How do you choose trapezoidal steel sheet by profile and thickness for a finished roof surface?

T18 and T35 can be used in roof solutions, but they are not interchangeable choices made on appearance alone. A taller profile may have different load-bearing capabilities on supports. The correct answer is given by the load table for the actual thickness and steel grade.

Load-bearing capacity depends on several data at once: section, thickness, material, loads and support arrangement. There is no single purlin spacing for every T18. Nor should a thicker sheet be chosen as an automatic way to increase the distance between supports in any case.

In the example of one T18 variant, thicknesses of 0.5–0.7 mm occur. This is the range for this product, not confirmation that every version can be used on a specific roof. Check snow and wind loads and edge zones. The fixing design is part of the solution.

The example T18 has an overall width of 1173 mm and a cover width of 1128 mm. The cover value is used to cover the area because part of the sheet participates in the overlap. For a roof width of 7 m, at least seven such strips are needed: 7/1.128 gives approximately 6.21.

The last sheet will require adjustment according to the edge detail. The length is determined along the roof slope, taking into account the eaves and other connections. Do not buy square metres from the plan area without checking the actual cutting layout.

The minimum pitch depends on the system and the type of joints. A run made from a single sheet is assessed differently from sheets joined along their length. Laps, seals and drainage conditions must be taken into account. The profile name alone does not determine all these details.

Do not confuse percentages with degrees. A 10% slope means a height difference of 10 cm per metre of horizontal projection. It is not an angle of 10°. Where the available roof height is small, it is worth checking the geometry before purchase. Reducing the pitch later may require a different covering.

Felt under the steel sheet can temporarily retain condensation and limit dripping. It must have conditions for subsequent drying out. It is neither thermal insulation nor an unlimited water reservoir. With constant high humidity, an additional layer alone will not solve the problem.

Check the ventilation and how the felt is terminated at the eaves and laps. The requirements depend on the solution. The material should not be laid in such a way that it creates a path for water to be drawn into the wrong place. In a heated roof, a full layer design is needed, not just felt on the steel sheet.

The coating is selected according to the environment and the expected durability. Colour does not confirm corrosion resistance. The entire type of protection and the service conditions matter. Compare samples with the flashings to avoid a difference in appearance on the finished roof.

Cutting to size uses methods in accordance with the material instructions. Do not cut with an angle grinder or any other tool that strongly heats the steel sheet. Swarf is removed as work proceeds. Leftover particles can corrode and cause stains on the covering surface.

In addition to the sheets, the set includes flashings, fastenings and the intended seals. Count the ridge, eaves and side terminations separately. Elements with a nominal length of 2 m may have less coverage after lapping. Do not add up metres without taking joints into account.

Establish transport and unloading. Long sheets must be carried without bending or scratching the coating. On delivery, check that the profile, length and material match. The complete set should follow from the roof drawing. This ensures the price covers a covering that can actually be installed.

For a roof squeezed between other buildings, check the route for bringing the sheets in. The width of the entrance is not enough if long material has to be turned in a narrow passage. Shorter sections may make transport easier but create additional joints. Their cost and permissibility must be calculated before changing the order.