grooves/channel on the bottom near the edge. 
 

It appears they have, as it’s only happening on the joint. Should some sort of drip groove have been formed in mortar? I don’t know to be honest, but seems sensible to me. 

[Daniel3103]

Thanks to those who have responded!
I was never notified of the responses, so I only noticed them today.
Now, to answer your questions.
The render is Silicone K Rend TC15.
As for the grooves in the coping stones, I have now figured out what the problem is, with the help of some advice I have received. These grooves should be above the overhang, but they are actually above the wall, as you can see from the photo below. So, it now seems to me that I need to cut new grooves into the lower surface of the coping stones, between the wall and the edge of those coping stones. Does this seem sensible to others here? What kind of distance should the new grooves be from the wall, or from the outer edge of the coping stones?

[scottishjohn]

It might be easier to stick a piece of piping to the underside of the coping stones away from the wall 

i cannot see you recreating the groove very easily 

 or of course you could replace coping stones with correct size as these half moons should out past the wall edge

what width is the wall and what width are the coping stones  

and you will see then if them are correctand mis placed or wrong width for the wall

 you window sills will only have one shape  in them as the inside of them would not need it 

or could it be that coping stones are not central on the wall so one side works and other does not

[Daniel3103]

I’ve now done the job! I’ve cut a new groove in the underside of the coping stones, alongside the existing groove that was in the wrong place. As Scottish John said, it wasn’t a very easy job and I haven’t done it very neatly, but I’ve done it well enough to make it work. I’ve already seen water being caught by the new groove and dripping from the edge of the coping stones to the ground - so, my work seems to have produced some effect! As for appearance, I’m not worried about it because the grooves are not visible. The first photo shows one of the new grooves I have cut – one of my neatest and straightest ones – alongside the groove that was already there.

 

Now, my next job is going to be to clean the rendering. Do people here have any advice about that? This rendering has localised water stain marks as you can see from the second photo, and there are also a couple of small red marks on it, that I haven’t been able to wash off – the third photo shows one. It has also lost its shine more generally because of diffuse water staining and diffuse dust. So, what is the best way to clean this rendering and give it its shine back?

RE the photos - the first one shows the underside of a coping stone, the second one shows the top and side of my wall and the third one shows the side.

[ProDave]

To clean such stains from render, I squirt with 50 50 water bleach mic using a garden sprayer bottle, just where the stains are, and let the rain water rinse it off afterwards.

[Mr Punter]

I am very impressed that you managed to cut a new drip groove.

[Conor]

Well done. I had to do this once with granite copings that didn't come with a drip groove. Horrible job. Learnt my lesson and we cut them before fitting this time round.