On most roofs, the blanket insulation does its job of insulating the roof and the metal roof sheeting keeps the rain out. On steep roofs, the termination point of the end of the blanket insulation is not critical. But as the roof gets flatter - a simple thing like leaving the blanket insulation too long, can generate a roof leak that will baffle most roofers...
One of the reasons that corrugated roofs are not suited for low pitch applications is the 'dribble effect'.
The 'dribble effect' is the tendency for the rainwater to hang around the downstream edge of the roof sheet and dribble underneath. It can run backwards under the trough quite a bit - especially if there is wind involved.
The gutter on the roof has to be wide enough to catch the dribbles and prevent it from going into the roof cavity - and when this is the case, the roof does not leak.
The exception is when there is a building blanket insulation involved.
I was taught to always to cut the blanket insulation well back from the back of the gutter. I thought it was just to save on materials - until I discovered the real reason why...
... If the water dribbling under the roof sheeting manages to meet the end of the building blanket - a funny thing happens. Capillary action sucks the water up into the blanket and soon the blanket insulation is soaking wet!
Then two things happen.
The wet insulation seeps water into the lap joint of the insulation blanket and drips over the edge into the ceiling cavity - causing a leak that will baffle most roofers.
AND also the wet blanket insulation starts to dry off under warm weather and 'steams' the underside of the roof sheet. Premature corrosion commences and the life span of the metal roof is drastically shortened... even the roofing screws rust away.
This is one of the dangers of using corrugated roofing on low pitched roofs.
Recently, I was called out to a roof in St Peters, Sydney - because there as a leak that could not be tracked down. As the video below shows, it was a typical case of a flat metal corrugated roof with wet blanket insulation underneath...