RabidStaot,

For the default 45-degreed gabled roof by the roof tools, it takes 3 rows of grids for the roof to rise up to a default floor height of 16 steps/16-click-high.
Surely, if the roof angle is larger, it may take fewer grid rows to be high enough for sims access.

the roof tool roof is pretty interesting:

1. the attic walls by it is bound to the roof-tool roof instead of the floor grid layers, so they can ignore the floor grids of foundation and the default wall1 walls or others to create an illusion as if they're fused together. EA Maxis did a bad job on it coz graphical glitch can result, so in my instances, I use invisible recolours of Numenor wall-windows to silence some of the highly glitchy overlapping regions or simply remove the walls. And it takes a while for me to figure out a relatively appropriate lighting from the light choices.

2. Still the roof-tool roof is one of the two major factors to limit sims access in terms of room building... (I'm not talking about objects ) Such a room is sims-accessible when all the following major criteria are met.
- the tiled floor grids are at least 16-step-high/16-click-high
- the <16-step-high/16-click-high floor grids are untiled
- the roof-tool roof is at least 16-step-high/16-click-high

3. for the rooms by roof tool, they're not a completed closed room, the lowest end(s) of the roof quite often lack walls for them to form a closed room.
To solve this, one can choose to mod the wall.txt or not to enable the attic wall to show up in the wall tool section.
If so, then one can build the attic walls at the very edgy sides to close a room. One may save some spaces for decorative objects including lights as long as they don't poke through the roofs unless that's your expected design. As in this lot, I added an extra roof piece on the upper level, so the fire place is accessible to sims because the upper roof by-passes the roof-tool roof height limit. I've been using attic walls to build A-frame lots for >1 year and it appears to be stable in unmodded wall.txt-based games.
If not, leave out 3 grid rows for default walls to close that end, or use reduced wall segments to fill any line to the 3 grid-rows.

Indeed, it's because I built the gabled roof on the ground so that the counterpart of the building on the foundation can be relatively high enough to match up the gabled roof's top.
I've built some A-frame ones from the ground up in 3x3 lot while making bigs rooms for sims under 20k. That's why I said the priceless roof tool @ build mode is the cheapest way to build a lot! Also, I did some A-frames on foundations before. They're really big budget-savers

The foundation wall appears stable, too. N, I've made it out-cuttable for my eyes' comfort. I normally use it for foundations of a floor! But, it may be deformed a little bit different from the default wall1 walls a bit.

I've not dug deeper into the screen deck walls and the invisible column deck walls. But, the former seems to be as stable as the foundation walls in my very limited amount of experiences.

The only catelogue-enabled wall appears quite unstable is the swim-pool walls, lots made out of it may crash the game in an unmodded base game and probalby cause un-reusability of a lot.

The enabled OFB stage wall (501?, right? I'm not in that HD.) seems to cause stability issue with the the foundation stage. But, I've not seen it as a big issue as the swim-pool walls.

After all, I need no more of the swim-pool walls since I can make custom caustic wallpapers already. :D

Sorry, I've said too much. Let's end here.