If your goal is to make the surface have a thickness that results in a uniform (flat) bottom then there is a fairly simple way you could accomplish this. I did it with a really simple plane for an example.
- Select All (Edit Mode > A ) so that everything in your mesh is highlighted
- Extrude (E). Cancel the extrude but do not cancel select.
- Translate (G) along Z axis only (if your mesh is up with respect to Z)
- With the new bottom selected, Scale on Z (S, then Z) to 0
- I would also reccomend adding a loop cut (CTRL + R) around the sides if you plan to edit the top and bottom independently or modify the topology etc. You can make that loopcut parrallel to the bottom by scaling it to 0 on the Z axis like you did with the bottom
As @Haunt_House Suggests in the comment below, you can also align a loopcut with the following method
You can align the loopcut to the bottom by hitting E for even and F for flipped after deciding on the edges to cut. CTRL-R, LMB, E, F, RETURN
You can do almost the same thing with the solidify modifier, but if you do it this way, you can't garuntee that the edges are perfectly flat.
- Start with a plane like you have, choose the solidify modifier (Generate > Solidify)
- Choose a thickness that you like, exit edit mode and apply the modifier.
- You can then flatten the bottom by selecting it all and scaling it to 0 along the axis representing the height (Z in my case) similiar to the above method.
As you can see, the sides are not perfectly flat but if you dont need or want flat sides, this will work fine too.