Large rock under footing?

[Jim McClendon]

I had a builder ask me what I thought about leaving a rock the size of a large car (maybe we should call it a boulder) under the space he wants to pour a footing for a concrete slab. Raised block (three high), filled and a slab poured. What is the ruling on this?

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[Gunnar Alquist]

Since I am a mere home inspector, I would leave it up to the engineer.

Having said that, it seems to me that if this rock/pebble/boulder goes as deep or deeper than the footing, then it is probably OK. I have seen perimeter foundations constructed over large rocks/boulders with the blessing of an engineer. However, it might make a difference what kind of rock (say, sandstone or granite). And, as I am not a soils or structural engineer, I would defer to one in my report if I saw something like this on an inspection.

[Dom D'Agostino]

I imagine the reason he asked is because it's expensive to remove.

[John Goad]

It would also be a good idea to spread sand over the rock before the footer is poured over it, but like Gunnar said, let an engineer have the final say.

[Jerry Peck]

As stated by others 'let the structural engineer address it', but here is one problem with leaving it:

That boulder has been there a real long time, it ain't a gonna go nowhere.

The rest of the foundation, however, will likely have some initial settlement, which is all fine and dandy ... 'ceptin' that area on that rock ... that area of the foundation ain't a goin' no where neither.

Result = "differential settlement"

[wayne soper]

I agree with Jerry, I as you all have seen many times where the large rock left under the garage slab has caused the slabe to rise and crack as the rest of the structure settled.

[John Goad]

In Tennessee when you see cedar trees you can count on there being limestone rock below.