Choosing T18 or T35 sheeting for the roof of an open shelter

A carport is open on at least some sides, so wind can act on the roof from below as well. Fixings and anchoring of the structure must suit these conditions. A small area does not mean small forces in every detail. A fixing scheme from an enclosed building should not be copied without checking the difference in loads.

Location, height and surroundings matter. A carport standing against a wall may be exposed to snow accumulation at a specific point. A roof with a low pitch places different demands on connections than a steep slope. This data should reach the designer or the person selecting the covering before comparing T18 and T35 prices.

The designations refer to the profile height, but the full cross-section depends on the manufacturer. T35 has higher ribs than T18, which may provide greater rigidity under otherwise comparable conditions. However, load-bearing capacity also depends on thickness, steel grade and flange width. The name alone does not determine the permissible distance between supports.

T18 may be sufficient for a smaller carport with a suitably designed frame. T35 is worth considering where requirements lead to a higher profile. Both options must be checked in the tables for the specific configuration. Do not buy the thinnest available sheet on the assumption that a larger number of screws will solve the support problem.

The minimum pitch results from the documentation of the selected product and the way the connections are made. A continuous sheet from the top edge to the eaves may have different requirements than a slope with a transverse lap. With a low pitch, the lap length, sealing and direction of water drainage are particularly important. A single value should not be applied to every trapezoidal profile.

The roof should have properly executed side flashings and a wall termination if the carport adjoins a wall. A gutter or other controlled drainage must also be provided. Water should not run incidentally onto the passage, façade or parking space. Drainage is worth planning before the posts are set and the ground is paved.

Condensation may appear on the underside of an uninsulated roof. This does not always mean a leak. The risk depends on humidity, temperature and air exchange. In a carport under which you store sensitive items, this problem must be taken into account. A greater profile height alone does not prevent water vapour from condensing.

The anti-condensation layer can temporarily retain moisture if it is correctly selected and has drying conditions. It is neither thermal insulation nor a sealed membrane. It also requires end details to be carried out in accordance with the instructions. Where clearly better protection is needed, it is worth comparing the full layer build-up or a suitably designed insulated roof sandwich panel.

The screws are selected for the wooden or steel substrate and the expected forces. The edge zones may require a different arrangement than the centre of the roof slope. A distinction must be made between fixing the sheet to the support and joining the sheets to one another. The correct seal and contact pressure are just as important as the screw length. The layout should be recorded before installation.

Determine the sheet lengths after checking the geometry of the completed structure. Allow for the overhang at the eaves and the required finish at the upper edge. A long sheet requires safe transport and handling. On a lightweight canopy, the covering can easily be damaged by point loading when stepping outside the support. The work organisation must be adapted to the chosen material.

Ask for a quote for the same usable area, coating and agreed thickness. Include flashings, fasteners, drainage and transport. If T35 allows a different support arrangement, take this into account only after the design has been confirmed. The difference in sheet price is only one component. What matters is the cost of the entire canopy with the required parameters.

Before purchase, approve the colour and condition of the surface visible from below. In an open canopy, the inner side of the covering is part of the appearance of the structure. Keep the storage and maintenance instructions. A sensible choice between T18 and T35 is based on load-bearing capacity, tightness and conditions of use, and only then on the lowest price per metre.