Complex sentence: The third addition was completed before 1882, when the front roof was raised to a full story in height by replacing the original common rafters with new vertically-sawn two inch by ten inch common rafters.
Split into simpler sentences: The third addition was completed sometime around 1882, when the front roof was raised to a full story in height. The original common rafters were replaced with new vertically-sawn two inch by ten inch common rafters.