Old lime plaster question

[keithevans595]

Hello,
First post on here.
I have a 1880's semi-detatched house that had gypsum plaster part way up a wall that was exhibiting damp.
The damp wall has a chimney breast and is the party wall to the adjacent property.
I have removed all traces of gypsum plaster up to the point where the old lime plaster is dry and the walls are currently drying naturally.

I have 2 questions:
1. I am getting the walls re-plastered with lime plaster, if one corner is taking longer to dry, is it still ok to go ahead with the re-plastering?
2. One wall, which is dry because it was not connected to the damp wall, was skimed with a thin layer of gypsum plaster. This wall did have a door way which was blocked up.
This gypsum skim comes off easily with a scraper leaving the original lime plaster underneath. If I took off all the skim I may be left with a reasonable finish wall.
The wall has a paint finish which looks original from the small section I have removed, def not emulsion.
I intend to eventually re-paint the room using claypaint, do I need to do any prep to paint over this old paint?

Looking forward to your colective wisdom.
Thanks, Keith

 

[MatthewC]

Hi Keith, welcome to the forum.

You say that one corner is taking "longer to dry". I presume you are refering to the new lime plaster that has been applied, and also that there was no obvious sign of dampness before starting.

How long is "longer"? If the wall is slightly damp in that corner then it will take longer than other walls, just leave it and wait another week or so. When my house was plastered there was one lath wall where we plastered both sides on successive days, and it took a couple of weeks to dry enough for the next coat.

If one wall at that corner is an external wall, I would be checking what you have outside - is there high ground there? Or a water butt? Or concrete or paving right up to the wall? Is the gutter above overflowing down the wall in heavy rain? Is it a favourite place for a dog?! If the wall is inherently damp for any reason, it will take a long time to dry out properly - like, months. Sort out any possiblle issues and then just leave it to sort itself out.

Gypsum plaster on damp lime simply pushes the dampness further up the wall..

Best of luck, and keep asking questions!

Matthew
http://houseintheenchantedforest.blogspot.com/

 

[Cubist]

Just a thought after noting that your damp wall is in fact a 'party wall' between a pair of semi-detached houses and thus 'internal' to both properties.

Either, the damp is rising from the ground below somehow, or it is descending from the roof, but it could also perhaps originate from a water leak from say your, or your neighbours, central heating / water supply/waste system.

Is the damp only present in a ground floor room or is it found upstairs as well?

If upstairs, and you have access to the loft space, check around the the exit of the chimney stack for any signs of rain water penetration and perhaps get your neighbour to do the same. If you have a head for heights and access to suitable ladders/scaffolding check any flashing around the chimneys exit at the roof or broken tiles in the immediate vicinity.

 

[paulc]

I intend to eventually re-paint the room using claypaint, do I need to do any prep to paint over this old paint?

If you want to avoid painting, you could always mix some pigments in with the final skim coat. Mike Wye (and probably others) do what they call a Regency plaster that can be supplied pre-coloured. But that rather assumes you want the same colour all over the walls..

 

[worms]

...you could always mix some pigments in with the final skim coat.... But that rather assumes you want the same colour all over the walls..

Oh, I don't know...Michelangelo managed to vary his colours quite well!

 

[keithevans595]

First of all, a thank you for your welcome and thoughts MatthewC, Cubist, PaulC and Worms.

Just to confirm,
The gypsum plaster has only been off the walls for a week now and the walls are starting to drying out.
No new lime plaster has been spread yet.
The area to be plastered is approx 5 sq m.
I am gauging the drying process by the colour of the lime mortar between the bricks, light = dry, darker = still drying.
I put a de-humidifier in the damp corner last night and over 18 hours it has collected nearly 2 pints. bearing in mind the ground under the suspended floor is earth.
The mortar is begining to dry now, so I could turn it off and see what happens to the mortar colour overnight.

Regarding Cubist's thoughts about water pipes and roofs, there are no water pipes in the area, there does not appear to be any damp upstairs or next door.
The roof has a valley between the front and back, effectively 2 pitched roof's, our part of the valley was re-done about 20 years ago so should be ok.

My only thought is to take a brick or two out near the damp area and see whats happening in the cavity?

Just to confirm a few other points, I haven't got a dog, would to cats suffice!
Mrs Keith wants the walls painted!

Thanks

 

[worms]

Is the terrace built along a slope, with your house lower than the neighbour where the damp wall is? It's not uncommon to find damp issues along the base of a party wall, on the uphill side of a downhill house.

 

[stretch]

Is the damp corner a party wall.? If so then maybe your neighbours walls are cement rendered.? This would mean that the cement would be holding the dampness and your breathable lime plaster would be the breathable side.... Just a thought.

 

[88v8]

I have a 1880's semi-detached house that had gypsum plaster part way up a wall that was exhibiting damp.
The damp wall has a chimney breast and is the party wall to the adjacent property.......
I intend to eventually re-paint the room using claypaint, do I need to do any prep to paint over this old paint?

Ahhh, the fun of a new house and the sins of past owners....

If the fireplace has been blocked with no cap on the chimney and no vent in the fireplace, that will eventually make the wall damp.

It takes roughly a month per inch of wall thickness for a damp wall to dry thoroughly, once the cause of damp has been removed.

Underlying paint just needs to be sound and not dusting before you apply the clay paint. Thin the first coat, even 50% would not be too much. Clay will not stick to damp surfaces.

Your suspended floor... check that there is a good airflow beneath, no airbricks blocked.

I would be surprised if the wall has a cavity. 1880, more likely solid. From the outside, do you see headers and stretchers or only stretchers?

Ivor

 

[MatthewC]

My only thought is to take a brick or two out near the damp area and see whats happening in the cavity?

Cavity? Are you sure there is one? I'd be surprised if an 1880s house had one. Cavity or not, I would think more than twice about taking bricks out of the wall.

Have you asked your neighbour if he has dampness on his side? Or if he has gypsum on the wall, see if it feels cold to touch. Also, check what he has outside. It might be his ground level introducing the dampness (compost heaps have been known!). The list of things to consider include the same on his side of things, as the damp will happily spread from outside to the party wall (are you sure the outer wall is dry?).

The other thing is how far below the suspended floor is the ground? It sounds like it is VERY damp if your dehumidier got that much water.

As Ivor says, it really does take a very long time for dampness to dry out - you have only just removed the gypsum so it seriously can take months. However, if it is dry enough to apply lime plaster, then the lime will allow that drying process to continue, slowly. Hence you could go ahead now but <disclaimer> I haven't seen your wall!

I'm afraid Mrs Keith might have to wait a good while for the wall to be painted...

 

[keithevans595]

Hi Worms, the house is built on a slight slope at the rear, but there is an old extension on the back which has a concrete floor. The wall adjacent to the back of the house are relatively dry.

Is the terrace built along a slope, with your house lower than the neighbour where the damp wall is? It's not uncommon to find damp issues along the base of a party wall, on the uphill side of a downhill house.

Stretch, the damp corner is part of the party wall, I have checked with the neighbour and he just has lime plaster on the other side.

Is the damp corner a party wall.? If so then maybe your neighbours walls are cement rendered.? This would mean that the cement would be holding the dampness and your breathable lime plaster would be the breathable side.... Just a thought.

Ivor, the fireplace is used occasionaly for an open fire, the chimney is not capped.
Underfloor at the back of the house was soil but has been covered in a 3 inch concrete layer with a DPC plastic membrane, which is well below the old slate DPC and there is a gap between the concrete and the bricks, leaving a soil gap. There is approx 18 inch gap between concrete and suspended floor.
The front room and a small middle room still has soil as the base and the depth increases to 2 feet at the front of the house.

I checked the original house documents and the land was sold in 1884 and the property was built shortly after. The house is located on the Isle of Wight and it definately has cavity walls

If the fireplace has been blocked with no cap on the chimney and no vent in the fireplace, that will eventually make the wall damp.

Your suspended floor... check that there is a good airflow beneath, no airbricks blocked.

I would be surprised if the wall has a cavity. 1880, more likely solid. From the outside, do you see headers and stretchers or only stretchers?

Matthew, as above there is definately a cavity. Neighbour has lime plaster on his side of the wall and the problem area is in the middle of the party wall with the opposite of my layout.
As above, there is concrete and soil under the floor.
The garden, all 250 feet of it is on a slight slope, maybe 5 feet difference over 250 feet. After heavy / prolonged rain springs do start running but the garden has french drains and most of it runs into the main drain. The amount of water being collected in the de-humidifier could be the damp from the soil base, it has rained a lot recently.

Cavity? Are you sure there is one? I'd be surprised if an 1880s house had one. Cavity or not, I would think more than twice about taking bricks out of the wall.

Have you asked your neighbour if he has dampness on his side? Or if he has gypsum on the wall, see if it feels cold to touch. Also, check what he has outside. It might be his ground level introducing the dampness (compost heaps have been known!). The list of things to consider include the same on his side of things, as the damp will happily spread from outside to the party wall (are you sure the outer wall is dry?).

The other thing is how far below the suspended floor is the ground? It sounds like it is VERY damp if your dehumidier got that much water.

 

[keithevans595]

Time to eat some humble pie!
While I did correctly say the house has cavity walls, I discovered yesterday that there are cavities on the outside walls, but the interior walls including the party wall are single skin walls.

 

[worms]

In any old property I've experienced, I wouldn't find that rate of water accumulation in a dehumidifier at this time of year surprising! But if you have the ability to light an open fire (or any sort of combustion appliance that draws air from the room and vents the exhaust to the outside!), I'd be running it 24/7 as, in my experience, nothing dehumidifies a property quite like the combination of warmth and constant draw of air through the property.

 

[worms]

I've just re-read this thread and realised that I don't see any mention of whether the plaster is directly onto the bricks or onto lath? I think we are all assuming that it is directly onto brick, but it would be good to know for sure.

Also, what happens in your loft space at the top of the party wall - does it extend up as a double layer with cavity, single layer, with the top of the cavity visible on one side, or is there no wall up there at all and the top of the cavity is visible on the floor (or covered with insulation and junk)?

 

[keithevans595]

Hi worms,
A previous message should come before this, awaiting approval.

There is no plaster yet, I need to order it for delivery from the mainland, me being on the Isle of Wight.

New plaster will be onto brick and as previous message the wall in question is a single skin up into the loft.
The dehumidifier does seem to be drying out the surface of the wall slowly after being exposed for about 2 weeks now.

I have some pics I can put on when I have worked out how to do it!

 

[Cubist]

As water ingress from above, either from a leaky pipe or rainwater from the chimney and surrounding roof, or even a dog with serious bladder problems, seems not to be the culprit here, there can, to my mind, only be one other cause. Capillary action in which water is effectively being drawn up through the brick-work and its mortar from the soil below. Many people make the mistake of thinking that soil immediately below the house and covered for over a century would be bone dry to some considerable depth but in truth, and depending in part on the height of the local water table, digging down a few inches would reveal wet soil quite quickly. Even without the assistance afforded by our heating systems, that water will try to percolate upward from the its source and will constantly be looking for a means of escape into the atmosphere. Commonly that route is through the exposed surface of the brickwork or the plaster with which the wall has been coated. Typically our heating systems aid this process by creating circulating air currents of warm and cooler air that carry the water away from the wall and, hopefully, out of the house through the ventilation system.

Gypsum plasters offer more resistance to this process and many paints also add to this resistance. It could be Keith, that in removing the plaster from the offending wall, you have opened up an easier escape route for the water and its molecules are just making a mad dash for freedom while the opportunity presents. Leaving the masonry exposed makes that journey easier and it may 'appear' that in doing so the wall will become thoroughly dry. That, I think, would be a false perception and, with a change in local weather conditions you may see the damp return to, or even exceed, the level you are currently observing. Replacing the plaster and paints with 'breathable' products will help to reduce/eliminate the risk of damage resulting from this kind of rising damp, but despite the claims of some, I doubt it can ever be thoroughly and absolutely beaten.

 

[a twig]

Double check the valley - when I moved into my project the fall of the valley went the wrong way, so all the water collected against the “closed” gable end rather than flowing the other way through the open end and into the down pipe.

That water held against the wall caused us no end of problems until we re-did the roof - it used to find all the failed bits of mortar and then work its way down inside the wall, and also get drawn into lots of weird places. for e.g we had a joist or two whose ends were in that wall and those joists used to draw that the water along them so I had damp spots in the ceiling a good metre or two away from the wall itself

Also check where your gutters are actually going - it turned out at one of my neighbour’s house - the down pipe discharged directly underneath the living room suspended floor…

If the wall is still a bit wet, you can lime plaster - you may get some salts coming through though (I did when I had to plaster a bit too quickly when an Xmas bedroom deadline loomed) but these can just be brushed off

 

[keithevans595]

Thanks Cubist and Twig,
I am now sure it is, as you say dampness from the ground escaping upwards.
I think, to use a current popular phrase, I'm going to have to live with it.
I have ordered the lime plaster and it should arrive next week ready for my plasterer to start spreading it.
It can't be any worse than looking at blown gypsum plaster or bare brick and hopefully will have dried enough to paint it in the spring.

As the old plaster on the walls have been cut back to dry walls, what technique do you suggest for joining new lime to old lime plaster?
My plasterer said they used to use scrim.
The plaster supplier said it was difficult as I may get a hairline crack form as the two move in different ways as the temperature changes.

Finally, at a corner would you use angle bead? Again the plaster supplier said traditionally the corners are formed without beads.

 

[malcolm]

Plastering onto a damp wall shouldn't be a problem other than it will take the lime plaster more time to dry out. I've never had any problem plastering new lime plaster up to existing. The main rule is to to keep spraying the wall and the existing plaster with water until the water isn't sucked immediately in. Leave that to dry and make sure the new plaster is properly compressed and you should be fine. I don't skim over the existing lime and have never seen a crack appear at the join.

I don't use corner beads. Anything less than stainless will be attacked by the lime, and nothing sticks to stainless. You've got workable time with lime so don't need corner beads. Though doing one face the day after the adjacent face makes things much easier.

 

[Cubist]

My experience mirrors Malcolms so nothing to add to what he has already said except....

Some might say that the odd fine crack in the plaster, if such arise, may add some character and charm to the wall. Not to my senses though so if you do get some crevices opening up they can be closed again very quickly and easily with a fine filler like that supplied by Ty Mawr - I can personally recommend this as I've used it for both internal and external repair work very successfully.