Roof coverings
T18 or T35 trapezoidal steel sheet for a roof?
In brief
Choosing between T18 and T35 is not about which profile is better, but which suits the specific roof. Check the span, loads, pitch and the full geometry of the product, because the name alone does not determine load capacity or minimum pitch. Compare technical data sheets and complete roof variants, including coverage width and overlap edges. Note whether the intended use is roofing or cladding, and which side is coated. Leave the support spacing decision to the designer, and when replacing sheets, compare the entire cross-section with the existing material.
What do the symbols T18 and T35 really mean?
The symbols refer to the profile family and its nominal height, but do not describe the entire section. Field widths, corrugation spacing, overlap edges and cover width may differ. Even the actual height is not always exactly what the abbreviated name suggests. A technical drawing of the specific product is therefore required.
It is also worth checking the roof and façade variants. They may have a different orientation of the visible surfaces or decorative side. Not every sheet may simply be turned over. When ordering, the application, colour and coated side must be specified. This avoids a situation in which the delivered steel sheet has the correct symbol but does not match the planned layout. When replacing several sheets, the entire section must be compared with the old material. An identical symbol does not guarantee that the corrugations or the side overlap will align. It is safest to use the drawing and a joint sample. Ordering by height alone may result in reworking a larger part of the roof slope.
How does profile height affect support?
A higher corrugation may increase the stiffness of the section. However, this alone does not allow a larger spacing of battens or purlins to be assumed. Load-bearing capacity also depends on steel thickness, its properties, geometry and the number of spans. Snow and wind loads and the method of fixing to the structure are also significant.
The choice should follow from the design or from checking the tables for the specific product. A thicker T18 should not be compared with a thin T35 by height alone. In some situations, closer support of a lower profile may be justified; in others, a higher trapezoidal profile will be more favourable. The whole solution is assessed, including the costs of the structure.
Is T35 suitable for a lower pitch?
Some T35 variants are approved for lower pitches than certain T18 variants. However, this is not a universal rule arising from the name alone. Manufacturers specify their own requirements for the profile, overlaps and any seals. These must be checked in the documentation for the selected material.
On a low-pitched roof, pay particular attention to the joints between sheets, how water drains and the detailing around obstructions. A deeper profile cannot compensate for unsuitable roof geometry. Adding sealant does not make it acceptable to go below the manufacturer’s minimum pitch. If the roof does not meet the requirements for the chosen sheet, use a suitable alternative or revise the design.
How will each profile look on a house or garage?
A lower profile usually gives a more delicate pattern, while a higher one emphasises the shadow more strongly. On a low garage, the difference will be visible from a short distance. On a tall roof slope of a house, it may matter less than the colour and shape of the flashings. Samples should be viewed at a similar angle, preferably next to façade materials.
Do not choose a profile on aesthetics before checking the technical requirements. If both variants meet the requirements, the decision can be made on appearance and cost. If only one suits the support spacing and pitch, choosing the other requires an agreed change to the layout. A more delicate appearance does not justify using a material outside its permissible range.
Which variant will be cheaper across the whole roof?
T18 may have a lower price per metre of effective coverage, but the result depends on thickness and coating. T35 usually has a different effective width, so the same unit must be compared. The price per total metre is often particularly misleading. It is worth calculating the exact quantities of sheets, laps and flashings for both variants.
Include the support structure, transport and labour in the comparison. A more expensive profile can be advantageous only where the project confirms a saving elsewhere. This must not be assumed in advance. If the roof has already been built, changing the purlin spacing also incurs a cost. The cheapest sheet does not always give the lowest total for the complete covering.
What data should be provided for selection?
Prepare the length and width of the roof slope, the pitch, the support spacing and information about the building’s location. Its intended use and the layer build-up are also needed. For a heated room, insulation and moisture must be resolved. For a canopy, condensation will be important among other things. A single trapezoidal sheet does not replace these decisions.
Ask for the full variant, thickness and fixing conditions to be specified. Check that the offer matches the documentation. If a seller or contractor gives a single universal support spacing for every T18 or T35, it is worth requesting the basis. The selection is correct only when the figures relate to the specific section and the actual roof conditions.
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