Fall for 6m garage.

[Ency]

I need to replace a garage roof. The existing garage roof is 6m x 2.5m and is attached to the house on the long length. It has 6m furrings to provide a fall. These are attached on the wall plate and on the opposite outer wall. The 2.5m joists are attached above these furrings. However the furrings only have a fall of 5cm to 0cm. This only gives a fall of 1:120 which is not enough for this 6m length. What is the solution? Is a solution to run additional 6m furrings perpendicular to and above the joists. What fall would these need considering there is already a 5 to 0 fall? Would 6 to 1 in addition be ok. This should then give 1:60 fall if I’ve got the calculation right. The garage would have an EPDM membrane instead of the existing felt.
Any advice would be appreciated.

 

[noseall]

I need to replace a garage roof. The existing garage roof is 6m x 2.5m and is attached to the house on the long length. It has 6m furrings to provide a fall. These are attached on the wall plate and on the opposite outer wall. The 2.5m joists are attached above these furrings. However the furrings only have a fall of 5cm to 0cm. This only gives a fall of 1:120 which is not enough for this 6m length. What is the solution? Is a solution to run additional 6m furrings perpendicular to and above the joists. What fall would these need considering there is already a 5 to 0 fall? Would 6 to 1 in addition be ok. This should then give 1:60 fall if I’ve got the calculation right. The garage would have an EPDM membrane instead of the existing felt.
Any advice would be appreciated.

1:80 is standard.

Lesser fall is ok as long as everything is precise. The lesser the fall the more diligent and skilful all the other jobs need to be. I remember sorting through a load of diminishing strips on site once, so that they worked most favourably for the roof.

 

[^woody^]

You may want to design it to 1:40 to allow for the eventual joist deflection over a few years.

So that's 150mm higher at one end.

Furrings should be no less than 5mm at the narrow end, not zero, and extend to the end of the fascia drip not just the end of the joists.