Ceiling joists are the wood structure that support the weight of the ceiling drywall or plaster finishes. Roof rafters are the wood structure that usually run from the gutter up to the ridge or high point of the roof structure. Roof rafters are usually one single run from the gutter all the way up to the roof ridge. The roof ridge in most style roofs just serves as a lean to for the roof rafters. When the roof rafters are only continue run the roof structure is relatively easy to evaluate. When roof structures are complex so is the evaluation of the roof structure. In this home changes were made to the roof structure for some reason. I have no idea what was done or why it was done however I can assure you that vertical 2″ x 4″ columns like these will not support the weight of heavy snow on a roof. Even if the vertical 2″ x 4″ columns were to support the weight of the roof the columns are now imposing weight on the ceiling joists that are usually only strong enough to support the weight of the ceiling finishes. Often when temporary roof structure repairs like these are made we find cracks in the ceilings and walls due to the additional weight of the roof structure and snow loads. Corrective actions for complex roof alterations like these are complex and expensive. One concern is the roof rafter on the left side looks like it was repaired after it split. One split roof rafter adds 50% to the loads carried to the roof rafters on either side of it. If one roof rafter spit because it was into strong enough and the rafters to each side get increasing loads odds are the problem can and will spread with the potential for a catastrophic and sudden collapses next time there is a heavy snow accumulation on the roof.