no damp proofing on external wall

[randomrob69]

Hi, i'm just after a bit of advise about my external wall which does not have any dpc.

My house is an end of terrace built in the early 1900ish, the builders ran out of money when building the terrace leaving the external wall with no dpc as there was supposed to be another house built next to it ( hope that makes sense).

I have checked under the floorboards to verify there is no dpc, there is however a dpc under the floor joists that are resting on the external wall.

The neighbours have a concrete floor around their garage that touchs my external wall. Should i remove a channel of the concrete next to the house to reduce the amount of water ingressed by the concrete and possibly add some airbricks?

Do you think that it would be worth getting a chemical injection?

Is there a specific type of plaster that could be put on the internal walls as most of the plaster is blown and will need to be replaced?

Any advice on what the best way to stop the damp will be gratefully received. I have a few photos in my folders named 'damp'

thanks rob

 

[RedHerring2]

A few more bits of info', Rob, might help with some appropriate advice:

Where, heightwise, is the external ground level in relation to your internal floor level/bottom of your floor joists?

How thick is the wall?

Is there a cavity or is it a solid wall?

What was the plaster on the internal wall, lime, cement/sand or gypsum?

Is that a field stone/ rubble wall with some dressed blocks for appearance?

 

[randomrob69]

Sorry to take so long at getting back to you but have been pretty busy, i hope this information usefull.

The wall measures just over 40cm in depth, it looks as though its a cavity wall but i'm not 100 percent certain.

The level of the external floor is about the same as the level of the joists, the level is slightly higher where the internal damp problem appears worst. The joists dont look to old and have got a dpc and dont look affected by any damp

The wall seems to be a field stone with dressed blocks for appearance.

I'm not sure what the internal plaster is but it seems fairly old and has blown in alot of areas so will more than likely have to be replaced.

It looks as though someone has put a really hard plaster on the wall in the worst affected area, it seems similar to cement.

I know i will have to get the professionals in once i've got some money but in the mean time i just want to see what you think the best options would be and if theres anything that might slow down the rate of damp until i can get it fixed.

thanks

 

[RedHerring2]

First of all, I'm just a DIYer so anything I say, not only may be contradicted by others, but, what you have is one of the classic problems that falls into two different ways of fixing it:

1. Prevent the damp entering the wall or prevent it reaching the decorated surface, This approach requires some modern (relative to the age of the house) treatments to the internal wall.
2. Admit that it's not possible to prevent the damp in the wall so allow the wall to breathe. This approach recreates the original finishes.

A couple of undisputable points first:

a) the external ground should be minimum 150mm below where the dpc would have been (or is on the internal stonework in your case). This is to prevent rain splashes up the wall above what would have been above dpc.
b) may be not so indisputable, but a chemical injection on a 400mm thick wall is pointless, it's not going to penetrate to the middle of the wall (assuming you're approaching it from both sides).

Finally for this post, (I'll add more later, or hopefully, others may chip in)
I would guess that, although your wall is, in effect, built of two skins, the middle is usually filled with a mix of rubble and mortar. So, I guess, there is no cavity

 

[RedHerring2]

One method of correcting the problem of the external ground level being level with internal floor, that I have seen, where it's not been possible to lower the ground level, is to provide a water-proof render to the lower part of the wall, perhaps the lower 300mm or so. Normally this would bridge the dpc but obviously in your case this would not occur.
I'm no expert on this and there is always going to be a weak point along the top of this render. There is also a danger of driving any damp higher, for it to be able to evaporate from the wall. All of that is debatable.

If you were going to remove some concrete along the wall, you need to think about 150mm wide by 150mm depth, minimum. You could go deeper and/or wider and fill with gravel to ease the run-off of rainwater.
This 'french drain' resolution needs a little more info'.

If you wish to recreate the original finishes and allow the wall to breathe, you need to remove any cement based render internally and replace with a lime based render/plaster. Ideally, any external pointing would be a lime mortar.

If you prefer to go down the more modern route of preventing the damp from reaching the finished decorated surface, there are basically two options:
1. cement based, water-proof render with a gypsum based finish on top, or
2. dry line the wall.
There may be other treatments that I haven't mentioned.

Either of these more modern methods have the same risks associated with the external render, ie. driving the damp higher.

Another point worth mentioning is the endgrain of the joists. You say the condition of the joists is good, but are the ends in contact with the wall and/or is the endgrain protected? IMO, it should be.

Finally, for now, airbricks? Where were you thinking of installing these?
The solum (below the floor area) should be ventilated anyway.

 

[randomrob69]

Thanks for the advise, i guess i will lower the external ground level and replaster the internal wall and see if that helps.

I just wondered about 2 more things concerning the internal wall.

a) Do you know what would be the best mix of plaster to go on the internal wall?

b) Is there a special type of paint that can go over the plaster and allow it to breathe?

thanks

 

[randomrob69]

disregard the last message as i think you have already answered it.

As for the joists. I'm no expert but i'm sure they should be attached somhow to the wall, at the moment they are just resting on the dpc level, they are not touching the external wall and seem to be held in place by the screws going through the floorboards only. As it is mainly a corridor/cupboard the joists are only about 1 metre long

 

[RedHerring2]

The joists used to be built into the wall, so that bricks/blocks/stone between the joists fixed their location. You could simply fix timber blocking between the joists to recreate that efffect and to prevent any turning motion of the joists. Think about 2/3 of the depth of the joists. Ensure the timber blocks are not in contact with the wall. Other than that, the joists are not likely to go anywhere while there are sufficient floor boards to restrain them.

If you intend to use gypsum plaster, use a first coat of sand-cement render, possibly with a waterproofer in up to 1 metre high from floor level.

If you intend to use a lime based plaster, no cement based undercoat and use a breathable paint on top. eg a limewash.

 

[randomrob69]

just wondered if you knew what the best mix for external mortor was as there are quite a few gaps in the external wall between the bricks.

thanks

 

[RedHerring2]

Good point, having looked at photos again, it certainly needs it.

I, personally, would use a mildly hydraulic lime- sand, about 1:4.
I'd guess that whoever has done it before has used some lime based and some cement based, possibly some other kinds of stuff in there as well.

Is the stone a sand stone? If so, I would guess that a cement based pointing is going to be bad for the stone.

 

[randomrob69]

nice one redherring thanks for the information, most of the bricks are sandstone and theres a mix of all sorts of mortor on the wall.

i've heard that a cement based mortor retains water?

 

[RedHerring2]

I think it's worse than that, the cement based mortar does not allow the water to evaporate, so the sandstone ends up being eroded.

I don't know the chemical/mechanical/physical details of the process, but I'm sure you'll be able to find some examples of the damage that can be caused.

Edit: just found this, it might be helpful.
http://www.angus.gov.uk/devcontrol/advice_note_30.pdf

BTW, sand should be a sharp sand with a lime mortar,not a soft/building sand.