You can only calculate the roof cost once all layers and works have been itemised. The price of the metal sheet per square metre is just one element of the calculation. You also need to add the structure, insulation, flashings, drainage, transport and installation. To compare quotes, you need the same quantities and scope of works. Below we show where to get the data and how to spot items that are easily overlooked. The building footprint does not correspond to the roof area. The roof slopes are pitched, extend beyond the walls and may have dormers. For a simple roof, you calculate each plane separately. You need its actual length and width. The dimensions from the design must be compared with the completed roof truss before ordering sheets cut to size.

Compare the cost of a roof as a complete system, not the price of the covering per metre. Before you set offers side by side, establish a common roof standard: the same plan, pitches, dimensions and list of openings, plus the scope of insulation, drainage and accessories. Check whether the quotes cover the same layers and works – if one includes only the covering while the other also covers removal and the substructure, their totals are not comparable. Pay attention to the unit of measurement and how waste is calculated, because a lower price per metre does not always mean cheaper covering.

The price of a wall insulated panel and a roof insulated panel should be compared within a specific order. The profile, facings, core and connection details differ. The square metre alone does not describe the entire expense. First count the elements for both surfaces, then add flashings, fixings, transport and unloading. Prepare separate lists for the walls and the roof. On each, record the panel type, core, thickness, sheet parameters and coating. State the thermal requirements and other properties adopted in the project. The same core thickness does not make a wall panel and a roof panel equivalent items. Their geometry and method of use are different.