Water proofing over pebbledash?

I doubt it will be effective over paint

Dunno about that one but I can heartily recommend Liquid Plastics K501 for what you want to do. It really is the dog's whatsits for that job

Its quite the opposite. What many people dont seem to realise is that water content in a wall is an equilibrium between water entry and evaporation. Waterproofing products reduce water entry, but they reduce water evaporation even more, so the wall gradually ends up wetter.

The standard advice a la SPAB etc is to replace cement render with lime, which allows much more evaporation. However its best to only do such work when the cement render can be removed without damaging the bricks.

This is one of those topics there is much argument about. If in doubt check out SPAB's info on the topic, or ask on the periodpropertyuk forum.

Having said all that, obviously you need to check the rainwater goods etc before deciding bigger work is needed.

NT

Crazy logic. I have a SW facing gable end wall that was permanently damp until it was sealed 10 years or so ago. Now it doesn't get wet internally, so it doesn't need to dry. I really don't see the problem.

i knew youd be along, thats why I said where to get the info from the experts

NT

pebbledash is more or less water proof. I strongly suggest you look elsewhere..like higher up a wall with a cavity. .g. I had water dripping out of a window lintel, due to failed bargeboards about 15 ft higher..it ran down the cavity..

oh god, sermon time.

In modern houses, (post the ark basically) there should be a waterproof outer coating, and the indside breaths to lower any moisture, and you have ventilation to exhaust it from

which you aint.

Thanks, will do. We had the guttering and the roof replaced a year ago, and it's a bit uncertain wether the damp had got in and damaged the wall before or after that operation (it's a dimly lit room).

Though, where the water is coming in is the exposed gable end and there is brickwork for two chimneys that I suspect will need looking at. A fellow popped his head through the door and reckoned some sprayed on sealer on the render would do the job - which to me looked a bit doubtful.

I'm more interested in the cause of it and am now heading for a cheap 'prongs in the wall' damp meter (screwfix? ebay?) and a closer look, once I get access.

SPAB advice:

You can depend on me

But often the render supporting the pebbledash is crazed. Not always easy to see

Although I'd agree your general point, you can't extend this to, "all walls will become wetter if their outer face is sealed" without knowing more about the wall and the weather.

How much water exposure does the wall suffer?

Where does the evaporation take place, inside or outside face? (mainly an issue for sheds, but it's significant for single-skin rear wings in Victorian terraces too).

Can you reduce the water exposure on the outer face or top surface by fixing some other problem, such as guttering or inadequate waterproofing around the eaves?

We've got a big Victorian pile with a damp problem that's almost certainly caused by a leaking roof valley. Yet the previous owners spent money on having a "coating" applied to everything outside. It's a "coating" rather than mere "paint", because paint doesn't cost over =A311 thousand! No fix, a lot of money, and yes it's probably going to make that specific problem worse rather than better. However the coated section of the house (apart from this small area) _is_ distinctly drier inside than the unpainted section to the rear, which has the same wall construction and is actually more sheltered from the weather.

I also like the colour, although I wouldn't have spent =A311k for it!

SPAB, and the Scottish equivalent, are largely talking about large- scale buildings with very thick walls. Their obsession with lime renders as a panacea works fine on those, but it's not the solution when you're a cheaply-built fisherman's cottage facing the worst of the sea's weather.

If we were talking about "modern" houses, we either wouldn't be having this conversation, or we'd be asking how to sue the builder for getting it wrong. "Modern" in this sense is really quite modern, and much newer than where many (most?) of us live. It's certainly far more recent than the Ark.

When I said post the Ark, I sort of had a reason.

Meows stuff applies to soft brick or timber houses with lime mortar and no DPC.

Once you move to portland cement, the mortar is more or less waterproof compared with lime, and the bricks have to be strong enough to be used with it.

It was about the same time that relatively hard brick and portland cement came in that DPC's were also becoming used, and tee design of houses shifted from allowing water to get out, to stopping it getting in.

Use of open fires also tended to remove internal moisture adequately.

Given a proper overhanging roof and drip boards around windows, such houses are perfectly dry in the absence of strong driving rain or other persistent soaking.

Us of a impermeable coating on such a house is either decorative, there to act as a barrier in the presence of driving rain, or possibly to prevent spalling when frost hits a slightly prmeapble surface of poor brick. Its often applied where frost damage has already happened.

Needless to say if the ingress of water is elsewhere than via the coating or cracks in it, the coating merely makes the symptoms worse.

Hence th myth that applying such coatings makes the PROBLEM worse. It doesn't. Its simply makes it more obvious.

I repeat, in a post 1900 style house, control of damp by structural breathing is the wrong approach. The structure isn't permeable enough with or without coating.

You need to fix the leaks in the impermeable surfaces..and generally those are failed guttering, timberwork, lead valleys and the like. Or bridged DPC's.

Condensation inside is curd by ventilation, insulation, and heating.

Even modern bricks are not impermeable, and cement mortar used in brickwork ditto.

The house wall being discussed is non-cavity, so the wall: a) conducts water/damp through it b) has no cavity surface from which to evaporate damp c) is much more likely to experience transient surface condensation d) due to all the above has a much higher risk of water problems than a modern cavity wall e) Has warm humid air on the inside and often cold air outside, which routinely is _below_ the dew point of the interior air.

And now, its being proposed to add a vapour barrier *on the outside* of this wall. That simply isnt an effective solution. To use that approach is to misunderstand how water is handled by such a wall.

SPAB's advice re avoiding impermeable coatings is not aimed at huge rambling residences, its aimed at walls that dont conform to modern construction standards, ie dpc & cavity. SPAB's advice re lime is a different topic aimed at not exactly the same types of wall.

And far from SPAB's advice being 'obsession', they actually are the experts on this, and have learnt this stuff from both a huge amount of experience and understanding the theory behind it. For someone to claim they know better than spab simply because thats how it used to be popularly done and they dont see what the problem is, is dubious at best.

NT

I would venture to suggest that, with central heating, most homes have warm, dry air on the inside, especially in winter.

No surface treatment is ever a "vapour" barrier. It isn't like covering your house in cling film, it simply acts as a barrier to water droplets, which is a completely different issue. Take a piece of new wood and allow the rain to pour down on it for a couple of weeks. On planing you will find that only the surface is wet. However, that doesn't mean moisture isn't going in and out in the form of vapour, and there is no way to prevent that. There are figures somewhere detailing the vapour permeability of various surface coatings, and IIRC they are way up the scale.

It might help if SPAB defined "impermeable" in terms of coatings. AFAIK there is no such thing short of full scale encapsulation. The siloxane type products outlined earlier in this thread don't form a film at all, so describing them as a "coating" is very wide of the mark.

I don't mind listening to experts, even the self-appointed ones, but I stop when their arguments cease to make sense.

Exterior coatings of many kinds greatly reduce rate of evaporation. Its not necessary to prevent it completely to run into trouble.

Even those reduce evaporation. After repeated application over time, they heavily reduce evaporation. It isnt the solution.

They make sense just fine, but naturally one needs to study and understand them properly. The understanding of how old buildings handle water has moved forward in the last 2 decades, and unfortunately not everyone is willing to learn what they thought they knew all about.

NT

Condensation shouldn't be a problem in a properly heated and ventilated house. The usual problem in winter is that cold air coming into the house contains too little moisture, is heated to 20degs or whatever, and is then much too dry for comfort. In other words, there is generally a shortage of moisture, not an excess. However, even if you discount that, you have to balance vapour going out against rain pissing in. I am simply not prepared to have wet walls so that a minute amount of possible condensation can go in the opposite direction.

I'm not sure they reduce it at all. Water vapour is air, and it just isn't that easy to stop. What can be a problem with surface coatings is when rainwater, not vapour, gets behind it.