Stephen Chapman, technical consultant of rolled architectural zinc specialist elZinc, explains how to achieve best practice standing seam roofing
What is standing seam roofing?
Traditional standing seams date back to medieval times when they were originally used on ecclesiastical buildings. Nowadays, standing seam roofs still use malleable metals which are profiled into standing seam trays and then welted in situ either manually or with profiling machines.
Standing seam roofing can be used for any building type and any project where budget constraints aren’t too severe. Its main advantages are aesthetic appeal, versatility of use on different roof forms, durability and the use of weathering materials that don’t require a painted barrier. Zinc is the most popular material for this roofing, followed by copper, aluminium and stainless steel.
The standing seam joint
A typical joint requires 70mm of material to make and is formed by seaming together profiled trays of zinc running longitudinally from ridge to eaves. A small gap at the base forms automatically and allows for lateral thermal expansion. Joints are formed with a double lock that first links the trays horizontally and then folds the seam down to a vertical position. Seam centre dimensions normally range from 430-600mm. Each tray is anchored by a combination of fixed clips and sliding clips that allows longitudinal expansion.
Choosing the right cross joint
Cross joints are needed to introduce expansion joints on large roofs or around details such as chimneys, windows and other roof protuberances. There are several different common joint types (see box right) which vary in terms of complexity. The main factor in determining which detail should be specified is the degree of roof pitch, with different joints nominally considered suitable for different pitches. Another important factor of course is whether the joint needs to function as an expansion joint as not all of them can.
Other site-specific issues are roof orientation and weather conditions in particularly exposed locations – it’s always a good idea to look at the installation site with an experienced installer and take all these factors into account when determining joint type. Useful guidance on UK installations is also available from the Federation of Traditional Metal Roofing Contractors.
A combination of fixed and sliding clips is used to anchor the roofing trays to the substrate while allowing the zinc to expand and contract. This thermal movement is accommodated by a gap in the detail at the foot and at the head of the trays.
The position of the fixed clip zone depends on the degree of roof pitch. At low pitches such as 3° roofing trays can be anchored in the middle but as the pitch increases they need to be anchored progressively further up. By 30°, for example, they should be hung from a band of fixed clips positioned at the top in order to prevent the trays from buckling when they expand up the roof.
Dealing with wind loading
Properly installed standing seam roofing is suitable for the windiest of sites. As wind loads are transferred from the metal sheeting to the substrate and the structure of the building via the clips within the seams, the heavier the loading the more clips per square metre are needed. In addition, the bay width of the trays needs to be narrowed in windy locations, otherwise an unwelcome fluttering noise can be generated by the movement of the pans of the trays or, at worst, the standing seams can be lifted.
Higher wind loads therefore entail either more clips per linear metre, or a reduction in the distance between the standing seams or both. As wind loads are generally heaviest along the edges of the roof and at the corners, these are the areas where more clips are most likely to be needed. In particularly windy sites, it may also be advisable to increase the thickness of the metal itself from a typical 0.65-0.7mm to 0.8mm for zinc.
Always consider the implications of the pitch and environmental conditions when specifying the roof details and the thickness of the metal. Architects also need to ensure the roof can drain properly so that problems aren’t encountered during installation.
A not infrequent issue is low-pitched valley gutters that can cause particular difficulties if no proper provision has been made to recess them into the roof substrate. The correct ventilation of a ‘cold’ roof, or the proper choice, location and installation of vapour barriers in a ‘warm’ roof, are also of paramount importance. From a visual point of view, drawings need to set out the position of seams, especially on facades, to make sure these achieve the desired aesthetic effect.
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