Box profile metal sheet maximum unsupported overhang?

[Dunc]

I have a 45° pitch box profile metal roof which will intersect an EDPM-covered flat roof.

The metal roof will be supported on 25x50mm counter battens and battens (to provide ventilation) on top of sarking boards.

The EDPM laps up the sarking a good 500mm and my roofing membremane on the sarking laps onto this.

To avoid screwing through the EDPM too low, how far up can I set the battens? i.e. how long can the unsupported end of the box profile sheet be?

I can't find anything in manufacturer installation guides.

[Dunc]

Anyone able to help with this? Thanks.

[Mr Punter]

If the metal roof manufacturer is not willing to assist you may just need to make an educated guess.  @saveasteading and @Gus Potter may give you a steer.

[Gus Potter]

On 22/07/2025 at 18:59, Dunc said:

To avoid screwing through the EDPM too low, how far up can I set the battens? i.e. how long can the unsupported end of the box profile sheet be?

I can't find anything in manufacturer installation guides.

Yes you'll struggle to find bespoke details like this. 

 

Let's say we kind of want to follow the ethos of the building regulations, introduce some practicality and so on. 

 

The regs often require a minimum vertical upstand on a flashing of 150mm. You want to be able to keep the roof clean and get your hand in under the edge of the roof sheet to clear leaves and keep any ventilation clear.. Say measured vertically we need 75mm for getting your hand in to clean.  Now we have a vertical measurement of 150mm + 75 mm = 225 mm. That translates to a slope length measured up the 45 degree roof slope of 1.41 x 225 = say 320mm and that is where your first batten will be from the EPDM when measured up the slope. 

 

As the box profile does not go all the way down to the EPDM the length of box profile cantilever is 150 x 1.41 = 212 mm not a lot for say a Steadmans 32mm box profile. I could calculate it out but I think you'll be fine under normal conditions to have a cantilever of 300mm. Provided no one stands on it at the leading edge of the cantilever. 

 

The weak spot is maybe at the roof edges where you get high wind suction. One belt and braces way is just to put an one extra fixing at the bottom of any gable verge cover flashing to prevent the very edge of the roof sheet pealing off.