Hello folks.

I'm converting an old brick garage into an office. Currently I'm levelling up the brick courses, as the old (rotten) roof achieved its slope via the bricks. These slopes weren't very pretty though and I'd been informed that the correct (modern) way to achieve this slope would be through the use of firrings.

 

Mulling it over I've been wondering if I shouldn't just cut the angle I need into the upper side of each of the rafters though, being oversized at 195mm (I couldn't source 170s at the time, that could be delivered to my area), I wondered if I could sacrifice some of that extra depth, that I don't really need, to achieve the slope instead.

 

Just wanted to know if I might be missing something, like bowing/sag, and that it's actually a bad idea?

 

Thanks in advance.

Depends on the clear span between walls and ceiling joists centres as well as load, as to what depth joists you need. Avoid sag at all costs! 

 

Minimum fall I think is 1:80 for a flat roof. You can still run into issues if your roof sheets ply/osb sag a little and form shallow depressions.

 

The rule of thumb I think is to design for 1:40 and you'll likely end up with a bit less due to build inaccuracies etc but it'll work out right. 

@WoodWorrierwhat is the span between walls..? And what spacing have you got the joists planned at ..?

min 10 deg slope for flat roof, have you given consideration as to how you will cut the firrings? the easiest way for you to do it would be to put the joists down and then cut graded/diminishing firrings which are square/rectangular stock fixed at 90deg to the joists at the correct spacing to give the fall.

Sorry I missed a few of these!
It all worked out in the end. Firrings were cut separately from lengths of C16 on the table saw. Roof seems very stable and strong. And no leaks! Quite happy with how it turned out, thanks for all your input!

cheers