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We are trying to partly level out our slab to build a subfloor. Most of it is level enough but a section that is about 20ft x 5ft starts to slope and fairly aggressively in some parts. I see self leveling concrete says it's only for up to 1-1/2". For anything beyond that depth should I fill with gravel? Use pressure treated wood? Regular cement?
Thank you kindly
If the goal is to install a subfloor, do the leveling in the supports for the subfloor. Once the subfloor is installed on leveled sleepers (joists but not spanning, since they sit on the concrete, whatever you want to call them) nobody needs to know or care what the concrete under them happens to be shaped like. And SLC is relatively expensive.
Sika 125 can go to 2".
Some SLC allow you to add aggregate of the thickness of the pour you are trying to achieve. This is typically done to make the SLC go further but probably also adds strength.
If you are building a subfloor on top then I think the leveling details do not require perfection. The biggest advantage of SLC is that it doesn't crack and become loose over time and lead to popcorn noise it also can be troweled to a feather edge for perfectly flat substrate. SLC is quite a bit more expensive than concrete.
Concrete lifts generally need to be over 1" in thickness and anything thinner is guaranteed to crack.
In your situation where you are building a subfloor on top of the existing slab and just want to prep the slab for the subfloor I'd probably do a lift of concrete for anything more than 1.5" have a hard stop where it gets thinner and then top that with SLC (depending on your subfloor construction details). You'll need to properly prep the slab for the new lift of concrete and properly prep the new/old concrete for the SLC.
