The choice of trapezoidal sheet for a roof depends on matching the profile to the supports, loads and building conditions, not on the number in the name alone. When comparing T18 and T35, check the steel thickness, cross-section geometry, coating, pitch and method of joining sheets. The designations refer to families of sheets with a specific rib height, but they do not guarantee identical geometry across suppliers. Differences in flange width, pitch and cover width can affect the cutting plan and number of sheets, so a specific product data sheet must be used in the design.

The choice between T18 and T35 depends primarily on the support spacing and loads, not on the roof area alone. To decide, establish the dimensions, pitch and support layout, then check the distance over which the sheet spans between purlins or battens. Local snow and wind loads, fixings, anchoring and drainage must also be taken into account. Condensation should be addressed separately. The design of the whole structure, including columns and connections, remains the designer's task.

Begin by marking out the zone the roof is actually meant to shelter, rather than relying on the terrace's floor area. Indicate the table, chairs and main passage, since wind can carry raindrops under the open sides. Match the size, pitch, structure and drainage to this. T18 or T35 profiles form a lightweight covering, while a sandwich roof panel limits heat loss through the roof surface itself. Keep in mind daylight in the room, condensation beneath the metal and the junction with the wall. Load capacity and fixings need calculation, not floor area alone.