I have has similar like you but cant give you any solution as still happen to me.
9 years ago when I moved into my house and noticed this, hence have file a complained to the developer since it still under warrenty.
What they does is
1) Removed all existing marble (Yes, Marble)
2) Removed the slab included sand till the bottom of the base (structure)
3) Relay a layer of waterproof
4) Put in new sand as suspect due to the sea or river sand which causing those oily sign (wood / dust...)
5) Put paper type of waterproof (paper like when we put chicken rice

)
6) Put in new tiles, yes tiles, as I want to replace with marble...
But after 5 - 6 years, this problem back again till now and it will keep growing...
* More than 6- 7 houses that I know is impacting within this 2 row
This post has been edited by Jo_da48: Apr 20 2011, 09:22 PM Attached thumbnail(s)