When is it worth switching from T18 to T35 trapezoidal steel sheets on an existing roof?

The T18 range in this example comes in sheet thicknesses from 0.5 to 0.7 mm, and the selected T35 range from 0.5 to 1.0 mm. Availability depends on the combination of thickness, finish and length. Confirm the exact specification when ordering.

A deeper profile can give a sheet greater stiffness and load capacity in a given support arrangement. This can matter on roofs with wider support spacing. Check the result for the actual sheet thickness, steel grade and section geometry. There is no fixed factor for converting the capacity of T18 into that of T35.

Read the load tables for the number of spans, bearing width, direction of loading and permissible deflection. Both the fixings and their connection to the supports must transfer the design loads. Even with a deeper sheet profile, the supporting structure or the fixings may govern the capacity of the roof.

Take a slope 10.80 m wide. At 1.128 m cover width, ten T18 sheets laid side by side cover 11.28 m. At 1.05 m cover width, the same slope needs eleven T35 sheets, covering 11.55 m. The last sheet in each layout needs trimming to fit. It is the effective cover width that determines the number of runs across the slope.

If each sheet is 6 m long, the order represents 67.68 m² of effective cover for T18 or 69.30 m² for T35. The rectangular roof slope measures 64.80 m². The example assumes no end laps or openings. For a price comparison, multiply each rate by the quantity required for that profile.

Inspect and measure the supporting grid. Corrosion, cracking and local deflection need attention whatever profile is fitted. Check that bearing widths and spacing suit the new sheet. Its fixing positions may differ, so assess the layout before deciding whether any existing holes can be used.

Assess existing fasteners and sealing washers independently of sheet condition. New fixings must suit the substrate and the loads to be transferred. Screw length alone cannot establish suitability. Check the condition of timber supports or, for steel supports, the thickness of the section receiving the fixing.

Inspect junctions at chimneys, walls, ridges and eaves. A taller rib may need a different flashing shape or profiled seal. A T18 profile filler will not fit T35 simply by being compressed more tightly. Match closures to the actual section and junction detail. At a wall, measure the height available for the flashing upstand. Changing the level of the roof covering may require the wall junction to be remade. Establish this before stripping the old roof.

The roof edge may also move relative to the gutter. Trace the path of runoff and check ventilation below the covering. If the project includes new insulation, assess heat and moisture across the whole build-up along with the change in rib height.

Compare two workable specifications, including the supports, fasteners and flashings. Keeping T18 may make sense if it meets the requirements and fits existing details. Where the design calls for T35, include every consequential change in the estimate. The sheet price is just one item in a roof refurbishment.

Record the exact profile, steel thickness and lengths fitted. Recommended sheet lengths in the ranges compared can reach 9 m for T18 and 12 m for T35 at 0.7 mm thickness. These are sheet lengths for manufacture and handling. The permissible unsupported span requires a separate structural check.