T18 or T35 trapezoidal sheet: which profile suits your roof?

An illustrative T18 version is 18 mm high, with 1128 mm cover width and 1173 mm overall width. One T35 version measures 35 mm, 1050 mm and 1090 mm respectively. Narrower and wider versions also exist. Specify the complete cross-section when ordering: rib height alone does not define the sheet.

Available thicknesses differ. The T18 data sheet lists 0.5–0.7 mm, while the T35 sheet lists 0.5–1.0 mm. That does not mean the thickest version is always necessary. First check the loads and support arrangement assumed in the design.

Two profiles called T18 may have different cover widths, rib spacing and edge shapes. The group designation is not one common technical drawing. The same applies to T35. Dimensions, load capacity and price must relate to the exact version, especially when repairing an existing covering.

T35 cannot be assumed almost twice as strong simply because 35 is nearly twice 18. The whole sheet cross-section works together. Flange widths, thickness, steel grade and load direction matter. Tables also distinguish single-span and multi-span arrangements.

Check both load capacity and deflection. A sheet may satisfy the first condition but exceed the selected deformation limit. Do not mix design loads with characteristic loads. The designer should use a table for the actual product, not any T35 table found online.

For a simple roof slope measuring 10 m along the eaves, T18 with 1.128 m cover width needs nine sheet runs: the division gives 8.87. T35 with 1.050 m cover width needs ten runs, because the result is 9.52. This is an illustrative geometry calculation without detailed verge arrangements.

More runs do not necessarily mean a worse choice. A taller profile may work with a different supporting structure. On the other hand, sheet count affects labour and the final cut. Calculate the whole roof rather than comparing one linear metre of each material.

Check the sheet layout at the final flashing too. A very narrow edge strip may be difficult to fix correctly. A different starting position can then be preferable. Decide this on the drawing, not after nearly the whole slope has been laid.

For illustration, give both profiles the same price of PLN 41 excluding VAT per linear metre. At 1.128 m cover width this is about PLN 36.35 excluding VAT per m² for T18. At 1.050 m it is about PLN 39.05 excluding VAT per m² for T35. Equal linear-metre prices therefore produce different coverage costs. This is a worked example for the stated widths, excluding installation and accessories, not a current sales offer.

For 100 m² of effective coverage, the example difference in sheet cost is about PLN 270 excluding VAT. That is not the full difference in structural cost. Include cutting, supports and accessories. Also avoid comparing PS with another coating if you want to isolate the effect of the profile itself.

Some roof versions of both profiles are intended for pitches from 6°. This does not mean every T35 can be used on a flatter roof than T18. Other versions may have different requirements. Overlaps, seals and installation details also matter. Rib height does not replace checking the complete solution.

Low pitches need particularly careful drainage checks, including deflection, sheet lengths and roof penetrations. Changing profile cannot correct a poorly formed roof slope. If the design uses a lower pitch, choose a solution approved for those actual conditions.

A taller profile also needs matching seals and details. Do not use a T18 profile filler with T35 simply because it is flexible. A mismatch can create gaps or lift the flashing. Include accessories when comparing both options.

T18 can be a reasonable starting point for a simple roof with appropriately spaced supports. Consider T35 where different structural performance or a more pronounced roof profile is wanted. Base the final decision on calculations and the cost of the whole assembly.

Obtain two quotations for the same coating and thickness, then examine supporting-structure options. Check that seals and flashings fit the selected cross-section. Give the installer the instructions before ordering. The move from T18 to T35 then becomes an informed choice rather than an arbitrary substitution.

That cannot be established without the table, loads and structural arrangement. Greater height is only one parameter.

No. Compare the actual models. T18 alone does not specify width or the complete geometry.