# Your recommendations for digging, base and compacting



## PoleBarnsNY (Jan 18, 2014)

How far down do you recommend digging to remove topsoil for slabs on grade?

What material is best for slab on grade base? 

How thick should the base material under a slab be?

Your recommendations for compacting

1. Building footprints for slabs on grade
2. Backfilled trenches

Specs 4 inch slab, 4000 psi, reinforced with fiberglass mesh for light duty garage floor


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## rino1494 (Jan 31, 2006)

- Depends on how deep the topsoil is.

- clean, crushed stone. 2" minus

- 4-6"

- Large footprint, use a roller. Small footprint use a reversible plate.

- Trenches... depends on length, width and depth of trench.


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## muskoka guy (Nov 16, 2013)

Topsoil depth will vary from location to location. You have to remove any organics or roots completely from below the slab. This could be as little a few inches to a few feet. You cannot leave topsoil under the slab. That said I would recommend at least a few feet of A gravel below the slab, compacted with water and mechanically packed every four to six inches. If the soil below the slab is clay or any other frost prone material you might even consider going down below frost level. If this is the case you could use B gravel for the lower level of backfill. Does your building require thickening of the slab at the perimeter where the walls are? Reinforcing bar in edges ? These questions could all be answered at your local building dept. who will have to approve it anyway. I would suggest a trip there. Here in Ontario, any garage over 538 ft sq must have the slab designed by an engineer. I have found by the time you pay the engineer, add all the extra work that they require you to do, I prefer to put in a full foundation. It may cost a bit more but the end product is far better than a floating slab. jmho Good luck.


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## tgeb (Feb 9, 2006)

You must mean 4,000 psi concrete.

I prefer 5" thick slabs in a garages.

Everything else has been answered.


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## Morecoffee (Feb 23, 2013)

I would add... 

The width at the start point for your gravel fill should be no less than a 1 to 1 slope from the edge at gravel grade level.


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## Windwash (Dec 23, 2007)

PoleBarnsNY said:


> How far down do you recommend digging to remove topsoil for slabs on grade?
> 
> What material is best for slab on grade base?
> 
> ...


I would skip the fiber-mesh for a slab on grade and just use regular concrete if no other reinforcing. Without rebar or 6x6 steel mesh, the slab may start to migrate after it cracks due to differential frost heave, especially on garages or buildings where part of the structure has 3-4+ courses of backfill but other parts have no backfill.


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