Rafters. This is traditional "stick" framing. Some of the bracing resembles the same angles as trusses but there are no nail plates. Pretty sure most homes through the 60s were done this way.

The bottoms of the rafters are horizontally restrained by the attic floor and reinforced higher up with collar ties. If you remove all of the joists, you will need to add a ridge beam spanning the entire space supported at each end by posts to the foundation. If you remove the collar ties, you will almost certainly need to reinforce the rafters. The vertical members suggest the rafters might have sagged.

Its easy to figure out by measuring the rafter sizes and spacing and the width of the space. Take that information with any snow load and look up the rafter size in the building code table. You will need an engineer to design the ridge beam. You might be able to get a lumberyard to provide a stamped beam design from their LVL supplier.

^^^ hahaha!

Thank you all! Where I'm doing it faces a beautiful view so I feel like raising the 7'9 ceilings overlooking that would be worth it, just a matter of when based on cost.

It should be designed by an engineer and priced by a contractor. It will be expensive but possibly worth it. Consider skylights. What is the climate?

Totally agree, I have a structural engineer for another project so will consult them again once we can afford this. Skylights would be great! Seattle area, mild climate.

Yes these are rafters and the ceiling joists are performing the same function as the bottom chord of a truss. It's keeping the rafters from pushing the top of the wall out. Remove the ceiling and the house will collapse without some other structural intervention. In the past I've replaced a similar ceiling with steel tie rods which serve the same function, are very strong and thin and add a unique decorative element.

You will also need to figure out what to do with your mechanical system.

If I'm not mistaken, there is a name for the 2x4 posts, they called "kickers" and they usualy bear on the top plate of the interior wall.

@GN,

Posts are for vertical loads. Kickers are for lateral bracing. They're not the same.

@Summit, I did an addition a while back with an enclosed pool, instead of a tie rods that you mentioned we used cables. I will try to find a picture (that was before cell phones), I should have an album someplace, if I find it I will scan a few pictures.

P.S

Pictures a bit old and I thought I had a few where you can see cables, but I guess not, but on this addition we used them.

That said to open up that ceiling they will need more than tie-rods.

@GN this is a perfect example why we use structural engineers. I neglected wind loading. The tie rods would do very little for wind.

Considering the small rafters and the low slope, a ridge beam seems like the best approach.

@res2architect The ridge beam is there.

@Summit Studio Architects Funny you mentioned that... the homeowner had an engineer to design that for him, and he only had cables on the drawings and I didn't feel good just having cables there (1st time I came across using them) without other bracing so I told the homeowner I want to add some collar ties because without them I told him I will walk away from this structure, so after going back and forth with them I finally talked the homeowner into it doing it.

That said, it has always been my nature to over-design something a bit and feel confident about it than under-design and deal with consequences down the road, and not on one occasion I caught things on the plans done by "Some" architects or engineers that didn't make much sense and I gave them my calculations and it turned out that I was right.

I only see a ridge board in the OP's photos. Are you talking about a different building?

Sorry RES2 I thought you were talking about one of the pics I posted, I know it's hard to see anything in these photos.

Yes in the original post it looks like they have a 3/4" board or plywood jammed in there.