A roof needs doing, so three contractors come out. One paces it, one works off an old drawing, one measures it properly, and the three numbers that come back are not comparable. The cheapest is usually the one who measured the least. Nobody is being dishonest: there is no single measurement any of them can work from, so each produces his own and prices against it. In the twin the roof is one surface with one area, and everyone bids the same piece of work.
The request is a scope the roofing contractor can price without walking the building first. The copilot isolates the roof and reads it off the model: 412 square metres of two-ply modified bitumen, falling at 1:50 to four internal drains, eleven penetrations and thirty-eight metres of parapet. That goes out as a report he prices against, and the details that cost money — the penetrations, the falls, the parapet — are named in the scope before anyone visits. The numbers are read, not estimated.
