We ripped up carpet in our hallway and found multiple variations of cement heights. What is the best way to fix the issue before laying the flooring so it doesn't crack? The 3 triangles in the picture are all different heights. The triangle closest to the kitchen is level. The middle one goes at an angle side to side. The third one goes down.
If you don't have a severe slope (one room is below another room), then you can use "self-leveling concrete". It is basically a very runny concrete mix that flows and sets as a completely level surface. Look it up on youtube to see what the process looks like.
If you have a significant height difference between room edges, self-leveler will all just drain there trying to level the lowest room first, so it might be better to use "Skim Coat & Patch Cement" kind of product. It mixes into thick mass and you spread it around with a trowel and then go over it with a straight edge to knock down high spots. That allows for better control at the cost of fussier application (lots of time on your knees).
Notice maximum height when choosing the product, a lot of them can be built up to maximum 1/2".
This is what I used for my basement: https://www.custombuildingproducts.com/products/skim-coat-patch-cement-underlayment
