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.

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

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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

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Here is a drawing that kind of shows the roof shape enter image description here

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    The top 3 pictures seem to show the ceiling joists run parallel to the roof joists, but the bottom picture seems to show the ceiling joists run perpendicular to the roof joists. Questions: 1) Are all the ceiling joists in the same room? 2) If so, can you provide pictures of bearing locations of the ceiling joists at the walls for ALL situations? 3) Do all the roof joists attach to the same ridge? 4) Can you provide a picture of all ridge connections? 5) Are there any “point loads” (posts) on the ceiling joists? 6) Is there a hip in this roof? (Picture?) – Lee Sam Sep 4 '19 at 3:21
  • I updated the photos. The ceiling joists span the entire width of the house. There is a wall running down the middle of the house that all of the ceiling joists rest on. The roof is best described as a mansard with a flat top. I can't see what the rest of the framing looks like from inside because it is finished, but I can get a photo of the roof from the rooftop. It seems like all of the sloped parts use 2x4 lumber and rest on the perimeter of the box that frames the flat top. The flat top is 2x9 that runs parallel to the joists below. – EricSLC Sep 4 '19 at 4:11

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.

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    Sounds like your “answer” is to go ask someone else because you did. Should this be a comment? – Lee Sam Sep 4 '19 at 3:23
  • I will likely get an engineer. The house had no insulation and I want to properly insulate it. I suspect this means there will be a lot more weight on this roof when it snows now. So I will eventually bring an engineer in, but for now I'd like to make some informed changes. – EricSLC Sep 4 '19 at 4:28
  • @LeeSam the answer is there... see "or a Bone head that just cut them out, the roof sagged walls bulged"... – Solar Mike Sep 4 '19 at 9:19

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.

  • Thanks for the help. Can you mark where you would put the A35 clips so I get it right? I am in a seismic area with high snow, so I am planning on getting someone out here before I put a new roof on to make sure everything is ok, but I would like to get a jump start on it. – EricSLC Sep 4 '19 at 13:52
  • @EricSLC If there are only single plates that do not lap, I might use a Simpson RTA clip at each corner. See here: fastenersplus.com/Simpson-RTA2Z-2x-Rigid-Tie-Angle-ZMAX-Finish The A35 is good for the mansard roof joists connecting them to the plates (top and bottom). Be sure to bend the flange for proper insulation...as shown here: strongtie.com/resources/product-installers-guide/… – Lee Sam Sep 4 '19 at 15:12

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