Planning and costs
Estimating roof costs from the structure to the last flashing
In brief
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.
Start with the roof slope area, not the house
The footprint area does not match the roof area. Roof slopes are pitched, extend beyond the walls and may have dormers. For a simple roof, calculate each plane separately. You need its actual length and width. Compare the dimensions from the design with the completed roof structure before ordering sheets cut to size.
The geometric area is not yet the quantity of material ordered. Add laps and offcuts at roof valleys, corners and penetrations. Do not assume a single waste factor for every roof. Ask for a cutting plan for the roof slopes. On a rectangular plane, a long sheet can be economical, while on a roof with numerous folds, shorter modules can reduce unusable ends.
Divide the cost estimate into layers and elements
First separate the structure from the covering. The first group includes timber or steel, fasteners and carpentry work. The second includes the metal sheet, battens, membranes and flashings. Price the thermal insulation and the vapour control layer in accordance with the design of the building element. Purchasing thick PIR board alone does not yet include the airtight installation of the entire insulation.
Calculate gutters by the length of the eaves, and flashings by the actual edges. Also include corners, outlets, brackets, seals and fasteners in the estimate. For chimneys and windows, check the number of flashing sets. Every additional detail consumes material and time. Two roofs with the same area can therefore have significantly different costs.
Compare square metre of covering with square metre of covering
In a price list you may find a price per square metre total, effective, per sheet or per linear metre. The effective price refers to the covering area after accounting for side laps. When buying long sheets, you still need to check offcuts and any transverse joints separately. The lowest number next to a product does not always give the lowest cost for the roof slope.
A sample modular metal roof tile with a 25 µm polyester coating has a list price dated 28 October 2026 of €8.10 net and €9.96 gross per sheet. A sheet covers 0.798 m². This gives €10.15 net and €12.49 gross per m² of coverage. These are catalogue prices for the material alone, not a shop offer or the cost of a fitted roof. This example shows why the unit must be read together with the amount.
Check what the contractor calls installation
The labour quote should state whether it includes battens, membrane, flashings, gutters and cleaning. Removal of the old covering, repair of the substrate, scaffolding or a lift may be priced separately. For a renovation, hidden damage is difficult to price before the roof is opened up. It is good to establish how additional work is approved and settled.
Also ask about delivery arrangements. Unloading long sheets requires space and the right equipment. The material should not be left waiting in a wet pack. Buying too early can be a false economy when you later have to rent storage or transport the goods again. Match the delivery date to the readiness of the structure and the crew, not just to a promotion.
Build a simple budget for the whole investment
For each item, record the quantity, unit, unit price and value. Add separate lines for transport, unloading and labour. Agree whether all amounts are net or gross. Do not automatically copy the tax rate from another quote. Material delivery and a specific construction service may be settled differently depending on the transaction terms.
Link the budget reserve to actual uncertainty. In a new house, this may be a change to the eaves detail. In an old building, the condition of the timber and the substrate is more significant. Do not use the reserve to cover missing items. First complete the cost estimate, and only then determine the funds for work whose scope cannot yet be reliably established.
Choose a quote you can verify
In the comparison, keep the sheet thickness, type of coating, coverage area and complete set of accessories side by side. Record the price validity period and the expected delivery date. For a substitute, ask whether the batten spacing, fixing method or flashings will change. A cheaper sheet may require additional work that is not visible in the first comparison.
Before approving the purchase, go through the cost estimate with the contractor step by step, from the eaves to the ridge. It should be clear who is responsible for measurement, ordering and receiving the material. Such a review usually gives more than comparing the price of the sheet alone. The final budget should describe a roof ready for use, with completed details and water drainage.
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