One pass from substructure to roof covering, so the shell is priced as a single job.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Build to Roof Estimator tool.
Building to roof is the stage where money runs out quietly. The frame is up, the roof is not covered, and the gap between the two is where a self build stalls. This estimator prices the work up to and including the roof covering, so the number on the page is what it takes to get a building closed and dry. It works in the order the work happens. Footings, then blockwork with its openings deducted, then the roof structure, then the sheeting. Each stage produces its own materials, and each takes its quantities from dimensions you supply rather than from a rate per square metre of floor. What it deliberately does not do is carry on into finishes, services or joinery, because a stage estimate that runs past its stage is a number nobody can act on.
A single storey building ten metres by eight produces a hundred and ninety six metres of wall at a two point six metre height once the eaves overhang is included, less the openings you list. Blockwork for that wall, at a block covering just over a tenth of a square metre, comes to about a thousand eight hundred blocks. A pitched roof over the same plan at a thirty degree pitch gives a roof area of about a hundred and five square metres, which at a cover width of a metre and a sheet length of three metres is around forty sheets counting laps. The eaves perimeter, ten plus eight doubled, is thirty six metres, which is the fascia and gutter order. Four numbers, four deliveries, and none of them derived from the others.
A build to roof estimator prices the stages from the footings up to the roof covering: footing concrete, blockwork and mortar, roof members, sheeting and the eaves trim. It is a stage estimate, not a whole house estimate, and that boundary is the point. Builders use it to answer the question that comes before every other one, which is whether there is enough money to get the building closed and dry.
From the roof area, adjusted for pitch, then divided by the cover width of the sheet you are using and rounded up for the length of each run. Laps are part of the calculation, because a sheet that overhangs the eaves or laps the next one covers less than its nominal width. Enter the cover width the manufacturer states and the sheet length you will order, and the count comes back in whole sheets rather than square metres.
From the plan dimensions of the building, since the eaves perimeter follows the wall line plus whatever overhang you set. It is not the same as the roof perimeter at the pitch, and it is not the wall perimeter at floor level, because both differ once there is an overhang. Fascia, gutter and soffit are all ordered from this figure, which is why a roof area alone cannot produce them.
It stops at the roof covering. Windows, doors, finishes, plumbing, electrical, ceilings and external works are outside it, as are scaffolding, plant and labour. The stage boundary is deliberate: a number that runs into finishes has to make assumptions about specification, and those assumptions are where an estimate stops being checkable. The next stage is priced with its own tools on the same building dimensions.
Because cement content is a property of the mix, not of the concrete volume. A one to two to four mix and a one to one and a half to three mix produce visibly different cement quantities from the same cubic metres, while sand and aggregate move in the other direction. Change the mix on the page and the bag count moves with it, which is what lets you compare a specification change against its material cost before you commit to it.