Dreaded damp patches on plaster... Any ideas?

[paulalde]

Afternoon

I had the front room skimmed around 8 weeks ago and we have been waiting for 1 wall (only external facing wall in the room) to fully dry out. This area in particular was taken back to brick so I assumed due to the cold weather it was just taking time.

Any ideas what this could be? Please see the picture. To be fair I took this this morning and it was fairly dark outside still so the camera makes the patches out to be slightly darker than they are. I'll add another picture this evening. Some of the darker streaks towards the bottom are actually where the skim is really thin and showing render colour.

Next morning

• paulalde
• 30 Dec 2014
• 1

Some points to note.


* Late 1800s stone build house with external wall sandstone and red brick internally. No cavity.

*House is raised with steps to front door so outside these patches would be between waist and chest height.

*Internally under the boards there is a recently inspected and clear knee deep void with an air brick with good air circulation to another air brick in the cellar.

*Old single glazed sash windows where replaced 1 year ago with new pvc sash units.

*Closer inspection shows dry plaster under the patches which would suggest nothing is rising.

*Externally nothing is bridging to these areas that would allow in moisture. No drains or pipes in close proximity.


I wouldn't say the room humidity levels ever get excessive, we don't put washing on the radiators and the bathroom upstairs has a humidity sensing extractor that comes and goes as it pleases.
One thing I have noticed is if we've been boiling a pan or cooking in the kitchen for a long time then the patches may ever so slightly darken.

I added some direct heat from a fan to the area last night and it seemed to clear up nicely after a period of time. This morning it looks to have re appeared but somewhat less apparent as before. Picture below.

After heater

• paulalde
• 30 Dec 2014
• 1

Penetrating damp - Nothing I can see that would cause this.

Rising Damp - Internal ground level is higher than external ground level, though areas under the patches are fully dry.

Efflorescent Salts attracting moisture - I don't see any salt build up but I have noticed when cooking with increased humidity the patches darken.



Any advice on how to proceed would be much appreciated. I'm thinking give it as much time as possible and then add a stain blocker after mist and then colour.

Thanks in advance

Paul

 

[stef99]

don't use a heater to force dry plaster. it will make it crack.

There has to be something causing the damp.
I would check to see if the wall ties are bridging the gap, (which maybe impossible as you need to see down the wall).

Other than that, look for any cracks on the outside wall. the damp could be coming in from there.

 

[Alastairreid]

No cavity stef.

Can you post a picture of the outside?

 

[paulalde]

No cavity stef.

Can you post a picture of the outside?

Thanks for the responses, here are some pictures from the outside. I'm thinking it's no coincidence that it looks like the pointing has been touched up on all 3 areas that are patchy inside. The internal floor level is level with the door step and directly above the air brick. I've only been in a year and didn't have any involvement in the pointing. It looks to be sand and cement rather than lime.

On the centralised picture there is a stone to the right that looks to have a crack but this is just a surface crack and the image shadow makes it look worse.

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• paulalde
• 2 Jan 2015
• 1

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• paulalde
• 2 Jan 2015
• 1

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• paulalde
• 2 Jan 2015
• 1

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• paulalde
• 2 Jan 2015
• 1

Cheers

Paul

 

[stef99]

I think your damp proof course is gone mate.

the old damp prrof course (DPC) will be made of oil like strip. like bitumen.

I suspect that has worn through (as they do after about 60+ years)

Theres a number of things you can do about it, but I have to admit I have never done it on stone walls.

This is not the best video as you may need to do a lot more drilling and filing.
but this will give you the idea of what you may need.

https://www.youtube.com/watch?v=vp2BxSYOkGs

since your wall is stone, you may need to rake out a long line of mortar.

 

[ree]

On the basis of the text and the pics there are no grounds for saying that the "damp proof course is gone". Neither does a bitumen DPC wear through after 60+ (or a 100+) years.

Bitumen squeeze out snot(s) can be seen to the right of the air brick.

The pointing, as the OP has noticed, is the most likely culprit for penetrating damp - the pointing should be raked out to 20mm to 25mm and re-pointed in lime mortar.

Check the sill for a throat? Best practice, the sill should project a min of 50mm.

You have new floor boards, and new gypsum plaster - why?

The plaster should be taken back to brick again to a height of one metre, and rendered in 3:1 lime and sand, and a remedial skim. Check the return walls. for damp.

The patches begin 300mm above ground level.

Has anyone investigated below the flooring?

Forget about stain blocker and suchlike 5 mins remedies.

What is the circular metal device below the sill?

The recommended video shows the correct way of gun injecting a DPC - but then: the creating of a bridging plinth is wrong, and his siting of the render stop or bell cast is also wrong. There's another glaring fault that he's ignored but its not relevant to this post. He seems to be an excellent plasterer who has entered a field he knows nothing about.

 

[stevethespreader]

I watched the said vid hasent he run the stopbead along dpc height? And then sat the bellcast on top? Any damp rising from ground level will stop at the stopbead?and will not travel any further past the bellcast?

 

[stevethespreader]

I watched the said vid hasent he run the stopbead along dpc height? And then sat the bellcast on top? Any damp rising from ground level will stop at the stopbead?and will not travel any further past the bellcast?

in other words the damp will not travel past or through the metal of the stopbead so it Is not bridging dampcourse the stopbead acts as a barrier separating the above render

 

[ree]

Moisture will travel by capillary action up the plinth. Capillary action will then pull the moisture that has travelled up the plinth behind the stop bead (which is bedded in S&C) to travel on and up behind the bell cast (which is also bedded in S&C) and so on up the wall.

The purpose of the stop bead (metal or otherwise) is to determine the edge of the render or plaster, its not for stopping "rising damp" or capillary action or moisture of any kind.

The injected DPC will be irrelevant in all this capillary movement thro, & behind, the render because once the capillary moisture is above the DPC it will begin to penetrate sideways into the brickwork.

Best practice is that you never bridge a DPC, and never (not always possible) use a S&C plinth. Always try for installing the DPC 150mm above ground level, and set the stop bead or bell cast above it - not on it or below it.

 

[Micilin]

I think Steve is not saying a bead will stop damp ( eg if it is put across the middle of a damp section)

The positioning of the drip bead is to stop damp rising . The use of a bell cast bead allows not just for a drip (because of the sloping profile ) but usually hides the DPC itself from the viewer, allowing you to set the bead up a little from it.

So capillary action etc doesn't carry on up the wall

 

[ree]

"in other words the damp will not travel past or through the metal of the stop bead" - sounds very much like a claim that the bead will stop damp?

In the video: The first time that we see the plinth its already installed, likewise the stop bead and bell cast. The plasterer says that its a plinth, and the bell cast is sitting on top of the stop bead - both are embedded in S&C.

"The positioning of the drip bead is to stop damp rising" - No its not, it has nothing to do with rising damp - see my post above.

The bell cast has no function in this instance of blinding the DPC from the viewer, how can it if the DPC is already blinded and bridged by the S&C plinth? There's no clean gap at the DPC level to be seen in the video.

 

[Micilin]

"in other words the damp will not travel past or through the metal of the stop bead" - sounds very much like a claim that the bead will stop damp?

In the video: The first time that we see the plinth its already installed, likewise the stop bead and bell cast. The plasterer says that its a plinth, and the bell cast is sitting on top of the stop bead - both are embedded in S&C.

"The positioning of the drip bead is to stop damp rising" - No its not, it has nothing to do with rising damp - see my post above.

The bell cast has no function in this instance of blinding the DPC from the viewer, how can it if the DPC is already blinded and bridged by the S&C plinth? There's no clean gap at the DPC level to be seen in the video.

Ree, to clarify , I'm talking about the function of a bell cast bead that is installed correctly, not the one in the video. ie what the bell cast bead is 'usually' for

I'm not sure but are you saying that the positioning of a drip bead has nothing to do with rising damp, or hiding the DPC?

 

[ree]

I've explained at length above, perhaps reading my posts again will clarify things. However:

1. The bell cast (bell casting bead) is "usually for" casting off water thats run down the render. Shedding water away from the masonry below
2. To prevent retention of moisture in the render at the bottom of the render.
3. To give a mechanical stop line to the render.

The bell cast is positioned above the DPC - the fact that it might blind the DPC is irrelevant.

The bell cast has no connection with the DPC or rising damp - "rising damp" is the term usually used for capillary action in masonry, and "capillary action" is the term usually used in connection with render.

 

[Micilin]

Ree..bear with me, I'll get there...so you would be happy with a drip bead that was fixed such that it either

1:allowed the render to bridge the DPC, ie so that render was continuous from plinth to wall

Or

2:was so far above the plinth , such that you could see the DPC from eye level

 

[stevethespreader]

Moisture will travel by capillary action up the plinth. Capillary action will then pull the moisture that has travelled up the plinth behind the stop bead (which is bedded in S&C) to travel on and up behind the bell cast (which is also bedded in S&C) and so on up the wall.

you shouldent have render coming past the stopbead from the plinth if its done proper u should have a nice clean stopbead to sit ur bellcast on so there will be no capillary action going past the stopbead like you say, as it will be impossible for the damp to go past the bead because the render stops at the otherside of the stopbead also the render for the bellcast starts at the otherside of the bellcast so its nowhere near the render of the plinth https://www.youtube.com/watch?v=FSKwZk9GGV8 so look at the vid and yes im saying that the damp from the plinth render stops at the bottom of the stopbead