Second opinion please

[Jilly]

Hi all, 

 

I need an opinion on this to decide if it an issue or not. 

 

The stable we are converting has an existing 150mm floor slab. During the rain last week there was a collection of water at one point where the floor is lower and so the builder decided to drill through it to try to let the water drain away. I'm not sure how well it worked but the area is now swept and clear  and the weather dry. 

 

The internal walls have gone up but rather than build a couple of courses of brick onto the concrete (he muttered about floor levels) they have used wood and placed some damp proof under it (ie they have screwed into the concrete). 

 

The intention is now to put damp proof under the screed and UFH. 

 

What's stressing me is that in winter the water table is really high here and I can see that water pressure could force water up through these holes. It will then track under the damp proof membrane and potentially contaminate the screws/wood and cause future problems. 

 

The builder says I am over thinking this and it's a perfectly acceptable way of doing the walls. 

 

Periodically, it's keeping me awake as we had issues with a pit in the next door garage filling with water. 

 

Am I correct or being stupid through stress? I would really have to throw my toys out of the pram to get this changed. 

 

Thank you! 

 

 

 

 

 

Edited September 13, 2020 by Jilly

[tonyshouse]

Is the top of the slab above outside ground level? 

[Jilly]

No it's currently about the same but will have the new screed/UFH on top

[tonyshouse]

How will you stop the straw getting wet if it sirs at ground level, I would like to see 75mm min ideally 150mm drop from bottom of straw to ground. 

[Jilly]

Straw?

 

[tonyshouse]

Sorry, what are the external walls made of? 

[Jilly]

140mm block

[PeterW]

Are you worried the water will come up through the holes he has dug..? 

[Jilly]

Yes, I'm worried that the water pressure from a high water table will push out little plugs of cement he will probably put in...

[ToughButterCup]

18 minutes ago, Jilly said:

Yes, I'm worried that the water pressure from a high water table will push out little plugs of cement he will probably put in...

 

If there is a slight fall across the site, build a simple French drain to a spot lower than the foundation .... ?  Cheap, quick, permanent.

[joe90]

2 minutes ago, ToughButterCup said:

build a simple French drain

i second that, we have a high water table, almost ground level in winter. We created a French drain around the outside (by back filling the foundation trench outside the blocks with 50mm stone) and a pipe to a ditch, it’s worked very well and no sign of ground water near the house. Did the same with our detached garage.

[PeterW]

58 minutes ago, Jilly said:

Yes, I'm worried that the water pressure from a high water table will push out little plugs of cement he will probably put in...

Hydrostatic pressure would have to be pretty powerful as it would need to break the cement bond. As both @joe90 and @ToughButterCup said - French drain 300mm all round the outside with a perforated pipe in it will keep the water table low. 

[tonyshouse]

The little holes won’t or shouldn’t go right through the slab, 55mm deep should do and then water wouldn’t come up them 

[Nickfromwales]

Pour hot bitumen into the holes. End of problem. 

[Gus Potter]

Petew W makes a good point, Frenchdrain is a good solid solution.

 

To be safe just make sure you don't make it too deep. Say max depth to top of foundation. If you have clay soil the French drain could dry the clay out too much under the found, it will shrink and could cause movement. If you have a gravel / sandy type soil it could start to wash out the fine material thus reduce the capacity of the soil under the found to carry the weight of the wall.

 

If you have a sloping site then you may only need a drain on the high side, you intercept the surface ground water there.

 

Often you'll have new rain water drains with pipe bedding and this too acts as a French drain, sometimes when you don't want it to though. With a bit of thought you can get the right solution with a little extra cost, if any.

 

 

 

 

 

 

[Faz]

You need to have a proper look at this if your water table is high.  We are building on the Fen outside Huntingdon and have set our FFL at 600mm AGL for planning - it looks like we are going to drop them in at 0.00 AOD - yep that is Sea level which is 800mm above current chopped up ground level.  Spent the whole morning walking around site with the laser level sorting that out!  Still hearing the beeping now (set out the concrete levels in my foundation too - beep beep beep!)