When conducting a survey on a metal profiled roof with overlapping roof sheets, we use the “rule method” where we insert a 12 inch steel rule into the gap that is present between the upper and lower roof sheets.
A butyl strip seal should be present between the upper and lower sheets where they overlap. This is called a mid-lap joint. The seal is necessary to prevent water getting into the building.
If the rule can be inserted beyond the fixings, it means that no seal is present and that the mid-lap joint is susceptibe to water ingress through wind-driven rain.
It is not common for a butyl sealing strip to be missing across an entire metal roof, but we have seen cases where this is the case. In most cases however, the seal will only be missing in isolated areas, due to it being missed during installation of the roof sheets.
If you are coating a metal roof, or if you are treating cut edge corrosion, it’s important to know whether the butyl seal is present because if it isn’t, it will be necessary to seal the overlapping roof sheet joint during installation of the new coating or treatment.
Whilst we don’t recommend that overlapping roof sheets are sealed as part of a coating or cut edge corrosion treatment project, they definitely should be if no butyl strip is present, but bear in mind that silicone seals and even waterproofing tape can trap mositure and cause underside corrosion, which cannot be treated in future.