In the scene where Dan and Mark are on the roof putting up the decorations in the episode "White Trash Christmas" did they use the actual roof of the house that represents Roseanne's house or did they re-buiild a replica of the original house?

I presume it was done in a studio wasnt on top of the "Roseanne house" it was custom made studio house i guess.Brilliantly done though i must say,i love that episode!!