How do I fill a huge gap where my concrete stairs separated from the house?

You defintely want to avoid using rigid mortar or those pre-mixed concrete patches for this. Since your steps and the house foundation are two diffrent structures, they are going to move at different rates throughout the year. If you pack that gap with something hard like masonry mortar, it's going to crack and crumble the very first time the ground freezes or shifts. You're essentially dealing with an expansion joint that has widened, so you need a material that can stretch and compress.

The most effective way to handle a one-inch gap is to use a **backer rod**. Grab a roll of foam backer rod that is slightly larger than the gap—maybe 1.25 inches—and jam it down into the crack untill it's about half an inch below the surface. This stops your filler from sinking into the abyss and ensures you aren't wasting a ton of expensive sealant. It also prevents 'three-point bonding,' wich is when the sealant sticks to the bottom of the crack and the sides, causing it to tear when things move.

For the filler itself, skip the hardware store tubs and look for a **self-leveling polyurethane sealant** like Sikaflex or a similar commercial-grade product. These come in standard caulk tubes, so you won't have to lug a 50-pound bag of concrete around. Since you've alredy established the depth with the backer rod, you can just squeeze the sealant in and let it flow. It will find its own level and create a watertight, flexible skin over the gap.

Don't worry about the quarter-inch layer instructions you see on those patch tubs; those are meant for different materials. With a polyurethane sealant and a backer rod, you do it in one pass. It stays rubbery once it cures, so when the steps settle another fraction of an inch next winter, the seal won't snap. Just make sure the concrete sides are clean and dry before you start so the sealant actualy grabs onto the edges.

You must utilize a stiff wire wheel on a power drill to aggressively abrade the vertical faces of the gap until you reach the sound aggregate, as any lingering laitance or efflorescence will inevitably compromise the chemical adhesion of the polyurethane, leading to delamination when the substrate undergoes its inevitable thermal contraction. Furthermore, I highly suggest opting for a closed-cell backer rod rather than the open-cell variety, as the former will not absorb moisture should the sealant bead suffer a minor puncture, thereby preventing the freeze-thaw cycles from expanding within the joint and destabilizing your repair.

I had the exact same issue last summer and used the DAP 3.0 Concrete and Masonry High Performance Sealant. It’s a total lifesaver becasue it’s rain-ready in just three hours, wich is great if you can't trust the weather forecast. Since you're dealing with an inch-wide gap, this stuff is way better than standard caulk because it won't shrink or crack over time. It’s super thick and stays flexible, so it handles the movement between the stairs and the house perfectly. Just grab a heavy-duty caulk gun and you'll be finished in ten minutes.

Listen to the folks about the backer rod, but back in my day, if we ran out of foam, we'd stuff some old tar paper or even dry oakum in there to bridge the gap. Not saying you should do that now, but it's the same principle—don't let the filler hit the bottom. Also, before you squeeze that tube, get a leaf blower or a shop vac and make sure that crevice is bone dry. I’ve seen many a 'high-tech' sealant peel right off beacuse someone didn't wait for the morning dew to evaporate. If you want it to last twenty years like the old asphalt pours used to, prep is everythign.