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PostPosted: Sat Mar 18, 2017 12:09 pm 
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What are the pro and against of installing a Sump Pump (+pit) and digging a French Drain along the inside perimeter of a terrace house's underfloor? The underfloor is just wet dirt...
My main concern (apart from the labour involved) is that once the soil/clay dries out (for the drainage effect) then the cohesion and texture of the soil will weaken...(am I right?).
On one side we'll have a healthier house: drier, mould free with a usable underfloor storage space, but this can be obtained with sealing the underfloor space.
Another consideration - there is no standing water and normally even after a rainy day the water is to be found a few inches from the surface...
I've also noticed that these 2 methods: encapsulating (or sealing) the underfloor and/or using sump pump + French drain inside domestic properties are commonplace in the USA but not in UK...any idea why?


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PostPosted: Mon Mar 20, 2017 10:07 am 
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I have a similar (?) situation.

Semi-D built in 1920s
Small cellar = a coal-hole and a couple of cold keeping slabs.
Normally water free - neither brick walls nor concrete floor are tanked.
Predominantly heavy clay at about 500 mm below surface soil in garden
A couple of days after a heavy rain storm, the cellar used to flood = 30 - 60 mm
Drains rodded and checked with Fluorescein by me = OK
Local Council suggested it was water table rise. My rear garden is on a slope towards the house (10 - 15' max)

I've:
cut a sump
installed a float operated a submersible pump which discharges to the outside rain-drain.


Obviously, that doesn't stop the water ingress but it mitigates its effects - with the help of some bricks under the chest freezer.


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PostPosted: Mon Mar 20, 2017 11:24 pm 
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Any concern about the changes this new drainage system is going to have on the structure of your property?

I'm thinking of just using a French Drain and a Sump (no pump) to keep water away from the walls - a sort of mitigating compromise just to make sure the clay stays compact and the walls are drier...like having a mini underfloor reservoir - but I need to do some research about it.

It's amazing how little information is to be found in Britain's forums regarding systems like encapsulation of the "crawl space" and use of French Drain + Sump + pump - are these new techniques from the USA?


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PostPosted: Tue Mar 21, 2017 7:05 am 
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swimmy wrote:
It's amazing how little information is to be found in Britain's forums regarding systems like encapsulation of the "crawl space" and use of French Drain + Sump + pump - are these new techniques from the USA?

Thanks for raising this issue.
One of my recent "project-ettes' has been to improve access to my "crawl space" and even tho' the ground (bare earth) wasn't noticeably damp to the touch I laid a layer of heavy duty polyethylene film for cleanliness if nothing else.
Having followed your lead, I've Googled for more details and the hits involving UK based suppliers of the materials +/ the service to install 'encapsulation' seemed absent. Can't move for USA based ones tho'.


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PostPosted: Tue Mar 21, 2017 9:30 am 
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i would think you will need a pump off some sort
the fact the water is there in the first place means that is the path off least resistance
so without a pump it will just lie there or build up or gradually reduce :dunno:

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PostPosted: Sat Mar 25, 2017 9:01 am 
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The underground water is to be found in my property only after heavy rain...this might be collected by the perimeter walls from the foundations and create rising damp - a French Trench (about half metre from the perimeter) should temporarily collects this water which should be dispersed back to the soil when the latter becomes drier/less saturated and so more absorbent (e.g. in sunny days). Think of this concept as an extra security "dampness valve" which is less drastic and more "organic" than using a sump pump (and with less unpredictable side effects).

In fact, there might be another solution which is less laborious (and a bit off topic):
1) lay a DPM or vapour barrier on the soil only - so not a proper encapsulation (USA style) as the crawl space walls are left free to "breath".
2) install a dehumidifier in the crawl space - remember there is another layer of vapour barrier just above the crawl space, so the dehumidifier is only taking the humidity away from the crawl space walls (where it really matters).
Job done!

Total encapsulation involves the covering of the perimeter walls in the crawl space - this means we are basically covering (not seeing) a possible issue e.g. rising damp - in that case one needs also to install a French Trench and Sump Pump (according to the USA method) - but still we can't see the problem.
By Semi-Encapsulating we are not only able to see if there is any problem with rising dump, but we can also act on it. My DeLonghi humidifier does a great job for the whole property so I assume it will do wonders in such a confined space (7-8 cubic metres?)...it only kicks in when the humidity goes above a certain level.

At the moment I'm still considering both options:
1) total encapsulation of the crawl space (USA method) plus French Trench with no Sump Pump (my way) simply because the problem isn't so drastic.
2) semi-encapsulation + dehumidifier

Option (2) suits me better cos' I have already a dehumidifier...option (1) is the one I'd like to try most - but having already installed the joists it makes it very difficult digging out a trench take out and dispose of the soil, install the soil fabric and gravel etc. Not to mention, this is basically a guesswork an intuition which hasn't been tested yet (not as much as I know anyhow).


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