Blown render = likely cause of damp?

[Mathew Newton]

At the risk of entering the 'most obvious question of the year' competition within the final 10 days I have a question regarding the likely cause of damp and, more importantly, the best way to 'cure' it...

My Victorian (1905) terrace has a double skinned (not sure if it's got a cavity as such, were they built like this?) rear wall that is rendered and has a Tyrolean top coat.

We have recently noticed signs of damp inside a dining room cupboard, mounted on the rear wall, and initially put this down to the low temperature inside the cupboard (~12-14C) due to the rear wall position. However, it then dawned on me that temperature alone is presumably not sufficient to cause damp, water is required for this, and so given that the kitchen/dining room is well ventilated (okay, the cupboard isn't) and there are no signs of damp anywhere else in the room/house then it must be coming in from outside.

Looking at the back wall there are no obvious signs of a way in for the water, no cracks and no dodgy gutters/downpipes. However, knocking on the render around the area of the cupboard sounds hollow.

Thus, my competition entry question is: Does that fact that the render has blown mean this is likely to be the cause of water ingress and, ultimately, the cause of the damp? If so, how does this happen? Presumably there may well be hairline cracks that I can't see and the blown render traps a layer of water against the bricks which eventually seeps through to the inside?

If my assumptions are correct, what is the best remedial action, both short and long term (i.e. before/after Christmas!)? Replacement will presumably be the long term fix - it's a small enough area that I'm more than happy to do it myself - but what with? Would it be worthwhile removing the blown render in the short term, or will the bare bricks then suffer just as badly? Should the bricks, before rendering, be 'treated' with anything? Does the render on these old houses serve to 'waterproof' the wall, or is it largely decorative?

Grateful for any advice...

Mathew

[Ian White]

Depends where you are. Up here on a hilltop in Scotland, there's absolutely no doubt!

May I add a related question?

Rusting and swelling of old iron downpipe fixing spikes is a known cause of cracked render, and our house is a perfect example. But when removing these spikes, they tend to snap off in the bottoms of the holes. Is it OK to fill over the deeply buried bits... or is that storing up trouble for later?

In other words, does it need a simple filling or the full root canal?

[Stuart Noble]

IME, if the render sounds hollow, it will leak, whether you can see cracks or not. Given that it's presumably already painted, my short term solution would be to paint it again using a pliolite solvent based paint. Go to a proper trade paint outlet like Johnstones or Leyland. You can use it at sub zero temperatures, so no need to wait for fine weather. Standard masonry paint has virtually no water resistance. If yours is anything like my 1900 terrace, the render was probably never bonded properly to the wall in the first place. It looks like they slapped on the full thickness in one go. If you repair it, use the scratch coat method. Stand by for finger-wagging from the purists.

[Stuart Noble]

Water borne rust particles will permeate water based coatings, but not oil based. I don't know that they do any real harm though.

[Andrew Gabriel]

Stick a large sheet of aluminium foil on the inside of the wall with thick tape all the way around the edge, and then move everything (cupboard, contents, etc) back into place as it is now. After a week or two, inspect it. There will be moisture on one side of it... If it's on the room side, it's condensation. If it's on the wall side, it's penetrating damp.

I would expect condensation in the situation you decribed. What is the construction of the cupboard (fixed/movable, rear panel or open to the wall), and what's stored in it?

[Chris Bacon]

Theoretically, as the holdfast (or whatever) rusts, it will increase in size with an enormous force being created (to about 10 times as big as it was, IIRC). IRL I shouldn't think it will create a great problem, and you can always get them out anyway.

[Chris Bacon]

Possibly.

Water runs down between the render and the brickwork (various flora and fauna can exacerbate problems, e.g. ants behind render). The water can't evaporate because of the render and coating(s), so migrates through the wall.

Improve ventilation, possibly use a fan.

1:1:6.

Not in th short term, as the wall will be wet if the damp is due to blown render. It might help dry it out over time.

The bricks should be fine - these sort of houses (I've had one) are often rendered to "cure damp problems", which are often due to poor pointing or some other trivial cause. Sometimes the house will have been painted before it is rendered, which makes the problem much worse.

No, unless they've been painted.

It does help waterproof the walls, and it can help them dry out after rain. Sometimes paint is applied which can be a problem (depending on the type of paint).

N.B. if you're patching, tyrolean can be difficult to blend in, it can be useful to mask off, and stipple with a stiff bristle brush (broom head) to help lose the joint. Tyrolean is also a hard cement facing, which itself often detaches from the underlying render (if the render is hard, too, this can cause other problems).

[Lobster]

I think there's probably a bit of 'chicken and egg' here, in that if water does get in, it can freeze and expand, forcing the render away from the brickwork.

When I moved into my current house about 10 years ago, I found a quite bad damp patch on an outside wall: investigations outside found an area of blown render about 2' square, with a crack down the middle. I didn't have time to do anything permanent to it just then, as the to-do list was far too long having just moved in, so I just bodged it by filling the crack with Tetrion exterior filler, and slapping a coat of masonry paint over the area - thinking I'd probably do the job properly the following summer.

Well, the wall inside dried out quite quickly, and the round tuit still hasn't come home to roost! - the render is still blown, but it is cosmetically fine and all still waterproof. One day...

David

[Mathew Newton]

I'll give that a go - it has got to be prudent to not jump to conclusions based on potential coincidences (i.e. the blown render). Also, it'll give me a week delay on the tuit list... ;-)

As for the cupboard, it is a waste height built-in cupboard (in the alcove adjacent to the chimney), with MDF doors, a wooden top/shelf and no back or sides. The damp is only present on the side (exterior) wall). As for the contents, just a blender!

The ventilation in the kitchen/dining room is pretty good - we always use the (powerful) extractor hood when cooking etc, and there are two vented chimneys providing background ventilation.

Will wait and see the results of the foil test before taking further action - if it turns out to be the render I'll follow the advice given (thanks all for the suggestions there).

Cheers,

Mathew

[Ian White]

This is not theoretical. In Real Life the swelling *does* create a problem: long cracks in the render, where water can be driven in.

Obviously most of this happens close to the surface - but the water, rust and swelling penetrate down the whole length of the spike, locking it solidly into the wall. As soon as you try to pull these rusted spikes out, they snap, leaving a rusty end embedded in the wall. They won't come out without drilling away a substantial amount of wall around them.

My question was whether the broken-off ends can be covered over when the render is patched, or whether they really do have to be dug out to prevent a recurrence later on.

[Chris Bacon]

It depends on whether there's going to be a problem later on.

[meow2222]

yes, often

ugh.

probably, though it could be condensation.

Cement render is one of the main causes of damp on old properties. Its a real poor choice, but unfortunately is still being widely applied. Awaerness takes time to travel it seems.

yup, thats one part of it. The other is that old walls use soft porous bricks, and the bricks stay dry by evporation of whatever damp gets into them. Cement render stops this evaporation dead, thus eventaully causing a wet wall.

Remove any blown render. Do not try to pull off fixed render, as it would break the bricks, which would then deteriorate badly over the years. Patch with lime mortar. Lime mortar does not contain any cement. This will enable the wall to continue to evaporate dry, while keeping most of the rain off the bricks. If you want to paint it, use a lime based paint, these are porous unlike modern paints.

3:1 sand: lime putty. You can buy lime putty but I prefer to make it. Mix fresh bagged lime with water, store it airtight for 3 weeks before use. Lime mortar is on the dry stiff side, dont be tempted to make it thinner/wetter.

However this is the wrong time of year to do this. For now, just remove blown render. When risk of frost is passed, re-render. Bare bricks will dry out even in cold weather. Drying takes weeks to months.

Bare bricks will do much better than rendered bricks, because they can dry out.

definitely not, another popular mistake.

yes, and thats precisely the problem. The net flow of water on thesse walls is from the inside out, not vice versa.

Cement render is applied for a few reasons, but on old properties it is simply an inappropriate treatment.

If you want a fuller explanation:

[Chris Bacon]

Sorry, missed that bit.

If it's 1905, it ain't Victorian, it's Edwardian.

Some of these old buildings did have "cavities", but very, very few. Some examples date back to the late C.19th., but normally "cavity walls" were not built at all, any "cavities" being only by there accident of construction - "rat trap" construction as used to cheapen construction (bricks laid on edge) is a good example. If you've walls made of bricks laid with some going right through the wall, others being laid along it (sorry for the ultra-simplistic description!) then it is not a cavity wall, although there may well be hollows in it due to incomplete filling with mortar. Proper cavity walls only really became generally used later.

[Stuart Noble]

Am I alone in thinking this is utter nonsense? It rains, the wall gets wet. It doesn't rain, the wall stays dry. How come we're generating all this moisture indoors? Don't answer that.

[meow2222]

No Stuart, there are plenty of people like you, who havent got the brain to actually find out how it works, and assume the simplistic storise you've been brought up on must be correct.

NT

[meow2222]

Cavity walls were in the minority in Victorian times, but they were a standard form of wall construction. They were used primarily in high wind or wet climates, eg coastal regions. The advantage recognised was that penetration of water into the outer leaf did not make the inner leaf wet, and the outer leaf would dry out again before the inner became damp. Only in the 1930s was the insulation benefit widely appreciated, and only then did cavities become the norm.

Victorian cavity walls were not the uniform cavity walls we're used to today. Vic houses typically used a variety of brick bonds. Half inch cavities were common, often tied, often not. 4" bridged cavity ratbond walls were also used, though not very common. And of course there is also the random cavity stone wall, which dates back a very long way.

NT

[Chris Bacon]

Absolute, utter, complete, rubbish.

"any "cavities" being only by there accident of construction".

The fact that the walls contained small voids did *not* make them cavity walls. Rat-trap brickwork is not found on quality buildings.

[Stuart Noble]

I've had plenty of opportunity to observe damp over the years, having had it to varying degrees in every house I've ever lived in, and many that I've worked on. In most cases I have cured or greatly improved the situation with modern materials without any of the dire consequences you go on about. I repeat that, almost without exception, damp inside coincides with rain outside. It rains, the wall gets wet, and the water soaks through the wall. End of story. Go stir your lime putty.

[<me9]

My parent's house was built with cavity walls in 1902. It and most others in that area were the same. Certainly a lot of the later Victorian ones had cavity walls.

[roger]

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My first house (in Stafford) was built some time between the wars and had solid walls and no foundations.