I'd sister 2x8" to the current joists and fill in between them so the framing is more standardized.I have an addition that was built in early 1900's. THe addition is aprox. 12' X 15' 7", The current ceiling joist are bowed and twisted, due to water damage. Unfortunatly once I opened up the ceiling (Removed all the 1 x 6 x 5/8, tongue and groove) I found that the joist were 30 to 34 inches apart from each other. They are running the 12' length. I know that they should have been at 24" (MIN) on center.
My question is shoulden't they have been 2 x 6 rather then 2 x 4's?
Since I am going to have to replace them (Builders permit requires inspection prior to covering) and actual length is 12'-10.25", so I will have to get 14' is 2 x 4's enough? It has a flat roof on this addition. Roof Rafters are running 24" on center.
The oppiset side of the house has a 12' X 32' addition. Ceiling joist are running 24" on center, but they used syrafoam for the ceiling I want to use 1/2" sheet rock or 5/8" sheet rock, will the weight cause me problems on this ceiling as well?
You don't mention the roof rafter size or where you live. The structure has been up for 100 years, so it can't be that understructured.
Do you have snow loads?