Patio slabs must slope away from the building at a rate of 1/4 inch per foot. Improper slope can channel water back towards the house, which can wash out the dirt from under the foundation. Adding slope to a flat or nearly flat patio slab will eliminate standing water puddles after a storm. In areas where sloped patio slabs will not work, like between buildings, a floor or french drain may solve the water run-off problem.
Check the slope by placing a 4-foot bubble level on the patio surface with one end facing away from the building. Raise the down hill end of the level until the bubble centers in the level position. Measure the distance from the raised end of the level to the slab. The end of the level must have at least 1-inch clearance off of the patio slab. Check several areas using this method.
Use a floor scraper to remove paint and loose concrete from the slab. Use a broom to sweep the debris from the slab.
Lay a piece of lattice, a thin strip of wood, on the slab next to the foundation. Place one end of the bubble level on the lattice and recheck for slope. Add more lattice if the down hill end of the bubble level does not measure 1-inch off of the slab when the bubble reads level.
Mix concrete float material and water in a bucket with water. Use the manufacturer's instructions printed on the side of the bag to determine the proper amount of water to add. Stir the float material and water with a drill equipped with a mixing paddle. The float material will have the consistency of toothpaste when mixed.
Soak the slab with water from a hose. Wet concrete will help the float material bond with the slab.
Pour the bucket of concrete float onto the slab. Use a flat trowel to spread the float. It may take more than one bucket of float material.
Place one end of a straightedge on the lattice and the other on the down hill edge of the patio slab. Work the straightedge back and forth from one side of the patio slab to the other, carving away any high areas of the float material.
Place a clean straightedge on the patio slab. Fill any low spots with float material that has a consistency of a thick soup. Apply the skim coat with a flat trowel.
Finish the patio slab with a concrete finishing broom. Lay the broom on the edge of the slab and push towards the house. Pull the broom back. Do this across the entire slab.