Building advice - tanking.

[Tail End Charlie]

I'm having a garage built, two walls of which will be against an earth bank, which gets very wet. The walls have been built, properly strengthened and I'm happy with that. However the tanking is being put on the outside of the walls before back filling and I've noticed the membrane is a brick course above the damp proof course. Fortunately the back filling wasn't completely done before I got home. Logically to me this is wrong as the water/ dampness will run down the wall and come inside the garage when it gets to the bottom level, which is above the dpc. The walls have damp patches at the bottom now and the builder says that's the slab drying out (the roof isn't on yet). I'm not convinced and have asked him not to do any more back filling until the roof is on so I can see if the damp patches remain.
Am I correct in my logic?

 

[fossyant]

Most likely you are right. It's his job to build it right.

 

[Hicky]

Tanking never works properly or lasts very long. I’ve had a wall tanked twice, the outer had a lean to onto the wall and often the guttering failed(hence tanking) as the neighbour wouldn’t do anything(environmental heath forced them to repair guttering). Ventilation works better, the house was sold and the new owner knocked it down and I sorted the external wall properly.

 

[Tail End Charlie]

@Hicky what makes the tanking fail?

 

[Slick]

There shouldn't be too many issues with tanking if its done correctly, although ventilation is definitely essential in any building.
Do you have an architect or engineer involved, as they would specify exactly how it was to be installed, and does it not already specify on the drawings how this should be done?

 

[Brains]

Can you back fill with ballast stones or gravel rather than earth ?

 

[Hicky]

Water will eventually breach it depending on the force ie how wet the wall gets, the water has to go somewhere. Think of the ability of coats to be waterproof, Hydroscopic level. A backfill of ballast/stone such as a French drain will help.

 

[Dave7]

As @Hicky says water has to go somewhere. Is it feasible to put a gravel type drain at the bas of the backfill so as to take water away?
It seems logical that the membrane should go right to the base of the wall though.

 

[twentysix by twentyfive]

It seems logical that the membrane should go right to the base of the wall though.

Yep - that's logical

 

[Deleted member 26715]

However the tanking is being put on the outside of the walls before back filling and I've noticed the membrane is a brick course above the damp proof course.

It's cost cutting innit, he's saved himself a new roll of membrane & by the time the bank gets really wet you'll have paid him all his money & there will be little chance of getting him back. But also agree with providing an escape for the water, it does have to go somewhere.

 

[Ian H]

My garage wall failed at the base, where the wall sits on the plinth. I dug down to seal that, used bitumen paint to add protection, and put a flexible land-drain in vertical layers against the wall, with an outlet to the side. There have been no problems over the 5 or so years since.

 

[Tail End Charlie]

Thanks to everyone for your thoughts. A land drain is being put in and the backfill will initially be gravel and similar. But for me, there's no logical reason why the membrane doesn't go below the dpc, the land drain is to ensure the water doesn't sit against it.
I don't want my bikes in a damp garage!

 

[Hicky]

There's damp everywhere...mainly the kitchen. It's our lifestyle, drying clothes in the house, cooking without lids(inefficient) etc.
In our old home we'd get mould on the walls until I took off the covers for the air bricks, filled with newspaper. Idiots!
Cue drier feeling home and warmer too. Some draft excluding elements are a good idea but ventilation is key in homes too.

 

[Deleted member 26715]

cooking without lids(inefficient) etc.

The number of arguments in our house over this!!!!!!!!!!!!!

 

[Beebo]

A photo of the wall detailing would help