I am getting a new A/C unit installed on Monday and when I asked them to put rock under the pad to level it out they said they would put what ever I supply.

They reason I asked was my previous experience was the installers just kinda level with what ever they have which is some kind of mixture of soil and mulch and will eventually settle.

The two units sit on a side hill that is subject to erosion.

I know I should use a rock that has rough edges and corners so it is less likely to move.

The question I have: should I use a smaller stone or a bigger stone or something else?

Small Stones

Large Stones

I picked those two because they were the cheapest at the local BB store and bagged so I can get them home.

In the end it probably wont matter much but I figured I would ask as I have a couple days to figure it out.

3 Answers 3


The smaller stone will compact better and you will want to compact it. Depending on the slope of the hillside, You might need to build a small retaining wall to keep the erosion from washing the stone away causing the pad to tilt.


I would use lightweight cinder blocks similar to this.

enter image description here


Most contractors in my area use “plastic pads” never set a unit on loose gravel If you are setting the unit on rock it should be compacted shale or crushed quarry! pea gravel will move “FOREVER “ ok just the opinion of a guy that did this for a few weeks over the last few decades.

  • I apologize, I should have made my post clear. I will want them to set their plastic pad on rock. Commented Aug 28, 2021 at 17:04
  • I would use crushed quarry or shale if a base for the pad. Things like pea gravel will not compact on a slope where crushed can be compacted and provide a level base that will allow the condensation to drain through the compacted rock. Similar to using a compacted rock base but a plastic pad will extend the life as the water won’t pool on the plastic and rust out contact points as quickly.
    – Ed Beal
    Commented Aug 28, 2021 at 21:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.