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Mansard Flashing Detail
last updated 25/07/16
Summary of this page

Flashing a mansard roof can be achieved by using a change of pitch flashing. An alternative to the flashing is to use a full length sheet and cut the ribs at the change of pitch. The gaps are closed by using purpose made rib caps and sealing with an approved sealant.

Irrespective of what style is used, the aim is to provide a long-lasting watertight solution.

In this article I look at the most common style, "The change of pitch flashing."

 
Mansard Flashing
Download CAD versions of this flashing

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.

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 'A'.

When detailing the CAD download for this flashing, the dimensions I used were 150mm for the overlap 'B' and 222mm for the underlap 'A'. The configuration of a 22 degree lower roof and a 5 degree upper roof meant that 150mm for the underlap 'B' would not be enough (we could have easily extended the underlap measurement to 250mm).


click to enlarge

click to enlarge

Download CAD versions of this detail
Download PDF versions of this detail

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 'B' back to the first purlin 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 'B' 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 purlin.

Considering these issues, my detail provided 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 fastened).

The overlap 'B' as mentioned is typically 150mm but in very severe pitched roofs(e.g. almost vertical) this can be reduced to as little as 50mm.

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 I think it adds to the detail.

To minimise the problem of wind driving water up-hill the top ends of the lower roof should be turned up (weathered).

Summary
The Mansard detail 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. I welcome any comments or suggestions you may have on these details - contact form.

 


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Technical Resources
  1. Material Selection
  2. Acceptability of Drainage from an upper surface to a lower metal surface
  3. Corrosion Technical Bulletin CTB 8 - Building Applications
For a complete list of BlueScope Steel Technical Bulletins click here

Resource Acknowledgements
Source: SteelSelect - author Richard Michael

Publication Details: November 05

Authors Disclaimers:  The information and advice contained in this article is of a general nature only, and has not been prepared with your specific needs in mind. You should always obtain specialist advice to ensure that the materials, approach and techniques referred to in this article meet your specific requirements. SteelSelect makes no warranty as to the accuracy, completeness or reliability of any estimates, opinions or other information contained in this article, and to the maximum extent permitted by law, SteelSelect disclaims all liability and responsibility for any loss or damage, direct or indirect, which may be suffered by any person acting in reliance on anything contained in or omitted from this document.