Double Joisting of Roof on Small Porch Questions answered and a range of solutions on problems and issues encountered with timber framed buildings

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Hi I was wondering if anybody can give me some advice about joinery. A firm of joiners were employed to replace a roof on a small porch. They clearly stated in writing that all roof joists would be removed and replaced with new timbers. Instead they double joisted the roof since the existing roof joist were ok, but they still want the full amount quoted. my questions are Is double joisting better or worse? Is double joisting quicker and easier to do using less timber? Any comments are very much appreciated, Thanks