Soil under garage floor strange question

[Bob49]

Hello,

Sorry if this is a strange question but can the soil under a garage floor increase in moisture content when it rains?

If we assume proper drainage and no gutter/downpipe/drains/water main/roof problems then can the soil for example under a concrete garage floor become "wetter" when it rains?

For example if you had a concrete/tarmacked drive that sloped towards the house and at the end of the drive proper drainage then would that keep the soil under the garage floor dry?

But if you had a gravel drive or block paved drive that let some water through to the ground then would that water seep under the garage floor? Does water "flow" under a garage/house if the house is on a hill?

The reason I ask is I have a concrete garage floor laid on a dpc sheet. But at the internal side of the garage (the cavity wall of the house) the bricks under the house dpc (150mm up from garage floor) have damp areas when it rains. Could this be considered normal/expected?

I've taken a brick out where the damp seems the worst and the inner leaf of the cavity shows no damp areas. So I've concluded that the soil under the garage must be damp.


Any thoughts? Thanks

Bob

 

[Leofric]

Well, the dpm should be on sand blinding on hardcore and not directly on to soil, but it is normal for damp to rise up brick walls until it reaches the horizontal dpc if that is what you mean.

 

[Notch7]

I suppose in a garage the dpm will be cut off at the concrete slab, which is likely to be below the damp proof course in the wall.

Inside a house the damp proof membrane usually gets folded back over the damp proof membrane -which usually ends up a bit below finished floor level so you dont see it.

 

[Leofric]

Not sure about the concern here. You can't stop ground getting wet and buildings aren't surrounded by a waterproof tank Look up water table .

I suppose in a garage the dpm will be cut off at the concrete slab, which is likely to be below the damp proof course in the wall.

This seems to be your situation, you are just seeing the damp in the bwk between the garage floor and the horizontal dpc in the house wall. There might even be some efflorescence (white salts drying out on the face of the bwk )

 

[Bob49]

Not sure about the concern here. You can't stop ground getting wet and buildings aren't surrounded by a waterproof tank Look up water table .
This seems to be your situation, you are just seeing the damp in the bwk between the garage floor and the horizontal dpc in the house wall. There might even be some efflorescence (white salts drying out on the face of the bwk )

Thanks, makes sense.