Bad design and specification is all too common in roof cladding systems. Clifford Dyer, director of MCRMA, takes a look at some of the considerations

The performance of any roof cladding systems depends on a number of factors; not least the correct selection of materials, design, detailing and assembly.

Equally the position and shape of the building, all types of loading, and the internal external environments will need consideration.

For the building design team, to consider the plan of the overall roof at an early stage will help incorporate the many performance demands on a profiled roof – particularly at low pitch. By the same token, if the roof’s design and specification is left to a late stage, the result will be performance compromises.

MATERIALS ASSEMBLY
The materials chosen for the roof must be assembled correctly. Of particular importance are slopes and end joints that avoid water and dirt traps. And, for maximum durability, all cut edges of steel sheets should be painted straight after installation.

Roof cladding sheets and their fixings must be strong enough to withstand the worst combinations of wind and snow, as well as imposed and dead loads. Profiled metal cladding manufacturers all provide load span tables for their products. This ensures that the building design engineer can select suitable purlin spacings to accommodate the relevant design loadings.

Poorly constructed roofs will leak as a result of ordinary gravitational forces; poorly secured fastenings and incomplete seals have often been the cause of leakage through profiled metal cladding. This can be overcome by ensuring that the cladding is complete, in that all joints are sealed where necessary and all fastenings are secured properly.

By careful checking of a roof contractor’s drawings prior to fixing, simple errors of design, such as lap jointing flashings that divert water into the building, can be overcome.

When designing a roof to resist rain penetration, it is essential to take account of the prevailing exposure conditions so that the appropriate methods of construction can be improved.

Weathertightness can be improved by increasing:

the pitch of the roof
the length of the end laps
the width of the side laps
In addition, laps can be fully sealed and the number of secondary fixings increased; wider troughs can be used to assist in shedding water, and end laps eliminated or reduced by using longer lengths of sheeting.

RELIABLE SERVICE
Metal roof cladding is designed and manufactured to give many years of reliable service, and will do so if a regular inspection and maintenance programme is put into action. Roof sheeting, fixings, flashings and gutters should be inspected at least once a year for deposits, and any areas of corrosion or damage repaired in accordance with the manufacturer’s maintenance manual. In addition, keeping roof traffic to a minimum will help prolong life.

BEST PERFORMANCE PRACTICE
Using information related to best performance practice is the key. Engineers should design projects to accommodate all loads - imposed and wind - safely. Skill, care and supervision must be used in installation to achieve the stated performance.

The encouragement of manufacturers to achieve high standards of workmanship varies widely. After all, it can be argued, they have no control on site and are simply suppliers in contractual terms.

Some choose a route through product training and registered installers for products that require tight tolerances and intelligent installation. Others simplify the system, provide site servicing or on-the-job tuition, and tools that make life easier for the fixer. What is clear is that high standards in the manufacturer of metal roof and wall cladding materials, and correct design and specification is of utmost importance.

This article was first published in Building Products magazine