Depending on how high you are building your stairs there is a build order to follow.

if you could provide a screenshot of the issue you have with stairs perhaps i can be of assistance.

Sorry for the late reply. So, I know that by building the roof first and then the stairs allows it but when building a two-storied house, not being able to put ceiling tiles (and therefore roofs) over the open spaces that stairs create is quite bonkers

I am unsure what you mean. If the issue is that you need to go up 2 sections inside instead of one up to the next layer of flooring, perhaps this option in this discussion may help ?

https://steamhost.cn/steamcommunity_com/app/1324130/discussions/0/4029094770951322256/?ctp=2#c4029094770951974360

If i am mistaken with your intent, could you provide a screenshot with the issue you are having with stairs perhaps ?

Okay, but that might take a bit, I'll have to build a new house first, big enough for the stairs to show ^^

Ah i hoped you had an (auto) save with the issue. If it's not to much hassle i would love to see it. I would very much like to help solve it for you.

That's weird. I built a wooden floor, wooden ceiling, then added stairs into that ceiling to create a second floor. Then I added the second floor ceiling and it completely closed.

Maybe it's because of materials? The one that didn't work was a concrete ceiling

Na, i think it must have been something else. But if it's fixed for now then that's good. If it happens again just screenshot it and post it on this forum somewhere. I check the forum often and if it pops up we will both have a look at it, ok ?

I run into this problem mostly when placing stairs between floors of differing heights.

The workaround is to use a balcony to create the floor above the stairs.

Sometimes doing this causes the area underneath to still have the 'outside' status, though.

@Vinny

If you need to go more than 1 level up indoors with stairs, perhaps try the setup i linked earlier in this thread ? It's enclosed.