Is this damp?

[yjones]

Hi

We had the rear elevation rendered earlier this year and left a portion of it exposed stone. The stone was repointed using (non-hydraulic) lime.

There looks to be a damp patch which is there permanently, no matter if its sunny or raining.

It doesnt look like rising damp does it?

Is it even damp? The fact that its always there is odd. Could it be perhaps the builder used a slightly different mix for the mortar in that patch?

Thanks

 

[blup]

Dripping gutter?

Blup

 

[Doppleganger]

A different mortar mix or even a different depth to the pointing can produce such results. A deeper depth of pointing can produce a darker colour to the finish.
It looks like the builder was a bit messy with the pointing and did not wire brush the stone to remove the lime afterwards.
It could still be done to improve the finish.

 

[yjones]

Dripping gutter?

Interesting - Ill take a look. But as I say - it looks like that even after a few days of sun.

 

[bobasd]

whats happening inside the house?
that pointing mix looks to be very sandy.
pointed random stone thats not been rendered needs to have much tighter "joints" - half your wall surface is weak pointing.
the rendering on the upper half of the wall just ends in a ragged line - ther should be a bell-cast fitting and a level ending for thee upper render, an bell-casts over the window heads.

 

[yjones]

whats happening inside the house?

Inside the house is a new concrete floor with a DPM. Im hoping its not moisture hitting the DPM and going into the wall.

Ill take a photo of the wall when its all wet and see if I can still make out the patch. If I can then it may not being damp causing it to be dark (ie a different mortar mix maybe).

 

[StephenStephen]

Just curious - do you mean non-hydraulic (ie;lime putty), or NHL (natural hydraulic lime) ?

 

[yjones]

Ah. The bag has "NHL 3.5" on. I thought that meant Non Hydraulic Lime.

 

[StephenStephen]

Ah. The bag has "NHL 3.5" on. I thought that meant Non Hydraulic Lime.

No - it means Natural Hydraulic Lime (confusingly!). (I think it would have been unusual to use lime putty outside these days)

 

[bobasd]

any damp signs in the house opposite side of wallor near to the patch?
DPM should have been flapped up the wall about 50mm to 70mm higher than the FFL - skirtin will hide it.
but you probly dont have a DPC in the wall.

 

[yjones]

Apologies for the tardy response (and poor interior photos).

The builder put gypsum plaster on the stone walls. Within weeks there were damp patches showing where the salts were attracting moisture and being trapped in the plaster. So the builder ripped off the plaster hence why you can see the bare walls below.

After the plaster was removed the walls looked/felt bone dry. I sprayed some salt neutralizing solution on the walls and since then they 'look' wet, but I dont think they are. Some of the stone does feel moist to the touch when its been rainy heavy, but that may just be water from the humid air condensing on the coldest stones(?)

 

[Doppleganger]

A bit daft putting gypsum plaster on a stone wall without a dpc.
At least it should have a waterproof cement render up to about 1 meter high. But a lime plaster would be better.

"Pink gypsum plaster is calcium sulfate, which is hygroscopic, and will therefore draw moisture out of the walls and then dissolve, giving that characteristic white efflorescence...
Only when the walls were thoroughly dry, should they have been plastered with a breathable lime and sand plaster" https://www.telegraph.co.uk/lifestyle/interiors/jeffhowell/3364227/Property-advice-On-the-level.html

 

[yjones]

A bit daft putting gypsum plaster on a stone wall without a dpc.

Yes, too put it mildy!!

I think lime plaster is the way to go. Keep it all breathable. But Im not sure how the damp patches/salts will behave with the lime plaster. Will it still show, or will it evaporate quickly? Should the plasterer use lime putty or NHL 2.5? Is there a salt neutralising additive that can be added to the lime plaster?

 

[Doppleganger]

But I'm not sure how the damp patches/salts will behave with the lime plaster.

I'm not convinced that it is damp patches. If it is, lime plaster is breathable and the damp will evaporate. But because it is hygroscopic it can also absorb moisture.
However, not sure how thick the stone wall is (insulation) but I suspect that condensation will always be an issue.

Will it still show, or will it evaporate quickly?

You may always get a slight difference in shade. Your dyed lime wash or breathable paint will mask that to an extent. The speed of evaporation/drying (if it is damp) will depend on many conditions such as internal temperature, air flow etc.

Should the plasterer use lime putty or NHL 2.5?

NHL 2.0 is perfectly acceptable. Horse hair in the first coat is required and cheap.

Is there a salt neutralising additive that can be added to the lime plaster?

I don't know the answer to that one. I suspect not. I suspect it is not needed.