How to Pour a Basement Floor on Old Work

Remove any loose concrete debris and sweep the floor surface clean. If there are visibly weak spots in the existing basement floor, scrape them with a chisel and remove as much damaged concrete as possible.

Decide how deep you want your new layer of concrete to be. If the previous floor is in fairly good condition, you may want to pour only a fraction of an inch; but if the floor has subsided significantly, a thicker resurfacing may be necessary. Use a level to ensure the perimeter of your ceiling is level, then measure your new surface's distance from the ceiling and mark it around the perimeter of the room with chalk. This will give you an even floor no matter how irregular the old one is.

Mix your concrete. If the new layer will be less than an inch thick, mix 3 parts sand to 1 part portland cement. Use fine sand for extremely thin layers, but coarse sand will do if you're pouring a half-inch or more. If your new basement floor will be over an inch thick, mix 3 parts pea gravel, 2 parts sand and 1½ parts portland cement.

Make a second, thinner batch of portland cement and water while the first batch sits. Use more water, adjusting the ratios until the mixture is the consistency of paint. Dampen the basement floor with water. Use a paintbrush to paint the surface with a coat of this cement paste to help the existing concrete layer bond with the one you're about to pour.

Pour the first concrete batch immediately after painting the paste, using the chalk marks on the wall as a guide to know when to stop. Wait a week for the floor to cure before using it.