Render, mortar and damp

[Gavin Willingham]

Hi

Got a bit of a multi-part query that despite hours of googling I'm no clearer on. Apologies in advance for the long post, but figured it was worth providing detail.

Bought a house a while back, and tidied up the decoration a bit inside. Now that the "easy" parts are done, I need to address a couple of damp problems before I can finish off those walls it's affecting.

The patch I decided to deal with first wasn't present when we painted the wall, but with autumn/winter and the rain a small patch developed, and it has since grown into a larger patch, approx. 1m wide by 30cm high.

As it started when I put a sofa against the wall, I originally figured it was a cold spot in the wall that was getting condensation, but since moving the sofa away, it has only grown. My next thought therefore is that the rain is coming in through the brickwork. The walls are insulation-filled cavity walls, so I guess it would either have to be a bridge in the cavity (mortar on a wall tie maybe?) that pre-dated the insulation, or the insulation itself. Or maybe still a cold spot where there is no insulation?

The outside of my (1939 built) house (in Cumbria) is rendered and painted. To get a better look from the outside, I chipped off a few sections of the render to look at the brickwork underneath. The first couple of sections were in a different area of the house to the damp patch I'm talking about here (a couple of different damp problems in the house). I chipped them off before xmas.

Underneath 1 small area, the bricks and mortar seem solid.

Under another - slightly larger - area where the render was hollow, there are holes where chunks of mortar has fallen out (possibly stuck to the render I chipped off), but the mortar that is left is hard at least...

In the section I chipped off today that is outside the damp patch in question, the mortar is just sand. When I say that, I mean I can scrape it out with a finger.

As you can see from a couple of the photos (below), this patch is just above the damp course. It is possible that the damp course is broken somehow and it's soaking up, but it's also directly above an air brick so this seems unlikely! Could be wind-forced rain coming in through the air-brick though?

I put some photos online:

(the first 6 are of the main area, the last 3 are of the other 2 areas I hacked off).

So my questions are:

1) Could this "sandy" mortar be the cause of the damp, given that it's beneath the render and the walls are cavity walls (albeit a filled cavity)?

2) If I need to re-point this brickwork, is it likely to be lime mortar, or sand-cement? Is there an easy way to tell without sending it off for analysis?

3) If I have to re-point, is there anything special about re-pointing around the DPC to avoid damaging it, or is it just a case of "go carefully"

If anyone can give me any pointers, I'd really appreciate it. Sorry again for the long post!

--Gavin

[Andrew Gabriel]

In photograph 6 (as best I can make out)... You've got a hard ground level surface far too near to the damp course. Rain will slash back up the wall, keeping it wet above the damp proof layer. Hence why you see the damp in the rain. My guess is the ground level has been raised here since the place was built, and worse still, it's been done with a hard surface which will slash back.

Also, the external render crosses over the damp course. That's not great unless it is a special mix with water proofer mixed in.

If what I can see there is a cause, then you might be able to remove the fillet of concrete between paving slabs and the wall, lower the level there, and top off with something like pea shingle which won't pool water on top to splash back (still need to be lower than the existing paving though). This is basically a french drain, and you will need to ensire water can drain out of it without filling it.

Another option would be to have a hard waterproof render skirt from the ground up to 18" high, but these can fail and trap water behind.

It might not help, but it's not the cause.

If the bricks are low water absorbtion type, then rendering with a waterproof mix might help, but it could also make the problem worse if the source of water is higher up the wall.

Also need to understand why it's getting through the cavity. A boroscope camera through some holes or a removed brick if the mortar comes out so easily might be useful to see what (if anything) is in the cavity here.

Unlikely to be lime mortar with a 1939 cavity wall, but not impossible. I would not worry about replacing with lime in that position anyway.

Looks like a state damp course? "go carefully", but it's stronger than many more recent ones.

The real key is identifying the source of the damp, which I can't do reliably from the pictures. Then fixing this cause. The wrong fix (because the cause was wrongly identified) will often make it worse.

[Dave Liquorice]

Was the render "blown" before you starting hitting it with a hammer and chisel? ie. Did it sound hollow when tapped or solid? Looking at the pictures most looks as if it was fairly firmly attached. Apart from the picture of the cormer where you can see algae growth on the brickwork and exposed edge of the render.

Render is blooming 'orrible stuff as water can track behind it for quite away before appearing some way from the source. The cavity walls you have complicate things as well, as you know. Is the damp coming from a bridge across the cavity or condensation due to no insulation, only action there is to take a brick out and have a look. Do you know what the insulation is?

[Gavin Willingham]

The terracotta piece at the bottom of photo 6 is the top of the air brick where it juts out. It's quite a close-up photo from a funny angle, and I cleared the dust you can see in 1,4 and 5 before taking 6 which is why it looks different. The DPC is 3 bricks above the ground (see photo

9 - although not clear it's at the top of the 3rd course). This is consistent with next door, as is the render crossing the DPC which is the same all round the estate. I'm not saying that's not the cause, but it's worth noting that that's how these houses were built.

When I had the floor up indoors, I thought it looked like a bitumen course, but I agree it looks more like slate from the photos. I shall investigate that a bit further.

I understand it's hard to do diagnosis over the internet, and I appreciate you taking the time to write such a detailed response, thankyou :-)

--Gavin

[Gavin Willingham]

The only bit that was blown was the bit on the corner, although more accurately, I chipped off a couple of inches around the corner, left it a couple of weeks and thought to check if it was hollow, so it's possible that water got in in the intervening time and blew it.

The rest, as you surmised, is pretty well attached.

I'm no big fan of render, but it seems every house (except the expensive stone ones) in Kendal is rendered :-(

The insulation is white fluff (mineral wool?). I know this because part of the inner leaf (hidden under the bath panel in the bathroom) wall is missing and I have a nice pile of the stuff.

It was put in by the Marks Group (before I bought the house), which looking at their website seems to use mineral wool so that's my best guess.

Taking a brick out to look into the cavity seems a good next step, although I was worried with the state of the mortar about causing a collapse!

--Gavin

[stuart noble]

At first glance I'd say the render is not bonded well with the brickwork and the paint has that "peelable" look suggesting it hasn't bonded either. If it was re-done with a scratch coat I'm sure it would be fine.

[Andrew]

The paint looks like that plasticky spray-on nonsense that commission-hunters love to sell. Was the wall dry when you removed the render ?. Is water being trapped behind this paint and getting through the insulation.

Have you tried getting a handful of the stuff that has 'leaked' and immersing it in water ?. It should not act like a sponge, whereas loft insulation will soak up water. Your local BCO can test it for water repellence. It's possible it is the wrong stuff, in which case in Cumbria where the rain is horizontal, it could be wet inside the cavity. Wet insulation is worse than no insulation.

Cumbria is a severe-weather area, one of those parts of the country where you need to be very careful before filling your cavities. Were the holes for the insulation drilled and plugged with a weak mortar mix, which is allowing water to track right through into the cavity ?. Or did they just shove some rockwoll into the holes ?. Can you see the dents where these holes were drilled.

I'm guessing that the house was painted to hide all the holes. When my house was done they just mixed a few handfuls of damp sand with some old mortar and plugged the holes with their fingers - definately a weak point where water could get into the cavity.

Andrew

[Gavin Willingham]

I took a brick out today, and it looks like a bridge was the cause as suggested. I've cleared that out and am now running a dehumidifier indoors to see if it starts to dry out.

Cheers

--Gavin

[Gavin Willingham]

Having removed a brick, it now appears that there's a short length of slate DPC above the air-brick, presumably to prevent rain-splashes. The rest of the cavity itself has no DPC. I'm guessing that there are separate bitumen courses in the inner and outer leaves with a void in between. This does of course mean that the insulation is bridging the DPC...

When I removed the brick, there was a bunch of mortar, rubble and sand that was bridging the cavity, so I suspect this was the cause of this particular damp spot. I've cleared it out and am running a dehumidifier indoors to see if that dries it out. I obviously now have a cold-spot with no insulation, but have called the insulation company out to rectify that.

The brick came out very easily, as expected, so I'm not really sure what to do about mortaring it back in - this will be the only brick in the vicinity not held in by sand! I plan to re-insert it later with sand-cement to at least plug the hole.

--Gavin

[Andrew Gabriel]

Dehumidifier won't make any significant difference. Masonary dries out at the rate of about 1" thickness per week, although at this time of year it will take longer. I wouldn't bother to put any insulation back for a long time. Actually, I would leave the brick out for now too, but put a grill over it so you don't end up with cats, rats, etc going inside. Wait until it's dried out before refitting the insulation and the brick.

[Dave Liquorice]

Agreed. Plaster/stone/brick dries at the speed it dries, slowly...

I'd add driven water (IIRC this house is in the Lakes...) to that list as well but make sure that air can still circulate to aid the slow drying of the cavity.