Ground supported slab level

[parrot77]

This is not a stricly Engineering problem but someone may offer some insight.
For under floor heating on a concrete ground supported slab I require 110mm of insulation to satisfy the U-value requirement.(Sourse- insulation manufacturer backed up by my own calcs)

Hense my ground floor build up is;
65mm screed with heating pipes on
110mm Insulation on
150mm Structural slab on
DPM/Radon Barrier.

Every detail I come across in books and literature shows the base of the structural slab level with the ground outside. This means however the floor level would be 325mm above the ground outside.
Traditionally a 150 step between inside and outside is formed by the structural slab (without any additional build up) In this case however a journey outside will require a parachute!!
My question; is there any consequenses of lowering the structural slab til the top is level or slightly below the ground outside? This will give a more acceptable step/ difference in level.
I propose to bring the dpm up the inside face of the wall and lap it with the dpc 150 above ground, to form a barrier.

Any thoughts?

 

[SlideRuleEra]

In coastal South Carolina (frost depth = 0 inches), structural slabs are routinely constructed with with almost all of the concrete below finish grade. However, this is based on having surrounding areas with positive drainage (typically a minimum of 1/4" fall per foot) away from the structure. The finish concrete finish elevation may be 1 or 2 inches ABOVE the adjacent grade.

Putting the top of concrete AT or BELOW surrounding grade is an invitation for "trouble" (flooding during heavy rain), IMHO.

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[parrot77]

Thanks SlideRuleEra,
I am in Ireland and flooding and frost are not uncommon.

Just for future reference, what would be the causes of trouble?
Is it your opinion that water may bridge the wall (which is masonry cavity wall) then find away through the damp proof mat and then penetrate the insulation and screed?

 

[SlideRuleEra]

parrot77 - Water is almost always the biggest problem. In many places we have sandy soils that drain well. If water can be kept out the structure DURING rainfall, it will be "gone" shortly after the rain stops.

If your soils are different, my comments may not apply to your location.

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