Rising Damp Question

[silver50]

I have a georgian basement flat with extensive damp proofing carried out in 1999. I believe it is failing. I have a specific question before a second visit by the contractors who did the work this week.

My question is regarding the tanking, which seems sound up until the level it stops, however on other walls within the property which were injected only, I have high moisture readings at low level at skirtings etc suggesting the injected areas have not been sucessfully treated.

The flat is "sunk" approx 800mm below external ground level. The outer walls were tanked inside, looking to me like not high enough and in one particular area definately not to specification (min 300mm above external ground level) hopefully giving me some good solid grounds (no pun intended!)

I have "salt band's" appearing approx 1m from internal floor level(where installed works stop) and highest moisture readings around 2m from internal floor (or 120mm above exterior floor) level. I appreciate 1.2m is quite high above ground level for damp to rise, leading me to my question..

Could tanking being in place, but not being installed high enough, actually force rising damp much higher than it may have gone without tanking being in place. The exterior walls were injected too.

The contractor came back a week or so ago but are willing to come again. They blamed condensation, and while I appreciate I have a small issue with this, I absolutely am not of the belief it is the sole cause of my worries. I don't believe this unsightly "salt band" would be caused by a small condensation issue and since their last visit I am now aware of the low level high moisture readings.

Many Thanks

 

[^woody^]

Any wall below ground has the potential to absorb moisture from the ground, and this moisture will travel horizontally and vertically. The greater the porosity of the wall and the greater the ground moisture, or the greater pressure of the moisture will determine how far it goes through the wall and up the wall

A tanked basement should cover all the walls which are underground and should meet up with a horizontal DPC above ground.

If say, the tanking and injected DPC are OK - ie installed correctly, intact, complete and appropriate for the ground pressure, then if the wall above the DPC is damp, then it can only be coming in above the DPC or it is condensation

Now, if the moisture is coming through the wall then you will get staining and the salt layer you mention.

Condensation would normally not produce the salts (it may in extreme cases but in a different pattern), and you would tend to get at least a little black mould spotting

Damp coming in could be causing condensation issues though

Its hard to say without seeing the walls in context, but if there is doubt, then if the cause can't be determined by an experienced independent survey, then tests could be done of the wall to check the moisture content and for ground salts - moisture coming up from the ground brings tell-tale salts with it

 

[silver50]

Thanks for your reply.

My next stage may be getting an independent evaluation if I remain unsatisfied with the response of the original contractors. There was signs of condensation, black spotting on window frames and a tiny bit near a ceiling and I agree this in itself may partly be being produced by moisture from the walls rather than the cause of the problems. I certainly feel I have now discovered a lot of areas very low down on walls I didn't see before (inside walls injected only, not tanked) and this certainly leads me to believe there seems to be a breakdown in the DPC.

Regarding my outside ground level question. I've a worrying medium to high moisture reading on and just above the tanking stopping/salt band area(1m from interior floor) but rather than it being very high here and tailing off as you go further up, it actually seems to steadily become higher from this level, highest at approx 1200 above the exterior ground height and over 2m from interior floor. I am just wondering if there was any possible explaination for this?

Thanks

 

[^woody^]

I would not go poking a moisture meter into the walls if you are not conversant with them and how to interpret readings. Its not just a case of reading the needle or lights!

These meters are not designed for anything other than timber. They will give false (high) readings in plaster or masonry and any salts will cause false readings too. Plus, you will only read the surface moisture - so you may well be reading condensation. Even paint on timber can affect readings

But DIY meters are very inaccurate in any case, and next to useless

In certain situations, I would expect to see elevated moisture content of the lower parts of many walls - up to 1200mm, and this would be due to naturally forming minor condensation on the cooler parts of the room. Further investigation of all other parts of the walls would then indicate if its normal, or not, or if there is potentially a rising damp issue

You really need this looked at by someone experienced, as there could be several possibilities for any dampness, and a proper diagnosis is not easy unless you can interpret everything in context

 

[silver50]

I had someone round on Thursday to give me a bit of insight (but not an official independent view). He was a surveyor from another recognised firm who has done work for us in the past somewhere else. Although he gave me some pointers, and to be honest his moisture meter probings were as much what i'm talking about as my own in finding high moisture levels, but I do appreciate what you're saying about taking and interpreting readings.

However, unfortunately I have mostly come away with the feeling there are problems, but not any great conclusion as to causes! This feels a bit unfortunate in that i'd like to have a few more facts in place prior to letting the firm who did the work back. The surveyor on Thursday seemed to think I should let this senior surveyor back before going down the avenue of getting a fully independent report.

It seems fair to say under my set of circumstances more thorough investigation may be wah tis required rather than quick visit's and probings with moisture meters!