Sizing a Concrete Pad for an AC Unit, Shed, Spa or Generator Before You Pour

There is a predictable mistake with equipment pads, and it is not structural. The pad gets poured to the footprint of the equipment.

Then the equipment arrives, and the technician needs room to stand. Or the manufacturer requires clearance for airflow. Or the inspector needs working space in front of a disconnect. Or the unit gets replaced in twelve years with a model two inches wider.

Pour the pad bigger. It is the cheapest decision on the entire job.

AC condensers

Two things drive the size. The manufacturer specifies clearance around the unit for airflow — a condenser starved of air runs hot and dies early. And a technician has to service it, which means room to open the panel and stand in front of it.

Practical guidance:

• Give the unit its specified clearance on all sides, and then give the service side more.

• Keep it out of a dead corner where two walls trap hot air.

• Get it up off grade so it is not sitting in the runoff from the roof or the irrigation.

• Leave room for the disconnect and the refrigerant lines to be worked on.

Also: think about where you are putting it relative to bedroom windows and the neighbor. Condenser noise is a real neighbor complaint, and moving it afterward means a refrigerant line change, not just a pad change.

Sheds

The slab should extend past the shed walls, not stop at them. A slab that stops flush at the wall line lets water run off the wall onto the edge of the slab and into the joint, which is how shed floors get wet.

Also consider the door swing and the approach. If you have to walk through mud to get to the shed, you will stop using the shed.

And confirm the placement against your setbacks before pouring. A shed that is permit-exempt by size is not exempt from zoning, as covered in when a slab needs a permit and when it does not.

Spas and hot tubs

This is the one where size is the smallest of the problems.

A filled spa with people in it is a heavy, concentrated load. The pad has to be designed for it — thicker than a walking slab, properly reinforced, and on a subgrade that was actually prepared. A spa on a four-inch patio slab poured for foot traffic is a settlement waiting to happen, and a settled spa is not a repair you make cheaply.

Beyond the structure:

• Access to the equipment side has to stay open. Spa equipment fails and gets serviced.

• The electrical has specific requirements — disconnect location, bonding, protection — and the inspector checks all of them.

• Where does the water go when it is drained? A spa holds a lot of water and it has to go somewhere legal.

• If it is near a pool, check the enclosure question first. See

pool decks and pool barriers in California for the rule that a permitted project can trigger.

Standby generators

Generators need clearance from the structure, from openings, and from combustibles, and the manufacturer specifies it. They also need to not be in a location that will flood, because the point of a generator is to work in bad weather.

The pad is straightforward. The installation is not: it is electrical work at the service, usually a gas line, and often a noise and setback question from zoning. Plan the gas route and the conduit route before the pad, because both may go under it.

The rule that applies to all four

Anything that is going to be under or through the pad goes in first. Conduit, gas, water, drain, low voltage. All of it.

Concrete is the least forgiving surface on the property to cut into. Cutting a slab to run a line you forgot means a saw cut, a patch that never matches, and a permanent weak line where the next crack will go.

Sleeves are almost free before the pour. Sleeve for the thing you are not sure about yet.

Height above grade

Equipment pads should sit above the surrounding grade, not flush with it and definitely not below it. Water follows grade, and a pad that is a low point collects it.

Two or three inches of exposure makes the difference between equipment that sits dry and equipment that sits in a puddle four times a winter.

One question that saves the most money

What else might live on or near this pad in the next ten years?

A generator next to the AC. A second condenser for an addition. A pool heater. An EV charger on the wall behind it. If any of those are even possible, pour the pad bigger and run an extra sleeve now.

That same logic is why we argue for planning the whole yard before the first trade arrives, even when it gets built in phases. It is laid out in the order to build a backyard in Riverside County.

Concrete is the cheapest thing on the job to make bigger and the most expensive thing to make bigger later. Book a free 15-minute call or call (323) 651-0635 and tell us what is going on the pad. We will tell you the dimension to pour.

There is usually a video for this. X Plan Nation is our construction channel on YouTube, built to explain the trades to the person paying for them, not to other contractors.