How to Prepare Cement Slabs

Measure, Layout and Sub-base

• 1

  Mark a rough outline of your project with a rope or hose. Drive wood stakes into the ground near each corner of your layout, aligned with the borders, but set 1 foot outside the project site. Tie mason's string to each stake to outline your project precisely, so that the strings intersect to mark the exact corners of the project. Use a carpenter's square to set perpendicular lines.

• 2

  Use the 3-4-5 triangle method to ensure your corners are square. Measure 3 feet out on the mason's string from one corner. Measure 4 feet out from the same corner on the adjacent mason's string. When the diagonal distance between these two points is 5 feet, the corner is square. Adjust the stakes and string as necessary until all four corners are square. Vigilant measuring and layout are the basis of a successful project.

• 3

  Use a line level attached to the mason's string to ensure all strings are level. Verify intersecting strings contact one another, so that the corners and entire string layout is straight and level.

• 4

  Adjust the string to reflect a drainage slope for your slab. This slope should be a 1/8 of an inch downward slope for every foot of distance. Ensure your slope angles away from any existing structure. Measure the length of your slab in feet and multiply that measurement by 1/8. For example, if your slab is 10 feet long, the required slope would be 10/8 or 1 1/4 inches. At the end of the slab, where the water would run off, move the mason's strings down on the stakes 1/8 of an inch per every foot of length of your slab. This slight slope will prevent water from pooling on your slab.

• 5

  Remove sod using a sod cutter or square-end spade. Confirm that this removal extends 6 inches beyond your mason's string border to ensure that there is room for wood forms.

• 6

  Create a story pole, a simple 1 x 2 inch wood plank used to mark vertical measurements. At the highest point of the slope, measure from the mason's string to the ground level. Add 7 1/2 inches, 4 inches for the slab's sub-base material and 3 1/2 inches for the concrete. Mark this total distance from one end on the story pole. Dig out your project site with a spade, a pointed tip shovel, using the story pole to ensure the depth is consistent across the breadth of your site and corresponds with the required slope. Check the middle of the site with a straight-edge and a level.

• 7

  Fill your excavation site with a 5 inch thick layer of compactible gravel to create a sub-base. Tamp the gravel to compress it down to 4 inches with a hand tamper, a heavy flat metal square with a vertical handle. Ensure the sub-base extends 6 inches beyond the project outline.

Frame and Concrete

• 8

  Cut a wood form, a frame made from 2 x 4 inch pieces of lumbar, to contain the concrete and determine its thickness. Use the mason's string as a reference to ensure the inside edges of your wood form equal the total size of the project. Drive stakes at 3 feet intervals along the outside of the form and attach them with deck screws. This will secure your form and ensure it does not move once the concrete is poured in. Use a level to verify that opposite sides of your form are level. Tack nails into the exterior of the form to mark the location of slab control joints. Control joints are straight crevices cut into the concrete to control where the slab will inevitably crack in the future. Coat the inside of the form with vegetable oil or a commercial release agent to prevent the concrete from bonding to the form. Lay down rebar or metal reinforcement if the dimenstions of your slab require it.

• 9

  Mix concrete according to the manufacturer's directions and pour it into the form. Distribute concrete evenly with a masonry hoe. Insert the blade of a spade between the wood form edges and the concrete to release air bubbles at the sides.

• 10

  Rap the outside edges of the form with a hammer to help settle the concrete. Use a screed, a straight piece of 2 x 4 inch wood, long enough to lay across both sides of the wood frame, to smooth out concrete. Move the screed in a sawing motion as the concrete levels and keep it flat.

• 11

  Cut control joints at regular intervals with a mason's trowel, using a straight 2 x 4 inch piece of wood as a guide. After bleed water disappears, the water that rises to the top of freshly poured concrete, float, or level, the slab with a wood float, a flat, wood smoothing tool with a handle. Move float in an arching motion.

• 12

  Draw a groover across pre-cut control joints to smooth out these edges. Use a straight 2 x 4 inch plank as a guide. Draw an edging tool between the wood frame and the concrete to create smooth edges. Cover the slab with plastic and allow to cure for at least a week, lifting the plastic and wetting the concrete occasionally. Remove the plastic and treat the concrete with a concrete sealer. Remove the wood forms.