DPM under carport concrete?

[Corazones]

I am having a 100mm concrete floor laid under my carport, which is at present only dry earth.
The local builder who is helping with this claims that, in view of the fact that part of the area is very occasionally affected by a flash flood, he should not fit a dpm as this would produce a risk of the concrete raft "floating" and cracking.
I have never heard of this before. In fact the 100mm rise in height (with a slight ramp in front) may well overcome this problem anyway. And it will still be well below the dpm of the house, incidentally.
I will be keeping my classic car under there and need as dry a floor as possible. What do other members think? Would you insist on the dpm in this case?

 

[SFK]

Corazones,
Some interesting reading on PistonHEads about DPM and Garages:
http://www.pistonheads.com/gassing/topic.asp?t=1281360
sfk

 

[^woody^]

You don't need a DPM in garages, and certainly not in open carports. That builder is correct, but probably not for that reason

 

[JohnD]

You don't need a DPM in a carport, but if you ever want to enclose it (for example I have a small shed at the end of mine) it will keep drier. Particularly if you ever put anything on the floor. Condensation and rust inside sheds and garages can be a big problem, and a lot of water can evaporate off a slab in contact with the ground.

The extra cost, at time of build, is insignificant.

 

[^woody^]

The extra cost, at time of build, is insignificant.

It's not the cost, but the prospect of a permanently damp slab with a DPM beneath it which would be significant.

And condensation in enclosed sheds/carports/garages is just the same with or without a DPM. Its condensation after all.

 

[JohnD]

...except for the water soaking into the slab from below and evaporating into the air of the shed. This naturally increases the humidity in the shed.

 

[^woody^]

This naturally increases the humidity in the shed.

And how does that occur in a cold unheated shed?

 

[JohnD]

Unless the air in the shed is already at 100% RH, water vapour will evaporate off a damp concrete slab and will make the shed more humid. The concrete slab is unlikely to be 0C or below. Increasing the humidy will increase condensation overnight when the temperature drops.

Additionally, I put some boxes on a (seemingly dry) garage floor. The garage does not suffer from condensation. But when I moved them, the bottoms of the boxes were wet. My neighbour has the same effect in his cellar, which has a modern slab floor.