I recently purchased a new construction home. The backyard came unfinished and the yard is sloping away from the house.
From what it looks like, the yard is subsoil, it looks like it's essentially all the stuff building debris/rocks scraped off the land when the developers were building/grading the foundation.
Grass obviously can't grow on this so I assume I will need tons of topsoil prior to grading/seeding (4 inches?) I live in the Pacific Northwest so we get a lot of rain during the winter season so erosion and runoff is generally a concern on a sloped backyard.
I recently finished spraying glysophate 41% (roundup) on the whole yard, I expect it to die out in a week or two. I assume I need to pull all the weeds out prior to adding the top soil, I will also have to remove the large rocks/roots/general debris from the subsoil.
My question is, should I be tilling the subsoil prior to adding the topsoil? If so, how much inches? Or should I be adding the topsoil then tilling it directly into the subsoil?
Any recommendations or techniques on how to make the topsoil stay on the sloped hill?? It's roughly 30-40 degree slope.
Any advice is greatly appreciated. My end goal is to have a grass seeded in September.
edit: Pictures here - https://i.stack.imgur.com/1vSpi.jpg You can see how the fence slopes to get an idea of the hill. I plan on putting a garden bed on a majority of the slope with trees/shrubs/mulch and maybe have stairs to the right of the house. The picture of the dirt was with the rake, I pretty much can't really "dig" at all. Water does pond up on some parts from what I have seen.