You could use the very same texture on both those sheets Remy just suggested, but just have an Adjust Hue/Saturation sheet effect on the more extensive one at the bottom, to make the texture paler.

You just need a constant stippling texture, preferably seamless, and to draw a polygon of it under the floor sheet on a new sheet of it's own. This polygon needs to extend an even width beyond the floor all the way around, and to have an Edge Fade Inner sheet effect on it that is about the same extent in width, so that it fades away from the building.

This might not give you as soft an effect as you are after, but you could always add a very subtle blur to that sheet to get rid of any hard edges and corner marks.

That's another way of doing it.

One thing to bear in mind is that 8-bit greyscale is insufficient and leaves steps in the gradients, a bit like that app which uses tiny pixel blocks to create 3D worlds. I'm not sure, though whether you need to set it to 16-bit, or even 32-bit.

Since I am not an expert by any means, and know even less about brushes other than the prescale offset brushes, I think that unless you have several backups saved that you might be able to work forward from again, you might need help from someone like jslayton (Joseph Slayton), or one of the other more seasoned FT3 users.

If this is a world you have re-imported back from Wilbur, having done something like CA155 to it, the only recommendation I can make that might help is to go back to the pre-Wilbur FT3 world and raise the lake using the prescale offset brushes at that stage and take it through the process again without the lake from the beginning.

At any rate, I recommend avoiding the tools that increase or decrease roughness. They are pretty difficult to control and can cause more problems than they solve unless you have an extremely light touch.

Are you using tools other than Prescale offset raise and lower tools?