Width or Span is the gable side of the roof, which is used in calculating rafter length and cost.Ģ) Enter overhang length: Most houses have overhangs (soffits) which are needed to protect the house exterior walls and foundation from water run-off from the roof. Use the diagram on the right to correctly enter your roof dimensions – this will help you calculate board length more accurately.ġ) Enter house footprint: Length is the roof eave ( gutter) side.