Basement Tanking

[ironman2009]

Apologies if this is a repeated thread but I have a specific problem, also apologies for the long explanation but I'd rather be thorough.

My house is built on the side of a hill, two storey's are visible from the front, three from the back. With this you will appreciate that the lower rooms at the front of the house are effectively below ground, by remarkable design the side walls are clear of ground as is the rear elevation. During the past few years there has been little if any evidence of penetrating damp. All walls are cavity walls, the house is built of brick and was constructed in the early 1930's

I have recently excavated the floors in the lower rooms to increase the ceiling height to 2700mm from 2100mm, doing so has exposed old brickwork by some 500mm below the original DPC, this brickwork is below the external land level and not only can I measure damp but is is visible to the eye, however it is not so bad as to run down the walls.

I am considering SBR tanking the full height of the front walls and perhaps 1500mm of the side walls prior to SBR render and then bonding insulated plasterboard to finish.

Does this make sense? Am I wasting my time (and money)? Should I be going about this a different way?

 

[dann09]

First, this is a structural question before being a plastering Q.
You would do well to post the digging out business on the Building forum.

By digging, you have revealed, and possibly weakened, the top 500mm of your foundation cum retaining wall. This is not good. Get a professional in to view as soon as possible - an experienced builder or SE will do fine.

Do not do any more digging. I just noticed that you have excavated other walls - have you had any professional advice before starting on your project?

 

[ironman2009]

The project has an architect, a struc. eng., and an experienced builder involved. Inspection holes were excavated before the work started and these confirmed that the foundations remain a minimum of 1200mm below the new lower floor levels, this was deemed acceptable by all parties. Please note that only one wall (the front) is backed up by land, the side and rear walls are clear all bar the lower 500mm which has been exposed when lowering the basement floors.

I am quite happy with the structural integrity of the alterations and realise that the best solution for the sides and rear elevation would be to lower the surrounding land and inject a new dpc, however this is not really practical for many reasons.

So my question is more about damp control as I am being given different advice from my architect and the builder. The architect is suggesting a membrane system running into a channel with sump pump (this seems excessive as we have never seen water (or damp) on the walls, although they do show as being damp on a meter). The builder is suggesting an SBR based cement slurry double coat prior to waterproof rendering. My basement is quite large at 2000sq/ft so I would like to get it right first time.

I should also point out that all parties agree that the property is considerably higher than the local water table.

 

[dann09]

1. has anyone suggested the possibility of a French drain on the outside of the building? On some parts of the perimeter at least?


2. Penetrating damp will be the major issue, an injected DPC will be almost irrelevant. All brickwork that was pre-buried will be sodden and degrading. All walls below the old floor level will be "solid".

3. In my experience, FWIW,( i'm not an expert in tanking etc.) the most successful results for similar situations have been tanking, drainage gulley and sump pump.Perhaps call a specialist company and have a heads up talk?

4. Why, with all your pro advice, are these issues coming up now?

5. Does your basement run thro to the front elevation wall?
If it does, you are most probably under the water table.

6. AAMOI: can you describe, or scan, a section view of your basement floor?

 

[joe-90]

You may be higher than the water table but there is a heck of a lot of water in the ground atm. I bet it dries out when the weather warms and we get a dry spell.