Lime mortar outside/gypsum plaster inside - damp problems?

[odilon]

We recently did a full rehab or our 1890's house in South London, including removing some cement render, replacing a surface layer of sand-cement based mortar with lime (the original was lime). Inside, wallpaper was removed and a new skim of plaster added in most rooms. I now realize that the original plaster was lime, and the new skim coat gypsum, something I didn't understand at all. A few of the walls are now having some stubborn damp problems, and I fear the combination of lime mortar outside and non-lime plaster inside is causing problems.

Can anyone advise on what might be happening, and what we can do to address it?

Thanks!

 

[ree]

You have an external render that was sand and cement but you replaced it with a "lime" render? What do you mean by lime - describe the mix please?

The interior wall surfaces were taken back to brick and "a new skim of plaster" was applied.
Do you mean that you floated the walls with browning for instance, and skimmed them with "skim coat gypsum" eg Thistle Multi-Finish?
No lime was involved?

When it becomes clear exactly what you have done then perhaps the damp issues can be approached?

And, as always, pics will tell a thousand words.

 

[ree]

Please dont double post it makes for confusion - for me anyway. Your other post is to an OP who posted in 2012.

 

[odilon]

No, we removed all external render - want to go back to the original bricks and lime mortar. Problem is that the guy who did the inside wasn't thinking along the same lines, and put a skin coat of Thistle multi-finish (no lime), and in a few places went back to brick, yes.

I've put some pics in an 'album' here:
//www.diynot.com/network/odilon/albums/

Thanks!

 

[ree]

So you've pointed up the exterior with "lime" mortar - could you describe the mix? Did you rake out the beds to min. of 30mm? Has the whole house been re-pointed?

What do you mean by "a skin(skim?) coat of Thistle multi-finish"? Skimmed over something else?

The double doors at the rear require a temporary barrier across. This is urgent.

On the gable pic the two purlin tails are exposed. This is bad building detail. Perhaps they could be flashed over - thats if they are still sound.

The interior damp on and near the chimney breast is possibly coming from the external shoulders on the outside c/breast, and possibly the stack and its flaunching and flashing.
It would need someone up there to inspect.

The suspect c/breast is redundant and one flue has some kind of "air-vent" terminal. Was this, and any other flues, swept since you've owned the property?

Some interior damp could be coming from the pointing. Given that its a solid wall house one has to wonder why was it rendered in the first place.

 

[Guitar_man]

Did you have the damp inside before you make the external changes? Is the affected wall the wall that was rehabbed?

 

[odilon]

Hi Jimbo,
There was a tiny bit of damp which only seemed to appear after the gypsum skim coat went on (presumably on top of existing old lime plaster, which in truth wasn't in great condition after all the wallpaper came off). We assumed it was due to cold walls, and a few days of dehumidifying seemed to dry it out nicely...though this was in the much warmer spring/summer months.

 

[odilon]

So you've pointed up the exterior with "lime" mortar - could you describe the mix? Did you rake out the beds to min. of 30mm? Has the whole house been re-pointed?

What do you mean by "a skin(skim?) coat of Thistle multi-finish"? Skimmed over something else?

The double doors at the rear require a temporary barrier across. This is urgent.


On the gable pic the two purlin tails are exposed. This is bad building detail. Perhaps they could be flashed over - thats if they are still sound.

The interior damp on and near the chimney breast is possibly coming from the external shoulders on the outside c/breast, and possibly the stack and its flaunching and flashing.
It would need someone up there to inspect.

The suspect c/breast is redundant and one flue has some kind of "air-vent" terminal. Was this, and any other flues, swept since you've owned the property?


Some interior damp could be coming from the pointing. Given that its a solid wall house one has to wonder why was it rendered in the first place.

ANSWERS:

**The beds were well raked out, and the mix was lime + sharp sand (no cement). I don't have the exact ratio, but the guy who did it knows his stuff.

**Sorry, typo, patching and then skim coat over the old plaster, which I assume was lime.

** We installed a railing/barrier. That is not a current picture

**They were like that, and indeed seemed non-ideal, but werent' causing any problems from what we could tell

**The outside c/breast was totally re-pointed, so shouldnt' be the source. One chimney flaunching was damaged in a recent storm, but the leakage/damp is far too widespread to be explained by that.

**No, the Chimneys haven't been swept. This chimney might be brought back to life later (there's a nice fireplace on the G/F)

 

[ree]

The pointing has been done in very wide beds and struck with a flat finish instead of a bucket handle finish. The flat finish might retain water.

The water could be entering at one point and showing over a wide area. Or it could be entering at various points and showing over a wide area. You simply dont know.

I would still suspect the exposed external c/breast pointing, stack and flaunching.

If water is entering through the gable re-pointing then, eventually, it will enter to some degree through all re-pointed elevations.

Sweep your flues, and perhaps, if you are willing to go over work, knock off all plaster on affected walls or c/breasts and render up with 6:1:1 sand lime & cement with an anti-salt additive. Skim with say board finish.

 

[stuart45]

Using a flush joint is the correct way of finishing a lime mortar joint. The next day it is hit with the churn brush to expose the aggregate, which allows the joint to breathe better.

 

[ree]

Agreed. Although i didn't know about brushing off later to expose the aggregate.

But with the damaged bricks and the loss of their edges what should have been a flush joint has spread as a "flat" joint - too much thin mortar exposure.
I would have kept the pointing in the rake out & bucket handled it.

I suspect that the elevation was originally rendered as a remedial measure against damp penetration, & that this method of pointing will now become a low risk factor for further penetration.

But i fully accept your interesting points.

 

[stuart45]

Agreed. Although i didn't know about brushing off later to expose the aggregate.
..

It's done because when a joint is tooled in with metal it brings laitance to the surface, making the joint more weatherproof.
This is fine with cement mortar, but not with lime when the joint needs to breathe.
Lime mortar for these kind of joints is normally NHL 3.5/well graded sharp sand. The lime shrinks slightly as it cures, making a honeycomb effect which allows air and moisture to pass through easily.
With those older soft bricks, they will absorb a lot of rain, so the joints need to allow the moisture to escape through them. The lime mortar also carbonates better.
This is now the standard method of repointing older building in the UK, even with damaged brickwork. Some are tuck pointed, but this is really expensive.
This type of brickwork is sometimes weatherstruck and cut, which can look a bit neater than the bucket handle.
odilon,
You shouldn't really be using gypsum on top of lime mortar, as gypsum tends to hold the moisture. Lime should be used with a fine sand.

 

[Bedrock9]

Hi Odilon, this is an interesting post as my house has been replastered inside using normal Gypsum based stuff and I was thinking of repointing outside with lime, have you had anything done to rectify this yet and is it showing any signs of success?

Interestingly the picture on the room showing chimney breast looks like it is very similar to a room in my house, and my brothers house has identical staining on the breast, I think this could well be sulphur which has soaked into the brickwork and will then behave like salt, drawing moisture from inside the house into the plaster.

Also what is the black bit on the floor in this room? Is it the hearth for the upstairs fire? (same in my house, upstairs hearth to one side of the chimney) if so is the chimney vent in the that room actually not going into the upstairs part of the chimney leaving that side unvented? Cheers

 

[Bedrock9]

Oh yeah, is the other chimney in your house active and still used as a fire? Otherwise shouldn't the stack be capped at the top with a vent that stops watering coming in but lets it breath?