You can level an existing concrete floor with a leveling layer of new concrete but you must prepare the old concrete floor first.
How to level concrete slab against a wall.
Over time concrete floors can settle unevenly or become uneven due to cracking and moisture.
Uneven exterior concrete slabs should be repaired for reasons of both convenience and safety.
Have a helper add or remove concrete in front of the screed as you pull it.
Pour concrete onto the existing slab until it is around where you want the final level to be.
This prevents water from leaking under the bottom of the wall to the inside of the building.
Neglecting to prepare the old surface will prevent the new concrete from adhering correctly resulting in a weak bond between the two layers.
Pour and level the concrete.
Add all of the concrete right away so none of the mixture has a chance to dry out.
An exterior wall must be placed on the outside edge of the slab so the exterior siding laps down and over the edge of the slab.
You can purchase concrete leveling compounds at home improvement retailers to level out the floor.
Make multiple passes if needed to create a flat evenly filled area.
Consider a wood shed floor treated lumber joists and plywood.
Drag a straight 2x4 screed board across the top of the concrete forms to level the concrete.
An even concrete slab is very important if you plan to build anything on your slab or lay down any type of outdoor tile.