Exterior rendering of houses

We have a problem with the exterior of our house! After being built for four and a half years there have appeared ugly black 'streaks' down the west facing walls. This is obviously caused by the rain, is not mould or any vegetable growth. It would seem to be the colouring granules within the cement rendering. Does anyone know the remedy? We have thought that it comes within the 10 year guarantee but so far this has been disputed by the builders (naturally). We have tried contacting builders but they all seem to be busy at the moment and do not reply to e mails!!! That is unless they do not want the work. Any help or advice would be welcomed. We live near Dax and if anyone knows a builder near there that would also be extremely helpful

Nigel Fisher

Once again my thanks to all and we are grateful for the response from all concerned. Will up date you as and when we have the answers from the inspections.

More or less what the father says about the mix.

I think you will find Brian that monocouche is far more expensive than traditional methods in terms of material cost. I believe it is the 'silicone' in it which causes the problem. Also, the design of eaves and verge have a bearing, small over hangs and no proper drip detailing don't help at all. You can often see the marks correspond with joints in verge tiles etc.

However, it is a purely cosmetic feature and cured by a little maintenance......

Exactly Richard, that why I said get somebody who knows to have a look. We are helping somebody finish his self-built house a little, when spring arrives it will be back to doing the crepi. His father is a retired mason who wants nothing to do with premixed monocouche because of its dubious qualities. Despite it being an entirely modern build, the crepi is 100% traditional mix.

Still no picture, you are hiding still?

Built 4 1/2 years ago the enduit is more likely to be moncouche premixed in bags then a traditional three coat system.Talk of poor ciment is a bit premature without knowing what material has been used.

Many builders use the cheapest available sand and skimp on adding the lime/chalk. If the mix is right then it should be fungus/algae free for many years, not soak and hold water and streaking not occur. It breathes with the lime, which is to say there are tiny little air pockets in the render around the acidic lime that protect and also stay 'warm' to condense out water. It should also not matter too much whether it is on traditional or modern material walls. Mind you, the red brick with polystyrene sheet insulation between skins is asking for trouble in and outside. If possible, get somebody who really knows about the materials to come at look really thoroughly rather than have anybody say definitively here. Hosing walls down if they have a bad mix that is heavy in poor quality cement will only make the problem worse and hydrofuge can also be a disaster if you have an insulation issue whilst a good idea if not.

I would be worried also about the cause of why the wall is staying damp long enough to go mouldy. If it is leaking gutters or blocked gutters it will happen whatever the quality of the render. You need to check and cure that before anything. Plus if there are micro cracks in the render that have not been filled and painted rain water will penetrate and then keep that part of the wall damp long after the rest of the wall has dried. This encourages mould. A textured mould resistant paint is often used to cure such problems once the wall has dried.

If the rooms behind the walls are highly insulated or not heated then this also exacerbates the problem. Pictures with good resolution would have helped to show the type of render.

I've been told by the locals that you simply spray the walls with eau de javel (weak bleach) with a garden sprayer on a dry day and just leave it to do the job....no reason to doubt what they had to say but haven't needed to put it to the test yet. For 10 euros for a couple of gallons though, worth a try.

The black streaks are, as you say, just rain marks. When we have them (usually at the end of winter) we just pressure hose them off.

Hi Nigel have a builder who works for us on occassions highly reccomend him, he will travel out here has Insurance, has worked with all kinds of materials, even French. If you need a quote and a chat with him then do get in touch, This is not a cowboy builder I can assure you, Jim www.gruedemoiselle.com or 0555602419

How to use the 10 year guarantee:

Say to the builder that you will with regret have to call the insurance company (you will have a copy of their certificate backing up the decenalle)

If necessary do that. I did for one supplier and finally the insurer sent someone round to inspect the problem. The insurer will ultimately get it fixed and charge the builder

Thanks to you all and sorry I did not reply sooner but really appreciate all the advice and will definitely try the 'hydrofuge' product.

This fact sheet might help section 9 http://www.pointp.fr/docindus/0001%20Gros-Oeuvre%20VRD-TP/01548%20PRB/prb%20%20SOLOUTIONS%20HD%20022007.pdf

High five Ben, think we might have got it for Nigel. Let's hope it is as simple as that and a hydrofuge will definitely help to restore and keep the colour.

Agree with Brian, given the vivid colors you describe. And yes, the walls can be treated after cleaning. They don't need to breath and the normal ventilation in your house will get rid of excess humidity inside your house. You need a "hydrofuge" product containing siloxane which leaves the original wall-color intact.

Blue grey - crap cement. People all over this area have had crepi work on new houses that never lasts as long as very old houses done decades ago. One major problem is that a lot of builders fail to use or skimp on lime which allows your render to breath, the poor quality cement then leeches out and stains. To be honest, you will never get the builder to deal with that because he will knock it on to the cement manufacturer, etc...

Thanks to you both. The reason I ruled out mould is that it appears to be 'solid' and not possible to scratch off. Also there are different colours. The colouring is a pale blue/grey and the streaks are black mostly but also with greyish, bluish streaks in between. At the lower level I used a pressure washer which easily removed the discolouration but also took a little bit of the cement, so am loathe to continue until I have professional advice. The difficulty is finding that advice!! I have renovated a couple of houses in the UK but never encountered this type of render before. Our last house in Enlgland was listed and we could not 'seal' the ancient render at that time. I was wondering if, after cleaning these walls, I could then use a transparent sealer (which should waterproof) or whether this render needs to breathe? Anyway thanks again and if you do come across anyone who deals with this type of problem please steer them this way

Ben, I agree to a point, but the mixture of detergent and bleach does eliminate fungus and then the point you make that I should have remembered (because I did that too often for my father when he did renovations for his customers) was that the entire wall needs to be done because of the effect on the colour and also because it will simply reappear somewhere else on that wall.

Hi Nigel: am just curious, how did you rule out fungi etc.? (Because this coloring is in almost all the cases due to veggies growing in the pores of your wall. Using bleach for example does bleach the color thus seemingly solve the problem, but doesn't kill of the fungi which will reappear in no time. Did you test it with a fungicide?).