How to Build a Concrete Pad for an AC Compressor

A man smooths concrete.
What You'll Need
Mixing bin
Form lumber
Premixed concrete
Plastic vapor barrier
Welded wire mesh
Screed board

Pouring a concrete pad is not much different than pouring any type of concrete slab. This article will explain how to build a solid, level concrete pad to support an outdoor AC compressor. Constructing the pad involves building the formwork, preparing the base, installing the reinforcing, and pouring and finishing the concrete.

Determine the Size and Location of the Pad

The size of the pad should be large enough that the compressor supports are at least 1 foot from the perimeter. Locate the pad on level ground in close enough proximity to the house to make the necessary hookups.

Excavate and Prepare the Formwork

Excavate to a depth of about 10 inches. Deposit a base layer of compacted gravel about 6 inches in thickness; the pad itself is typically 4 to 6 inches thick with a height slightly higher than the grade.

Use lumber equal in width to the height of the pad-1x4 or 1x6 will do to build the form. Nail the form boards to the interior surface of wooden stakes driven into the ground. Measure diagonally from corner to corner to ensure that the form is square. Make sure the top edge of the form boards are level front to back and side to side.

Install a 6 mil plastic vapor barrier onto the gravel, and reinforce the concrete with 4x4 welded wire mesh at the midpoint of the height of the pad. The forms should be coated with a form-release compound so that they can be easily removed when the concrete has set.

Pour the Concrete

To determine the volume of concrete you need, multiply the width by the length by the thickness of the pad. Concrete can be purchased as a pre-mixed, ready-to-use product that requires only adding water.

Mix the concrete in a wheelbarrow or mixing bin and add only enough water to give a fluid, workable mix. A high water-to-concrete ratio will result in decreased strength. Spread the concrete evenly and fill out the form completely. Use a mixing rod to remove trapped air.

Finish the Concrete

The first step in the finishing process is called screeding. A screed board worked front to back in a side-to-side motion will result in a roughly level surface. Next, you can use a wooden or metal float to get a smoother finish. Finally, use a broom or wire-brush to get a grooved or textured surface.


The concrete should be kept moist or damp and protected from rain for up to a week after pouring. Do not install the compressor until a week has passed and the concrete has attained its final set. You can then remove the form boards and backfill around the pad. It will reach its design strength in 28 days, after which it will continue to gain strength but at a much slower rate.