After you’ve prepared the base for your new patio, (see Planning and Preparing for Your Concrete Patio), you still have some upfront work to do before you can actually pour that concrete. Here again, rushing to get started (or finished) will ensure you end up with a concrete patio that just won’t stand up, and then think of all the work it will be to get rid of that big mess.
Drainage is important
The proper slope for a concrete pad is ¼” of slope for every foot of concrete. In other words, a concrete pad that’s 10’ wide should have an end to end slope of 2 ½”. If for some reason you can’t build in this much slope, consider install a drain in the center with the slope running towards it and a piece of pipe taking the water away underneath the concrete pad.
The time to ensure you have proper drainage built in is in when you install your gravel base. Build the slope into the base so you will end up with the concrete pad having the same thickness all over.
You need strong forms
Wet concrete is very heavy and for that reason you need to build the forms for your patio from strong lumber. If you’re pouring a 4” pad your forms should be e made from at least 2 x 6 lumber with supporting stakes driven firmly into the ground at least every 2 feet to hold the forms in place. Curves can be accommodated using plywood that has been soaked in water so it will bend into a curve. Be sure to provide extra support stakes around a curved section.
To prevent the concrete from sticking to the wooden forms, spray them with a form release agent (people used to use old motor oil) before you start to pour.
Some construction rental companies rent steel concrete forms that you know are strong and they could be an option to consider as well.
Reinforcing your slab
Putting proper reinforcement into your slab is important to help prevent cracking.
Reinforcement can be either steel mesh fencing (2” x 4” or 2” x 6”) grid or rebar. Either way the reinforcing steel needs to be in the middle of the concrete pad (not lying on the bottom of the pad).
If you decide to use rebar it should be placed in a grid pattern of 2 foot squares with the intersecting points tied together with wire to make it solid.
You can make sure your reinforcing metal is in the middle of the pour by placing small rocks underneath it (to raise it up to the middle) before you begin pouring.
Joints are necessary for a large patio.
If your patio is going to be more than 12 feet square, you need to plan on putting joints into the slab obviously after the concrete has been poured. Joints are basically grooves that act as release points and help prevent the large slab from cracking.
A proper concrete joint is ¼ the thickness of the concrete slab, so for a 4” slab the joint should be 1” deep. They also need to be 2 to 3 times in feet the thickness of the slab. So for your 4” slab, you should install joints at intervals of 8 to 12 feet.