Damp showing in Chimney breast

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I’m in the process of painting and decorating downstairs and have noticed this damp patch on the chimney breast. The wall used to be covered with wallpaper, and a few months ago, I stripped the wallpaper and had the wall re-skimmed.

Ever since I had the chimney breast re-skimmed, I’ve noticed this damp patch.
I assumed it was rising damp (the chimney has not been used for years), so I removed the skirting board, but was surprised to find it dry behind it.

I'm puzzled why it's damp above the skirting board section. The chimney is ventilated, but I'm not sure if water is seeping from the top of the chimney and making its way down to the hearth, It still does not explain why the base of the breast is not damp.

Any help, greatly appreciated.
 

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Lift a floorboard to inspect for damp below.

Typically chimneybreasts did not have a DPC on the assumption that constant heat and airflow would keep them dry.

Look through the hole. Is there builders rubble inside? Put your hand in. Is it damp?

Is the top of the chimney open and is there a discernible draught going in the hole?
 
Do you have an air vent to allow air on the ground floor to travel up the chimney?

I recently worked for a client that had a 3 story house. He repointed the whole side flank (£20K) and he still had damp around the top floor chimney breast. He spoke to a damp specialist that explained that the old soot is hygroscopic. It soaks in moisture, and during the colder months, that moisture travels through the bricks and plaster.

The (free advice) was to use sand and cement over the interior chimney breast.
 
Last edited:
OP,
Is the floor solid?
Is the back wall a solid exterior wall?
Your blocked off flue needs opening up and the fireplace cleaning out, and the flue itself sweeping to remove all remains of sooty contamination that leads to hygroscopic penetration of the c/breast.
I see you had an air vent hole in the face of the c/breast - after sweeping, install a hit & miss vent in the c/breast.
The redundant flue also needs ventilation at the top - typically a cowl in a chimney stack.

All damaged plaster surfaces need hacking off and replacing with a 4:1 mix of sand & NH lime.
Dont use cement in the mix.

Have you been in the loft and looked for leak stains around the loft chimney breast brickwork?
Pics showing the chimney stack might help?
 
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You have used the incorrect plater for a chimney breast. Fundamentally, you don't have dampness in the sense of rising or penetrating dampness, what you have is dampness from a combination of condensation and salts contamination. My advice is strip the plaster back to brick and re-plaster with a lime based renovating plaster.
 
Lift a floorboard to inspect for damp below.

Typically chimneybreasts did not have a DPC on the assumption that constant heat and airflow would keep them dry.

Look through the hole. Is there builders rubble inside? Put your hand in. Is it damp?

Is the top of the chimney open and is there a discernible draught going in the hole?

Thanks for your response, we did have remedial works, to the enclave of the left side of the chimney breast, a couple of the joists had rotted, due the outside grounds being higher than the dpc.

This has now been resolved, ground works lowered below the dpc, and poured some concrete to prevent pooling of water in future close the foundations.
You have used the incorrect plater for a chimney breast. Fundamentally, you don't have dampness in the sense of rising or penetrating dampness, what you have is dampness from a combination of condensation and salts contamination. My advice is strip the plaster back to brick and re-plaster with a lime based renovating plaster.

Can I used bonding in the brick wall, and SBR to tank it and then apply lime based plaster - or what should I use as the substrate on the bare wall prior to the lime plaster?
 
Best advice is always to follow the instructions of the product you use. I would say don't use cement:sand, which shouldn't really be necessary anyway. The critical thing is to use a product that will eliminate the action of salts in the masonry. All chimneys of a certain age accumulate salts over time and once embedded in the plaster the only option really is to remove and start again. Standard gypsum plaster is hygroscopic, which reacts with salts and will absorb and retain moisture from the atmosphere. You need a lime based renovating plaster, which is highly breathable and will not retain moisture. There are plenty available - e.g. Limelite.
 
Best advice is always to follow the instructions of the product you use. I would say don't use cement:sand, which shouldn't really be necessary anyway. The critical thing is to use a product that will eliminate the action of salts in the masonry. All chimneys of a certain age accumulate salts over time and once embedded in the plaster the only option really is to remove and start again. Standard gypsum plaster is hygroscopic, which reacts with salts and will absorb and retain moisture from the atmosphere. You need a lime based renovating plaster, which is highly breathable and will not retain moisture. There are plenty available - e.g. Limelite.

I only mentioned sand and cement because that was the (free) advice offered by the damp specialist.

I am not a plasterer... if the point of the lime plaster is to allow moisture to travel in both directions, what happens if the home owner wants to use a non-permeable finish such as waterbased eggshell on the wall after first lining it?
 
I only mentioned sand and cement because that was the (free) advice offered by the damp specialist.

I am not a plasterer... if the point of the lime plaster is to allow moisture to travel in both directions, what happens if the home owner wants to use a non-permeable finish such as waterbased eggshell on the wall after first lining it?

Use breathable paint?
 
Use breathable paint?

Such as lime/chalk paints?

And breathable wallpaper adhesive, or breathable wallpapers?

My post was not intended to be flippant.

If the chimney breast is in a kitchen (for exmple), why wouldn't the customer want a cleanable wall covering (including tiles)? I am simply asking if lime plasters are compatible with such finishes.

My, lay, gut instinct is that they are not, but I am happy to be educated/corrected. As things stand, I don't see the benefit of a breathable substrate if that substrate is covered by something that is not breathable. Or am I missing something?
 
Once a masonry base stabilises itself, paint finish and permeability becomes less important. On fresh plaster, and for up to a year afterwards, permeable paint is recommended. Trade paint is the best option - it's called trade paint because it is specifically formulated for use on new plaster.

PS. this would be different if the base was susceptible to dampness - but that chimney looks pretty dry. The apparent dampness is coming from the air, not from the base.
 
NOT APPROPRIATE
OP,
You say: "Any help greatly appreciated" I gave you "help" but you dont even take the trouble to reply to my post?
 
OP,
You say: "Any help greatly appreciated" I gave you "help" but you dont even take the trouble to reply to my post?
I thought I had replied, but for some reason, the page didn't save.

The floor isn't solid, it has a subfloor, which has been recently lifted due to a couple of rotten joists, This was due to the DPC being penetrated as the ground works were higher, but that's now been resolved.

The property is semi detached, and the chimney backs onto the neighbours chimney stack. The fireplace hasn't been used in years (over 20) and has been bricked up.

I've put my hand in the air vent, and it doesn't feel damp, I can feel a draft, and the room above also has a air vent. I have been to the loft and checked the stack there, and it's dry to the touch.

The neighbour and I had the chimney stack repaired a few years ago. I know there are vents, but I'm not sure if there is a cowl or pepper pot. I will need to get up there to take a look.

Thank you for your advice; I will try to remove old plaster, hack back to brick (1m from the floor), and hire a plasterer to do limelite plastering, which appears to be easier to do and requires less drying time.
 
Is there rubble in the cavity?
When we removed the floorboard, we removed any rubbish that was laying in the foundations, regarding the air brick I can not see any rubbish in there.
 

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