Tanking - Do I need it ?

Leanne1978_2

I am in the process of buying a house and have had a call from the surveyor to say there is a problem with damp. We knew there was a problem however the surveyor has said that as the rear extension was built below the DPC the room will need tanking rather than normal damp proofing. Does anyone know how expensive this is as I am starting to think it may be cheaper to pull the extension down (it is a pretty rubbish build to be honest) and rebuild it (also have a Dad who has built his own extensions !!)

Hyphae

If it's a rubbish build and has issues with damp already (and that you are already considering pulling it down), just get ideas for a new extension. Damp issues can be solved with tanking, but an extension bathroom I had with damp problems (no membrane beneath floor either!) caused no end of trouble- and after a few thousand £££ spent, still wasn't completely sorted. However, the shell of this extension was well buit so I didn't consider knocking it down!

ormus

all depends. basic tanking (with slurry) can cost about 1k/2k.
a membrane type with a pump, will be more like 5k and upwards.

i saw a job last year that cost the owner 17k!
(2 rooms).

Leanne1978_2

Thanks for responses. I am having someone have a look and provide quote and will see what they say. I might be being stupid here but can we not dig out the surrounding ground (outside)to the correct level ? The whole house is surrounded by very old slabbing that we were going to lift up anyway and level.

Mayflower10cat

It wouldn't hurt in the meantime! We bought our current Edwardian cottage complete with solid walls and damp... Did what the surveyor recommended; dug away the flowerbed at the front, down to the original slate DPC , replaced with gravel at a lower level, altered the levels at the front and a year on, we are dried out.

BluHed

It could be a fairly easy problem to solve or it could require some more invasive work to rectify. As you already probably know, tanking is only required when the internal ground level is lower than the external ground level so therefore your internal walls are being dampened by lateral moisture penetrating in from the external ground soil.
The previous poster was correct, you would need in the first instance to determine if it is possible to reduce the external ground level to at least 150mm BELOW the internal ground level which would let your existing damp proof course (DPC) do its work and stop any moisture rising through the brickwork via capillary action.
Even if the external ground level has been reduced to the level I stated, it is still possible that the internal plaster has become contaminated by hygroscopic salts from the ground water which will continue to make the plaster damp on days of high relative humidity and allow a certain amount of bridging over your existing DPC.
If this is the case, the plaster will have to be removed to 1.2m above the internal ground level and replaced with a waterproof render with an additive such as Sika 1. This is a tanking grade additive and is normally up to th job of providing a basic tanking although any chemical barrier has certain limitations with regards to water pressure and might not work in some circumstances which will require a physical barrier such as a polypropylene membrane or equivelent.
Going by what you are saying, I would recommend trying to reduce the external ground level first as this is the cheapest option and one that causes the least amount of work before looking at tanking.
Tanking with Sika 1 normally comes in at around between £60-100 per M2 which includes the price of hacking off old plaster.
If you have any further questions, please feel free to message me and I will advise you at no cost. I own a damp proofing company in London and see this problem on a daily basis.