Flat roof fall

[diybsp]

Hi, hoping someone here can offer some advice.

I'm building a flat roof extension with the help of a chippie to do the joists, firings etc. I'm just a bit concerned that the firings that he has fitted are too big.

The joist span is about 3.4m and he has used 150mm joist ripped to nothing. I imagine that too much fall is better than not enough I'm just worried how its going to look down the side of the extension when I do the fascia etc, it looks a massive slope which I haven't seen on other flat roofs.

Also there is a roof lantern, I was concerned about the water coming off the existing pitched roof onto the new flat roof behind the lantern and it just sitting there. Is this normal?

Am I worrying about nothing and is this acceptable?

Thanks.

 

[Notch7]

Have you only got gutter at one end then?

I think thats a bit naff these days, I personally would run gutter around 3 sides.

And no, if you have a roof lantern, a mono fall is not acceptable.

Between the house and lantern, the fall should be sideways.
Therefore the highest point on the roof is the middle of the roof lantern, going straght back to the house wall.

If it was me, I would do gutter on 3 sides.

Then make the fall from the edge of the lantern upstand down to the gutter on all 3 sides.
At the lantern upstand sides, the firrings will all hit at the same height, but as you work back to the house, the ones parellel to the house will continue to rise at the same height, meeting midway behind the lantern.

At the front corners, the firrings will form like a roof hip.

If you only have gutter at the front, the falls have to be worked out carefully to avoid pooling.

A fall of 1 in 40 is good for felt, so 85mm over 3400mm

 

[diybsp]

Thanks, will take me a while to get my head around that!

I'm not sure I can have gutter around all sides though as I think it might overhang the boundary. If I just had the gutter at the end and I wanted a fall left and right behind the lantern can I just fit firings perpendicular to the joists to meet with the ones coming from the pitched roof?

Another thing I'm not happy about is the joins of the boards being supported by a short bit of timber along the join with both boards screwed to it, instead of the boards being cut to both meet on the firings.

 

[Notch7]

If I just had the gutter at the end and I wanted a fall left and right behind the lantern can I just fit firings perpendicular to the joists to meet with the ones coming from the pitched roof?

Imagine a birds eye view with house at the back, lantern in the middle, gutter at front......

On far left have a 3.4m firring running from house wall to gutter.

Then have a parallel firring 3.4m long, butt it up against the side of lantern upstand and cut to finish at the upstabd rear corner.

Between lantern and house you can do the firring next to house wall, starting at the rear left corner and rising to the rear mid way point.

Then do one at the back of the lantern upstand -with the same fall, the result will be the flat roof behind the lantern will fall to the sides and towards the lantern upstand. If it was me, I would make the fall at the back steeper to make sure water runs away without risk of ponding.

Flat roof falls need to be a min of 1 in 80 for water to run off but that assumes a perflectly flat surface, generally 1 in 40 is enough to avoid ponding on felt. I do around 1 in 50 to 60 for grp or epdm..

 

[diybsp]

Thanks, that's very helpful. Going for epdm btw.