The influence of thermal movement on the performance of flashings

Whilst thermal movement is not normally a problem on the traditional pierced-fix profiles, due mainly to the fact that individual lengths are limited to between fourteen and sixteen metres by transport constraints, it requires consideration with concealed-fix profiles.

It is important to appreciate that the surface temperature of cladding and flashings is greater than the ambient during the day and under certain circumstances less than ambient at night. Dark coloured cladding laid over insulation can reach a temperature in excess of 80°C (50°C for very light colours) so it is possible that cladding and flashings can be required to operate over a temperature range of 80-90°C. Galvanized and aluminium/zinc coated products darken with age to a dark grey.  If the material is steel based this equates to 1mm/m, stainless steel 1.5mm/m and aluminium 2.0mm/m. A 50m length of cladding will move 15-25mm at either end on a flat roof compared to 1.0-1.5mm on a standard 3.0m length of steel based flashing.

Transverse flashings; these are comprised of ridge, apex, headwall, valley, transition and drip flashings. Whilst transverse thermal movement across cladding profiles is absorbed by the geometry of the profile these flashing are subject to linear movement which has to be accommodated by the fasteners attaching them to the cladding/structure together with transverse forces from longitudinal movements from the cladding i.e. biaxial loading. Assuming a roof slope of 20m there will be movement of ±10mm on either side of a ridge flashing, this could be sufficient to propagate fatigue cracking at the apex of the ridge and/or adjacent to the fasteners. On roofs with bull-nosed eaves the bullnose constitutes a fixed point and all thermal movement will be towards the ridge, apex or headwall. On concealed-fix profiles most manufacturers require the use of sliding connector brackets at the ridge, apex and headwall.

Longitudinal flashings; these are comprised of sidewall, apron and corner flashings. With the exception of corner flashing the thermal movement of the flashing is effectively equal to that of the cladding.

An important point to remember is that the vertical portion of sidewall and apron flashings are not to be positively fixed to the adjacent structure. This is necessary to accommodate the differential thermal movement between the flashing and structure. It is this differential movement that makes the use of membrane type flashings inoperable for this application.

Gable and barge flashings; these flashings are a hybrid of the previous two types. When attached to the gable cladding they are transverse and longitudinal when attached to the roof cladding. It is therefore of paramount importance that the connection of the flashings to the roof cladding is of a sliding nature capable of accommodating the anticipated thermal movement, both expansion and contraction.

The common practice of pierce fixing the outside edge of concealed-fix profiles, prior to rigidly attaching the flashing is not acceptable as it interferes with the performance of the roof profile and shearing of the fasteners attaching the flashing to the cladding.

This is also the portion of the roof subjected to the most severe wind loading.