Im going to be pouring my first ever slab in 30x26x4 dimensions.

This is in North Florida with a high & dry sand base. I've removed the top layer of sod, then some... All with a flat shovel.

After breaking up the ground underneath the 3inches of sod I removed there are thousands of little bits of grass and roots Ive been picking up. 1 small piece at a time...

My question is: Do I have to get all this out?

Hopefully slipping in an extra question: How important is it to get the existing gravel spread absolutely even (evenly dispersed) before putting this fill dirt on? (ie: rocks are more concentrated in some areas which Im trying to equalize)

Im wanting to start pilling on fill dirt Now but Im afraid that missing a toothpick size piece of organic material could spell an equal size crack later and hoping for input.

Considering unseen toothpick sized organic material within, Is this ready to apply fill dirt? Picture of ground before pouring concrete

1 Answer 1


All buildings (and slabs) settle. The key is to have them settle uniformly.

There’s several issues 1) type of fill, 2) compaction, 3) maximum size of slab pour

1) You mention adding dirt fill. Generally we use gravel, not dirt. Dirt does not compact as well as gravel. The type of gravel fill is “evenly graded crushed rock”. It tends to bind together good, it settles less over time due to decomposing, etc. Also, install the fill in about 6” lifts before it’s compacted.

2) Compaction is usually done with a vibration compactor, but depending on what the slab will be used for, you could drive a heavy truck over the fill. (Heavier buildings require better compaction.)

3) Concrete shrinks when it dries. Therefore, you want to pour the slab in square shapes (so it shrinks uniformly and eliminate cracks) and limit the size to about 12’ square maximum. Unusual shapes (like “L” shapes) will cause the slab to crack.

So, I don’t think a small amount of grass under your slab will be a problem. However, I’d use good construction practices by minimizing fill that will decompose over time and make sure it all compacted uniformly.

  • aggregate nerd: gravel can be about anything, base is specifically designed to support pavement, and spec'd base is guaranteed to do a good job.
    – dandavis
    Feb 11, 2020 at 18:28

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