Damp problems--Outside ground level above dpc

[Ratty80]

Dear all
I would really appreciate some advice on how to solve what's appears to be rising damp. In the back downstairs room there is damp right around the 2 external walls. I've bought a damp meter and it shows they are completely saturated at the bottom getting less higher up going up to about 3' high. They have White patches from the salts. On the inner skin of brickwork there is an old slate dpc at floor level. There is no visible dpc on the outerskin. However working out the ground level it becomes evident that the dpc is 2 courses below ground level. Is it possible that the outside dpc could be higher than the inner skin dpc?

I have removed a couple of bricks to try and clear the cavity though the section I did didnt seem to have any bits bridging the cavity above dpc (ie I remove brick from outside which allows me to reach down to ground level which is about 3 and half courses below the ground level around the house, and using a mirror i can see the internal slate dpc 2 courses below this level

I am intending to purchase dryzone or similar damp treatment. However I'm not sure what level I should inject this into. If do this at the recommended 150mm above ground level then internally this will be over 1' above floor level. When I removed the skirting it was saturated in parts so I can't really see how having a dpc at this level will solve the damp problem.

Any thoughts/help/ideas would be massively appreciates.

 

[RedHerring2]

Sorry Ratty, but due to your rambling style of explanation I, personally, am having difficulty visualising several important points:
1. Do you have an external dpc and if so where is it in relation to external ground levels and internal dpc? (It should be above external ground level by about 150mm minimum and it can be at a different level to the internal dpc, I believe.)
2. Is the internal dpc level with the internal floor level? (I assume from your post that is the case.)
3. What type of floor is it internally?

A diagram showing these levels and positions would be excellent.

An assumption that I'm making or what is obvious is:
Both skins are brickwork and you have a cavity which is not bridged.
You have slate dpc.

 

[Ratty80]

Thanks for your response, I must have got a bit carried away. In answer to your questions

I believe that I do not have an external DPC as nothing is visible (unless they used a layer of the dense damp proof bricks that I have read about)

Yes, the internal DPC is level with the internal floor

Internally the floor is made up of 4x2 joists with floorboards on top.

And yes your assumptions are correct, both skins are brickwork, there is a slate DPC on inner wall and the cavity is not bridged.

I have attached a diagram which should hopefully clarify things.

 

[Ratty80]

Diagram showing DPC

• Ratty80
• 8 Aug 2011
• 2

 

[Static]

Best to put in a drainage channel by the walls and while your at it put in some external tanking.. Alternatively lower the external level to about 150mm below the external dpc.

 

[RedHerring2]

Edit: As Static said. He's maybe a faster typist than me.


My guess is, that due to the high level of external groud level and it's make-up, that water is, perhaps almost pouring, into your cavity, possibly filling it so high at times to bridge your internal skin and internal dpc.

Another couple of questions;
Do they joists go into the wall below dpc level or stop short of the wall?
Does the external ground level fall away from the house?

At the very least, you need to prevent water getting up against the external wall. Some older houses often have a waterproof render down to ground level to achieve this, but it's not, perhaps, the best method.

It would be better to have a waterproof barrier going down to below the level of the internal dpc. This could be an air gap.

Even better is to lower the external ground level to below that of the internal dpc.

If any of that is not possible, you'll need to tank the internal wall.

 

[tim00]

For my two cents: you are sometime going to have to knock off the damaged plasterwork and reveal your inner skin of brickwork, so why not remove a few bricks from the exposed area and observe what the conditions are in the cavity over a few days esp rainy days? That's if you do indeed have a cavity wall.
What ventilation do you have for the suspended floors?
Have you lifted a few boards to investigate the sub-area?

Previous posters have made important points for you to consider.

 

[Ratty80]

Thanks for your help

Firstly in answer to the questions

No the joists dont go into the wall, they stop short of the internal wall
No the external ground level doesnt fall away from the house, infact the opposite, it rises away from the house.

The sub floor is vented with about 4 air bricks, that I was looking to add to when I remove bricks to rake out the rest of the cavities.

Yes I have lifted the floor boards as I needed to resupport the joists as the plate that supported the joists around the old chimney hearth had completely rotted away. This, I believe was unrelated tho as it appeared to be caused by rubble piling up around the old hearth and hence bridging the DPC.
Underneath the property it seems to be a very moist clay. At first I thought that it was unduely wet possibly due to water running underneath the house, though have since been told that because the ground is very clay like it is supposed to retain a high moister content.

In relation to lowering the ground level I think this would be impossible as the wall on the side of the house is basically a 2' allyway separating my house with next doors I could attempt to lower the ground level at the back of the property but the ground level already gets higher away from the property.

Thats an interesting point about the waterproof render as my neighbores have got what what i assume is a water proof render at the bottom of their opposing wall. I assume u think this is not the best method as it wont reach the DPC level required?

I should mention that the side wall of the property has a chimney breast in the middle. The chimeny breast bridges the cavity, which I assume is normal?

I had not actually heard about tanking before but have just googled it. Would this be an alternative to using damp proof plaster on the internal wall?

Thats a good point about removing the brick to observe the cavity. I should also point out that the wall along the side of the house shouldnt actually get very wet as the tip of my roof is almost touching my neighbores, so will be very interesting to see just where the water is coming from.

Thanks very much for your help so far.

 

[Static]

You need to channel the water away from your house and not let it collect in the cavity and subfloor as it will only cause damage over time..
Waterproof rendering the internal plaster is a waste of time as the water will eventually find a crack and seep in..

Create a new drain that takes the water to a soakaway or land drainage point and tank the outside of the wall.. then repair the damage internally or youll just be redoing anything you do now in 5 years time..

 

[Ratty80]

Thanks static, what product would you recommend using to tank the outside wall?

I will look into redirecting the water though I was hoping not to have to start breaking up the tarmac if at all possible. I need to have a really good look at things next time it's chucking it down to see exactly what's happening. I would be suprised if that much is coming in from the side of the house as the allyway at the side of the house is fairly dry as my roof and the neighbores roofs almost touch.

 

[Ratty80]

Redherring2 I just to clarify what u meant by an 'air gap'...presumably to dig say a 3" wide by 8" deep trench around the house to take me below the internal dpc?

 

[RedHerring2]

That would help enormously. You might still get some water splashing up the wall and you'll still need to prevent that channel filling with water.

 

[Static]

Thanks static, what product would you recommend using to tank the outside wall?

Normally spec something like synthaprufe 3 coats..

 

[timbo29]

Hi,
I have a similar problem to what has been discussed previously.
my house is built on quite a steep slope however the rear garden is graded to be level. This is done by having a retaining wall before the external wall of the house.
However for some reason the owner before decided to infill this with bricks ans soil which I am currently digging out which will mean that the ground level is not 900mm higher than the DPC!

 

[Vim]

Ratty80,

did you ever managed to sort out the problem? i have a similar issue to yours... ground level appears to be on exactly the same level as the dpc. my internal ground level is also moist, actually it very very wet guey mud almost like wet clay type.

im thinking of channelling the external ground level below the dpc by atleast 200mm but i want to know if that will permanent fix the problem?