Each of these was run through the estimator on this site at the inputs the job actually had. The left figure is what a rate per square metre would have said. The right one is the invoice.
Semi-detached, full strip and reroof
Victorian terrace, three bedrooms
Footprint
96 m2
Pitch
40 deg
Roof area
125.3 m2
Rate only
7,269
Invoiced
11,349
Days
7
Four thousand of the difference is tear off, waste and the scaffold. None of it is optional and none of it is in a rate per square metre.
Natural slate, hipped roof with two dormers
1930s detached
Footprint
138 m2
Pitch
30 deg
Roof area
159.4 m2
Rate only
15,297
Invoiced
26,571
Days
14
Two dormers and four hips carried a 26 percent complexity uplift. The rate-only figure was out by more than eleven thousand, which is 74 percent, and the shape of the roof is the whole reason.
Concrete interlocking, single storey extension
Rear extension
Footprint
34 m2
Pitch
25 deg
Roof area
37.5 m2
Rate only
1,576
Invoiced
2,331
Days
3
New build, so no tear off at all. Even with that line at zero the scaffold and the waste still added 48 percent.
Standing seam zinc, low pitch over a garden room
New build, garden room
Footprint
42 m2
Pitch
15 deg
Roof area
43.5 m2
Rate only
5,740
Invoiced
7,828
Days
6
The smallest gap on this page, 36 percent, because zinc is expensive per square metre and the fixed costs are a smaller share of a dear job.
EPDM single ply, flat roof replacement
Garage and utility
Footprint
58 m2
Pitch
2 deg
Roof area
58 m2
Rate only
3,946
Invoiced
5,657
Days
4
Footprint 58, roof area 58. A flat roof is the only one where those two numbers agree, and it is why a pitched roof priced off its footprint is always short.