Waterproofing the join on a dual pitch roof

The join between the upper and lower slopes of a dual-pitch roof can be a problem area for leakage and corrosion if designed inadequately. These joins are common on colonial style homes and on extensions, where a metal skillion roof is added to an existing tiled roof.

Where both roof sections are metal, many tradesmen simply fix the top sheets as tightly as possible against the bottom sheets (Figure 1). However, this type of join is prone to leakage because of insufficient overlap or "back distance". On a shallow roof, the back distance needs to be at least 250mm.

Rainwater travels faster down the steeper upper slope than on the flatter lower slope. At the join, rainwater depth is increased because of the dam effect of faster water merging with slower water. Also, the flatter the lower slope the more chance of wind driving water uphill into the back distance and over the turn-up.

If the top sheet touches the lower sheet, the protective coating of the roofing can be removed through rubbing caused by thermal expansion. This can lead to premature corrosion.

Stramit recommends that flashings be used for all such joins (Figure 2). A "change of pitch" or "transition" flashing provides a much greater back distance, protecting against water intrusion.

An advantage for the roofer is that less care is needed in aligning the upper and lower sheets. Although some roofers claim that flashing "doesn't look right" at the join, many people prefer the elegant appearance of a roof with a change of pitch flashing.