Do these "rafter ties" tie anything together?

Question

I've been wondering if I can open this ceiling up for a tall room. I've been reading that the ceiling joists are often also rafter ties so I went up there today to take a look. I can see a few are tied to the knee wall and have an angle brace, but a majority seem like they just span the house, but aren't really tied to anything. I would like some opinions on this. The plan is to cut them out and replace them with steel rods and turnbuckles where needed to keep things tied together.

The roof is best described as a mansard with a flat top. I've tried to show that here:

This is the only point load I see. It actually doesn't rest on a joist, it rests on that 3/4 board that is spread across the joists so I can't imagine it provides a ton of weight bearing

Here is a drawing that kind of shows the roof shape

Answer 1

Although I have done what you are asking several times, I had an engineer design and stamp plans, making a structural change this big is a big deal, actually the first one was a repair for a Bone head that just cut them out, the roof sagged walls bulged he could not sell and the bank shorted it, after repairing and turning this property I did several more on the side that had low ceilings.
All those places were similar, Recently helped a friend with a 2 story same issues but he did not have the $ for the engineering stamp. Good thing because after we repaired it it survived a huge snow storm that caused my barn roof to collapse. But again someone tried adding height with knowledge or experience and that’s why my friend hit a killer deal as I have in the past. So don’t change things without structural engineering review when they can cause massive problems including roof failure.

Answer 2

Yes, I think you can remove the ceiling joists.

There’s a lot going on here to consider:

1. span of the joists for the mansard, 2) point load (post) from the upper flat roof down to one ceiling joist, 3) lateral loads, 4) transfer of loads to footings, 5) verify knee wall supports, 6) single top plate, 7) a Building Permit will be required, so have plans prepared by an architect or structural engineer.

2. The reason the roof joists in the mansard portion of the roof have not failed is because structurally they are only 5-6’ or so. Structurally, the “span” is measured HORIZONTALLY, not the actual DIAGONAL distance, which appears to be about 12’-15’ long. So, the 2x4’s can easily span the effective 5-6’. (However, adding insulation (weight) could over stress the existing 2x4’s.)

3. The post supports the corner of the flat roof and about half the tributary load from the mansard. A new beam will need to be added to support the post, which supports the corner of the upper flat roof.

4. The ceiling joists do not provide any lateral load resistance. That is to say, the ceiling joists can be removed (except the two that supports the vertical post until you get a new beam installed to support the post). You will not need the steel rods, etc. that you suggested for lateral support.

5. Removing the ceiling joists does not change any of the transfer of loads to the footings, except the new beam you’ll need to add to support the post.

   Depending on where you are located, (high wind area or seismic area) I’d add Simpson A35 clips where the mansard roof joists connect into the upper wall and lower wall. (However, I doubt if you’re in such a zone, because you have “spaced” sheathing and not plywood sheathing. ) I also recommend adding plywood roof sheathing, as it will help “tie” the roof together.

6. I’d also examine the existing knee walls to make sure they’re laterally stable before you remove the ceiling joists. (It doesn’t look like they are true “knee walls” but rather two walls side-by-side with one slightly higher than the other, because in the second picture you can see that the ceiling joists does not extend all the way over to the “knee wall”...so it doesn’t provide any lateral support.

7. The single top plate is acceptable, as long as there are no splices along the wall. (I didn’t see any splices.) Again, I’d add a Simpson A35 clip at each corner to tie the plates together.