Planning the batten supports for trapezoidal steel roofing

Trapezoidal steel sheet is not restricted by a transverse module in the way many tile-effect steel roofings are. This does not, however, mean that the supports can be positioned arbitrarily. The sheet transfers loads between them. Its deflection and load-bearing capacity depend on the height and shape of the profile, the sheet thickness and the number of spans.

Two sheets described as T18 may have different crest widths and cross-section parameters. Likewise, a deeper T35 does not mean that an arbitrarily larger spacing can be assumed in advance. The correct value results from the documentation for the specific product and from a structural check. A table for another manufacturer is not a reliable substitute.

The snow and wind load, the roof pitch and the local roof conditions must be known. The span of the sheet, the manner of its laying and the width of the supports are also significant. The zone at the corner may require different fixing from the central part of the roof slope. The assessment therefore covers the entire surface.

In load-bearing tables, single-span and multi-span systems are distinguished. The more favourable column cannot be selected without reproducing its conditions on site. Both strength and deflection are checked. A roof that deforms excessively may have serviceability problems despite apparently strong sheets.

The battens form the supports for the covering, usually running transversely to the slope. In a typical roof on a truss, the counter-battens run along the rafters. They allow a space to be created beneath the covering and the substructure to be fixed above the underlay layer. Their roles are related but not the same.

The cross-section of the batten is also checked over the span between the rafters or other supports. The sheet alone may have sufficient load-bearing capacity, while a batten that is too slender will deflect. The entire system must therefore be selected: the sheet, the batten, the counter-batten and their connections to the roof structure.

Before installing the substructure, the geometry of the roof trusses is checked. The straightness of the eaves, the diagonals and any local irregularities are verified. Corrections should be made securely and in accordance with the structural design. Random thin packers may shift or hinder correct fixing.

The timber should have the required cross-section, grade and appropriate condition. Damage and cracks in the area of the fasteners are important. Joints of the battens must be positioned in accordance with the support design. A freely hanging contact beneath the edge of the sheet, which requires reliable fixing, should not be left.

The edge of the sheet at the eaves must work together with the flashing that drains water. The overhang of the sheet must not be arbitrary. It depends on the selected detail, the profile and the support. The first batten is positioned after agreeing the position of the gutter and the ventilation inlet, and not solely according to the crew’s habit.

Transverse laps, flashings and locations at roof penetrations may require additional supports. At the ridge, a functioning air outlet must be left. Densifying the substructure must not accidentally close the gap. For openings for larger elements, it must also be checked whether they cut through load-bearing parts.

Fixing the sheet to timber differs from fixing to a steel purlin. The correct screws, their length and drilling capability must be selected. The washer should be pressed down in accordance with the requirements, without crushing the seal and without deforming the sheet. Perpendicular driving of the screw also matters.

Fixing the battens and counter-battens to the roof trusses is a separate task. It cannot be omitted from the calculations, because this is where the forces from wind suction pass through. The number of fixings is not determined solely on the basis of the roof area. Schemes are needed for the relevant zones and connections.

Before laying the sheet, it is worth checking the spacings, cross-sections, evenness and fixing of the substructure. The roofer should confirm that the arrangement matches the ordered profile. The membrane, the clearance of the ventilation channel and the locations of the planned laps are also checked. After the roof slope is covered, access to them will be limited.

The best reference point is a drawing prepared for the specific roof. It allows a deliberate change in spacing to be distinguished from an error. If a different thickness or variant of the trapezoidal sheet has been supplied, the selection of supports must be checked again. A change that looks minor may alter the working conditions of the entire covering.