Choice of covering
T18 and T35 roof sheets: matching the profile to the roof
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.
What does the profile name really mean?
The designations T18 and T35 refer to families of sheets with a specific profile height. They do not, however, guarantee identical geometry across all suppliers. Flange width, pitch and cover width may differ. Within a similar name there may also be roof and façade variants. The specific product data sheet must therefore be incorporated into the design.
Load tables for one profile must not be mixed with the dimensions of another. A similar appearance of the sheet is not sufficient confirmation of interchangeability. When changing supplier, it is worth rechecking the cutting layout and the number of pieces. A difference of a few centimetres in effective width can change the order for the entire roof slope, the position of the last sheet and the amount of waste.
When to consider T18 and when T35?
T18 is worth considering for garages, carports and small canopies with suitably prepared supports. The low profile gives a calmer appearance and may be sufficient for the design loads. It is not, however, a universal solution for every canopy. An open structure may be heavily exposed to wind despite the small roof area.
The deeper T35 profile may allow different performance parameters of the sheet. A single permissible purlin spacing should not be derived from this. The same profile in thinner steel or with a different support arrangement will behave differently. It is worth comparing complete structural and covering variants, not merely the rib depth and the price per m² of sheet.
What role does steel thickness play?
The core thickness affects load-bearing capacity, deflection and susceptibility to damage during handling. It should be stated unambiguously in the order. It must not be equated with a dimension covering all layers. The steel grade is also important. Sheets of similar thickness may have different properties if they are made from a different material.
For roofs requiring calculations, a table corresponding to the thickness and static system is needed. It takes into account not only strength but also permissible deflection. A very flexible sheet may cause serviceability problems before reaching the load-bearing limit. Additional supports or thicker sheet are often more sensible than choosing the cheapest item in the price list.
How to choose a coating for the environment?
Conditions by a busy road, at an industrial plant or in an agricultural building may differ from a quiet residential environment. Moisture, contamination and the possibility of regular cleaning are significant. The coating is selected on the basis of intended use and documentation. A greater paint thickness alone does not ensure an advantage in all environments.
Both sides of the sheet must be considered. In an open shelter, the underside is visible and exposed to external air. In an agricultural building, it may be affected by a humid or aggressive interior environment. The underside coating does not always have the same decorative properties as the top coating. These conditions should be clarified before ordering.
Do the pitch and sheet length change the choice?
The minimum pitch depends on the specific system and installation conditions. Joining sheets along their length usually requires additional analysis of the overlap and sealing. A value known from another sheet should not be assumed. At a low pitch, the evenness of the substrate, deflections and the method of water drainage at the eaves are particularly important.
A long sheet reduces the number of transverse joints but makes transport and handling more difficult. It may require a different organisation of installation so that it is not bent under its own weight. Therefore, the maximum available length is not necessarily the best ordering length. The quote should take into account unloading, access to the site and the storage location for the material.
How can you tell which offer is genuinely better?
List the profile, thickness, coating and effective width in one table. Then add flashings, fastenings, sealants and delivery. The price per m² of total surface area is sometimes lower than the price per m² of covering for the same product. Without conversion, it is easy to choose a seemingly cheaper offer. It is also worth checking the maintenance rules and the scope of the warranty.
Before purchase, provide the seller with the support spacing, pitch, dimensions and intended use of the building. If structural data are missing, their verification is needed. Neither T18 nor T35 solves the problem of unknown structural load-bearing capacity. A well-selected profile should ensure safe performance and tightness at a cost justified for the specific roof.
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