repair to failed tanking

[singintherain]

I have a split level property showing damp due to negative hydrostatic pressure on the lower level where the back wall meets the outside wall.
I need some advice on the height of tanking I will need to sort the problem.
With all the rain we've had, I don't expect what damp is showing to get any worse, so if I hack off the plaster 6inches above the highest point and tank it, will the water beable to redirect itself over the top of the repair?
I plan to tie the wall tanking in to the floor by chasing out an inch square gully and filling it with hydraulic cement on top of the bitumen latex emulsion which will go on the wall before replastering.
Any advice gratefully received.

[RemedialExpert]

do you have a pic of the external element allied to this area..

[singintherain]

thank you for your time in replying.

i don't have an external pic, but the smaller portion of the damp which shows at the join of the 2 walls is on the outside wall, but only along that portion of it that is a retaining wall. as the outside is rendered to ground level, i guess the dpc might have been bridged, but the main portion is showing on the back wall which is fully earth retaining.

[singintherain]

external pic of outside wall for you with thanks

the damp shows internally at the start of the retaining wall, beyond the window.

[Tryanything]

Hi Singintherain

See if we can clarify things a bit

Is the layout as the drawing below

What is the difference in height between level A & B

What is the suface of level B ( Lawn,Earth,Slabs,Concrete )

What is the difference between the room floor height and level A

[singintherain]

Hi, Tryanything,

Thanks for that. Your diagram is accurate, apart from the outside retaining wall is positioned just before the window opening.

A to B measures 1.95m

The surface of level B is mainly concrete slab (80% at a guestimate)

Floor height in lower level room is 110mm above outside ground level.

The main portion of damp appears on the internal retaining wall.

[Tryanything]

Sorry more questions

How old is the property
How long have you had the problem
Do you know the construction of the walls (concrete,brick,cavity)
What drainage is there on levels A & B
Do the surface areas of levels A & B drain away from the house
What is the higher level room used for and is the floor solid

[singintherain]

The property is 70's and there is small amount of blown plaster at base of internal wall away from current damp suggesting previous problem?

Current problem is 6 months old and seems to have now got as bad as it will.

Walls are cavity.

Level A drains away. There is a pipe at bottom of external retaining wall, but always appears dry, so possibly blocked?

Level B can pool against wall of house in really heavy rain, but I always brush it off on to lawn at side.

Higher level room is bedroom on solid concrete base.

Thanks for your continued help.

[Tryanything]

Hi my thoughts are
A
Surface water from level B is getting through the outside skin of the cavity wall
B
Ground water or posibly a water leak is getting through the outside skin
of the inner retaing wall

In a lot of cavity walls there is a fair amount of cement slops and rubbish
where care has not been taken to keep it clean

As water maybe building up in the cavity it could breach the damp course
on the inner skin giving you the problem you have

Most probable is A in my opinion

I dont think the work you propose to do will cure the problem

Not sure if you mean level B is one slab of concrete or concrete paving slabs ?

My course of action would be

If a concrete slab clean out the joint between the concrete, house wall
and retaining wall and apply bitumen mastic and drill a 50mm drainage
hole at the lowest point that water collects through the retaining wall to
allow water to escape to the lower level

If its concrete slabs they are probably just sitting on a mix of sand and
cement dabs leaving voids below the slabs, best would be to relay the
slabs with falls away from the house on a solid bed with no voids and drill
a drainage hole

Hope this gives you some idears and is of some help

[singintherain]

thank you for your advice which i intend to follow, although i may have to cut a surface water drainage channel as the lowest point on the concrete slab at level B is against the wall, under the window, rather than at the retaining wall itself.
will a 230mm grinder with the appropriate cutting disc be up to the job?

[Tryanything]

What is the difference in concrete levels at the retaining wall and where it puddles

What is the difference of the bedroom floor height and level B

Any chance of taking a picture of level B, also of the pipe coming out of
the retaining wall that is always dry

How big an area is level B (square metres)

[singintherain]

difference in concrete slab levels between small retaining wall and where it pools is 25mm.

this pooling has been since the small wall with railings was put in to stop a wheelchair running off into the oil tank about 2 years ago.

there is no pipe in larger retaining wall at level A - must have been my imagination, sorry!

[singintherain]

my pics seem to have disappeared, so i'll try posting them again

[Tryanything]

Hi Singintherain

Looking at your other pictures of level B l would either relay that section
of concrete with falls away from the house or as you say cut a channel

From the lowest point to the joint in the blockwork form a channel with
falls about 50mm wide by making a series of cuts in the concrete and
then chip it out with a hammer and bolster

You could just cut out a corner of the blockwork aswell to allow the water
to run away

Would also drill a drainage hole at level A aswell in case there is a build
up of water behind the retaining wall

Another thing what is that outside tap all about where does the
supply come from, is it coming out of the concrete ?

And what is that to the top left in the picture of level B

[singintherain]

Thanks Tryanything, I'll get on it as soon as I can scrounge the use of a core drill and angle grinder.

The water pipe does indeed just appear out of the slab with no obvious (to me anyway) source!

That thing in top left is the german shepherd who just can't bear to be left out when there's a camera about!

best wishes and many thanks