People have actually done guides/posts with screenshots of every material/combination side by side with max building segments.

Its something like 8 or so connections from a blue for regular wood I think. Don't know if this is accurate but :

https://re-actor.net/valheim-definitive-build-height-guide/

One thing that helped me on 2 story structures was having a central firepit on the first floor with stone columns at each corner. These stone columns continued straight up through the second floor as well, so there was a central point that counted as "blue" stability (like the ground) all the way up.

Central firepits were an actual historic thing vikings did anyways.

The first floor has stone columns, and wood crossbracing near the ceiling that's so strong, the second floor was able to support another hearth in the center as well. So both rooms have a central firepits.

Use core wood for the corner support. The other wood has too many lengths.

Or make the walls one upright and a short upright. Any more and you will lose it at the top.

If that still won't help then you need to bring up the ground to meet the corner posts or any outer edge. Anything that should be in contact with the ground should be blue.

there are height limits . so the foundations need to be blue when you hover over with hammer in repair mode. and the colour changes the higher you go.. core wood helps. stone too .. and iron wood when you get past stone

should mention this is a second floor, the first floor however is made from stone so the wood at the bottom is blue

Then use core wood for the posts... the wood you are using now takes up too many segments.

You can only be x number of segments high before it breaks.

thanks y'all, simply but putting down a few core logs i fixed the problem

You can also use crossbeams and braces. Its orange and red on the 2nd floor but it holds.

Crossbeams and braces won't really help in Valheim. It's horizontal and vertical distance from a terrain item that determines stability. If you have access to iron, you can build iron beams. They can go much higher vertically and structures that are attached to them gain the blue "base" state.

Valheim structural stability can be simply described as "distance from the ground". You can see the structural strength of various items by stacking them head to toe. Each new piece gains more distance from solid ground(measured through the construction pieces), until it breaks. Other factors like weight, thick/thin beams, number of pieces, etc. have no effect on construction.

Most building materials have similar structure strength, and horizontal distance comes with a penalty that hits stone blocks very hard. The strongest building materials are wood-iron beams, and iron-gate doors. Hide them as skeletons inside larger builds.

Trees (normal and swamp) are very tall and count as "solid ground" to build with. Terrain can also be raised/lowered to create anchor points to build from.