The Design of the flashing
The flashing extends underneath the upper roof 'A' with a return fold, and then over the top of the lower roof 'B' with a break to suit the
roofing profile. To minimise the problem of wind driving water up-hill the top ends of the lower roof should be turned up.
On the overlap 'B', the break has been detailed as notched into the roof profile. Whilst this is not compulsory, it does give greater
protection from water blowing back under the flashing and aesthetically adds to the detail.
Typical size for the underlap 'A' would be 150mm minimum with a 20mm return fold; the overlap 'B' would be 150mm minimum with a break to
suit the profile (in this case 20mm to suit corrugated iron). These measurements are commonly accepted by the industry as standard sizes for
this type of detail. In saying this, there is no direct reference to the sizes for this style of flashing in SAA HB39-1997; the closest
reference would be for a sloping apron flashing at 30 degrees which has a recommended minimum cover of 150mm.
Where the detail is exposed to high winds or the upper roof is a low pitch then consideration should be given to increasing the underlap
Measurements for the CAD detail available for download:
Overlap 'B' - 202mm (overlapping the bottom sheet by 160mm)
Underlap 'A': 200mm
As with a mansard flashing, fastener penetration is an issue with this style of flashing. Firstly, the principle behind the return fold is to
prevent water running back up the flashing and into the roof space (by the wind driving water uphill or capillary action). Many roofing
contractors run the underlap 'A' back to the first purlin (as detailed above) to provide support for the flashing. When fasteners are installed,
they penetrate the flashing and the principle of having a return fold is diminished. There are detail examples that show the return fold
resting on the purlin with the fastener located just above. My biggest concern with resting the return fold on the purlin would be the
possibility of damage from foot traffic, with limited support being given to the underlap 'A' and the possibility of it dislodging under load.
The tradesman would also have little tolerance in fastening to the purlin and the strength would be diminished by fastening to the edge of the
Considering these issues, the alternative detail uses an extra support for the underlap 'A' and a separate purlin for the fixing of the roof
sheeting. For example, you could use a 70x45 purlin on the flat and a 75x35 on the flat as the support and the 10mm difference would
accommodate the return fold (the allowance for the return fold would prevent it from becoming squashed when the upper roof sheets were
Change of pitch without flashing
For aesthetic reasons, transitions are sometimes detailed without a change of pitch flashing. If this is the case you need to consider the
Reference from BlueScope Steel CTB-8
Under no circumstances should the overlapping sheet be simply fastened firmly down onto the underlying sheet, in the absence of a transition
flashing, in an attempt to waterproof the transition joint. Thermally induced stresses will lead to coating removal of the underlaying sheet at
the point of contact and thus can significantly reduce the life of the sheeting in this area.
The change of pitch is a relatively simple detail. The option that I have put forward addresses some of the issues associated with this detail
e.g. fastener penetration and flashing support.
The download provided can be imported into your CAD drawing, saving you time and making your plans more precise.