Start by establishing exactly what the contractor is pricing. The final cost is shaped not only by the rate per square metre but also by the roof's shape, the preparation of the substrate and the number of details such as valleys or chimneys. Ask whether the quote covers just laying the sheets or a broader service including the battens, underlay and flashings. Clarify what surface area forms the basis for settlement – where there are many cuts, the metres of purchased sheeting may differ from the metres of finished covering. Request a clear indication of net or gross amounts and units.

First establish whether the building will be heated continuously, periodically or not at all, as this determines the choice of envelope. Also check the working temperature, humidity, substance emissions and the method of cleaning the interior. A dry warehouse, a workshop and a facility with frequent interior washing place different demands on the building envelope, and universal sheeting in an attractive colour need not withstand an aggressive environment. A sandwich panel combines facings with an insulating core and reduces the number of successive operations, but the benefit depends on preparation of the structure and delivery of components in the correct sequence.