leveling floors and jacking up roof joists

I'm in the process of rehab'ing a single story brick structure that has seen years of abuse and neglect. The floor is in need of leveling and I intend to sister the floor joists to achieve this. There is a large window (approx. 8' x 5') in the back wall and the top of the window "frame" is a 6"x8" piece of timber running the 8' width. Over time, stress from the weight of the back wall & roof has taken its toll on this piece of wood and it has now taken on a sort of tilda (~) shape.
The sagging roof joists need to be jacked up and the brickwork in the back wall corrected. My question is simple. Does it make any difference in which order I perform the aformentioned tasks?
Should I level the floor, install new subfloors then jack up the roof? This would put a good deal of downward stress on the newly sistered joists. The sistered joists *should* be sturdy enough to avoid knocking the floor out of level again.
Should I jack up the ceiling first and place the stress on the original unreinforced joists (they're 100+ yr old 2x8's)? I'm afraid this may worsen their current condition.
Does it make any difference or am I just being paranoid?
thanks, -josh
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Talk with a reputable building engineer first. If you don't know of one, start with your town's building inspector - he should be able to point you in the right direction and ensure that you're following the process that the town has in place. Not exactly a DIY project. Need to find out why the house is sagging in the first place - could be poor soil conditions, bad foundation, etc.
Obviously you want to take care of any structural deficiencies from the bottom up before you move onto the rest.

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wrote:

First thing you do is protect the last thing you do from major problems while doing the other. I like to start at the bottom (as do most pros) so I'll put some temporary bracing in the weak spots of your upper structure while leveling from the bottom. The 'tilda' may be about to break and your roof may be about to fall thru that weak window so brace it up temporarily while you are messing with the floors.
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