How to Build a Concrete Patio Slab

Lay out the patio with batterboards and mason’s line. Remove the sod, dig out 8 inches of soil, and compact the exposed surface. Spread gravel on the surface in 2-inch layers, compacting each layer and adding gravel for a bed 4 inches deep.

Install a second set of layout lines on stakes. Position the stakes outside the first set of lines, offset from the first set by the width of the forms you’ll use. Level the lines with a line level or water level.

Drive stakes along the layout lines, putting them at the corners of the patio and every 2 feet in between. Drive the stakes straight up and down, firmly embedding them in the ground, trim them to height later.

The patio must slope away from the house at a rate of 1/8 to ¼ inch per foot-check local code for the proper rate.

Measure down from the batterboards and mark the two end stakes at the finished height of the patio. Hold a chalk like tight at the marks you made and snap it to mark the rest of the stakes.

Align a 2x4 with the marks on the stakes and nail it in place with two duplex nails. Brace the form board with a 3-pound sledgehammer while you pound in the nails.

Nail the rest of the stakes to the form board the same way. Cut the stakes flush with or slightly below the tops of the forms. You will use the tops as guide to level the surface of the concrete, and any stake sticking above the surface will keep you from keeping a smooth surface.

Cut a 2x4 long enough to span each of the remaining sides. Put each board in place so the top is level with the marks you made earlier on the stakes. Attach stakes to forms with duplex nails. Nail in the remaining forms.

On sides too long for a single board, butt two boards together. Cut a strip of ½-inch plywood and nail it across the joint. Drive a 2-stake at each end of the plywood strip and nail it to the form with duplex nails.

Place a 1/2x4-inch expansion joint against the foundation of the house and any other existing concrete that meets the patio. This will prevent the pad and foundation from bonding and then cracking if they settle at different rates.

Coat the form boards with a coat of commercial release agent or vegetable oil to prevent the concrete from sticking to the form boards, motor oil—an early substitute for the release agent—doesn’t work as well and contaminates the ground.

Reinforce the concrete with 6x6-10/10 wire mesh (6-inch squares of 10-guage metal).

Put the mesh on wire supports, sold separately, so it sits roughly in the middle of the slab’s thickness. Leave a few inches of space between the mesh and the edges of the forms to prevent rust.

Overlay sections by 4 inches and tie them together with wire. Wire the mesh to the supports.

If you’re using a wheelbarrow, build a temporary ramp over the forms so that the wheelbarrow won’t knock them out of place. Start in a corner and dump the loads of concrete against each other. Have a helper wit ha shovel spread the concrete into corners and against the forms and expansion joints.

Lay a wide board across the forms so you can reach the interior of the pour. If the concrete cases the wire mesh to sing, pull it up with a rake so that it’s in the middle of the slab.

Work a shovel or rake up and down to remove air pockets, especially alongside the forms.

With a helper pull a screed (a long, straight 2x4) across the forms to level the concrete. Tilt the screed forward and slide it from side to side as you push it forward. Shovel off excess concrete in front of the screed or fill in low spots.

Make a second pass with the screed tilted the opposite direction. Work a bull float back and forth with the blade flat against the surface to smooth and compact the concrete. For small surfaces use a darby instead of a bull float.

Work the concrete until water forms on the surface.

When you’ve finished floating, separate the concrete from the forms by running the tip of a pointing trowel between the two. Then slide the cutting edge of an edger along the forms to round over the patio edge, making it less likely to chip. Lift the leading edge of the tool to avoid marring the concrete.

Cut sallow grooves, called control joints, in the wet cement pad. These weaken the spots a little so that if the ground shifts, the resulting crack should form along the control joint. You need a joint every 8 feet, so make marks every 8 feet along the forms.

Place a jointer at each mark. Guide the jointer against a board that spans the patio. Control joints can also be cut after the concrete has cured using a circular saw with a cement or masonry blade.

After cutting the control joints, and when any water sheen has left the surface, run a wood or magnesium hand float over the surface to proved a final smoothing. Raise the leading edge of the float slightly as you work to avoid making likes on the surface of the concrete.

For improved traction, pull a stiff-bristle broom across the surface. Draw the broom in either straight or wavy lines. If you don’t like the pattern or if it is too coarse, trowel, wit, and broom again.

The longer the concrete dries, the finer the broomed surface. Make another pass along the perimeter with the edger.

Lay plastic sheeting over the finished surface. The moisture trapped inside eliminates the need for watering during the curing stage.

Let the concrete cure at least 48 hours, then remove the form boards.