# Forum Home Renovation Landscaping, Gardening & Outdoors  Water getting in under house slab

## snoobs

Hi I was wondering if anyone has any ideas about how to stop water getting under my slab causing my house to move and creating cracks. I was told to create a 900mm barrier of concrete around the edge of my house but I really don't like the thought of all that concrete. I had thought of perhaps putting in crushed sandstone compacting it with a whacker but i'm not sure how porous crushed sandstone is.  
Any ideas would be welcome. Oh and I have Termguard protection around my house.

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## rod1949

Sounds like its a new house  Yes / No? 
Has it been built on a swamp or underground water course and if so how would this have happened without appropriate engineering works being done? 
A concrete barrier I think would only be a short term solution. 
I would say you would need to be considering the installation of decent/substantial sub soil drainage.

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## GeoffW1

> Sounds like its a new house Yes / No? 
> Has it been built on a swamp or underground water course and if so how would this have happened without appropriate engineering works being done? 
> A concrete barrier I think would only be a short term solution. 
> I would say you would need to be considering the installation of decent/substantial sub soil drainage.

  Hi, 
Agree with Rod, above. I feel you need a substantial sub-surface drain around the perimeter. If you can identify the source of the water, or your house is on a slope, you can start on that side (probably the high side) and grade down and away from that. 
Cheers

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## cherub65

Don't concrete, that would cause more problems as you would be creating a dam behind your house. Footings weren't designed  for those sort of pressures.
Do as others said sub soil drainage designed for the lay of land, (not just at footings)

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## snoobs

Hi 
Thanks for the responses. The house is 3 years old and wasn't built on a swamp or wet lands. Does anyone have a ball park figure for putting in sub soil drainage? The house is 29sq. Also where can I find the specifications for th different slab types, ie. what is the slab depth of a 'H' slab etc? 
Cheers
Snoobs

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## theozchick

We are in the process of installing a subsurface drain for the same sort of problem.  Costs will depend on how much DIY you are prepared to do.  We did the digging ourselves but paid $130 for a bobcat guy to come and pick up the clay.  This was our only option as our local tip charges exorbitant rates (unless they need it).  Dumping the clay cost us $120 for roughly 6T (it would have been $500 at the tip).  Bunnings is currently selling 20m of 100mm socked ag pipe for $60 which is a great deal.  You'll also need 20mm blue metal or other aggregate to create the right drainage and protection for the ag pipe from silt build up. Our 13.5m L x 300mm W x 600mm D trench will take about 3 7x4 trailer loads from our local supplier.  Each trailer load costs us about $30.  Then there's geotextile fabric to cover the aggregate followed by backfill (not the clay you just dug out (if you have clay) and the turf or sods or whatever to reincorporate the site into your yard.  We're not really thinking about the cost.  You have to consider the value of your home and all the money you've got invested in that thing.  I mean, you are talking about the foundations and structural integrity after all.  It's money well spent.   :2thumbsup:

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## Val Driver

> We are in the process of installing a subsurface drain for the same sort of problem.  Costs will depend on how much DIY you are prepared to do.  We did the digging ourselves but paid $130 for a bobcat guy to come and pick up the clay.  This was our only option as our local tip charges exorbitant rates (unless they need it).  Dumping the clay cost us $120 for roughly 6T (it would have been $500 at the tip).  Bunnings is currently selling 20m of 100mm socked ag pipe for $60 which is a great deal.  You'll also need 20mm blue metal or other aggregate to create the right drainage and protection for the ag pipe from silt build up. Our 13.5m L x 300mm W x 600mm D trench will take about 3 7x4 trailer loads from our local supplier.  Each trailer load costs us about $30.  Then there's geotextile fabric to cover the aggregate followed by backfill (not the clay you just dug out (if you have clay) and the turf or sods or whatever to reincorporate the site into your yard.  We're not really thinking about the cost.  You have to consider the value of your home and all the money you've got invested in that thing.  I mean, you are talking about the foundations and structural integrity after all.  It's money well spent.

  Just found this thread. 
I was wondering if your going to lay ag pipe to prevent excess water getting to a slab would you leave space between it and the exterior of a building wall/slab footing etc. Otherwise wouldn't the water going in just as easily escape if it were covered with blue metal? 
I struggled to find a search term that would lead me to the propper installation of AG pipe. How deep below slab height would it need to be installed. Is this a plumbing issue. 
I'm asking as I have a sunken lounge that was  ground level. I have since excavated but still want to keep my living room dry. 
Cheers,
Chris.

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## Master Splinter

How deep does the ag pipe need to be depends on where the water is coming from and how much earth is behind it.   
If it is just mild surface runoff you are trying to control, then a bit below the surface (covered in drainage gravel) is fine.  
If it's behind a retaining wall then it's got to be at the bottom of the wall. 
The ag pipe then has to flow down to a stormwater connection so that the water that makes its way to the ag pipe can actually drain away to somewhere else. (its ok to have the ag pipe level along the wall, as long as the discharge end is lower) 
(nice pdf of retaining walls here - http://www.concrete.net.au/publicati...ning_Walls.pdf  )

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## cherub65

Best to use vertical drainage in situations where water can ingress to inside, Info here  :: AUSDRAIN Stormwater Solutions - Drainage Cell, Rainwater Harvesting & Stormwater Tanks ::

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