Flat roof

[Selfbuild2345]

Hi, got a question.
I have a single skin concrete block work build, which is level all the way round and have a wooden wall plate on top. The roof is going to be a flat roof but obvs need it at an angle so water runs off. Would i be able to plain the joists to an angle for the water to fall? I can’t use firring strips as this will take me way above 2.5m in height. My span from block work to block work is 3.8m and will have 300mm overhang on front and 100mm on back. The joists size i have used are 9 x 2 c24.

 

[Deluks]

Yes. Circular saw might be quicker as you'd probably need to lose minimum 70mm at one end.
You could also build up the wallplate at one end to reduce the amount you'd have to cut off.

 

[Selfbuild2345]

Can’t build it up as will be too far over 2.5m. 9 x 2 joists i think are over sized hence why i was thinking of cutting each joist down in situe. Plus i am wanting the ceiling on the internal to be flat.

 

[tony1851]

Cut them down to 7" at the low end.

 

[Selfbuild2345]

7”??

 

[Selfbuild2345]

Do you have any tolerance on the 2.5m? I could use a 50mm firring strip on each joist but with 18mm osb then rubber my build will be 2.7m

 

[Selfbuild2345]

2.57m

 

[Notch7]

Do you have any tolerance on the 2.5m? I could use a 50mm firring strip on each joist but with 18mm osb then rubber my build will be 2.7m

It's 2.5m from highest point of adjacent ground to top.

Arguably there's no tolerance in planning.....

 

[Selfbuild2345]

It’s all level ground around. So i cud build one side up as am going to flag stone it anyway, could build it up an will the measurements come off there?

 

[Selfbuild2345]

To the top of my joists am touching on 2.5m so really stuck for height hence why i was thinking of cutting joists to a slope as at the minute it’s flat

 

[Notch7]

It’s all level ground around. So i cud build one side up as am going to flag stone it anyway, could build it up an will the measurements come off there?

Possibly.....obviously it's from natural ground level, so it would need to look "natural"

In reality your only slight risk is if somebody complains....if that's unlikely you have nothing to worry about for a rather trivial amount.

 

[Notch7]

To the top of my joists am touching on 2.5m so really stuck for height hence why i was thinking of cutting joists to a slope as at the minute it’s flat

Well you can notch out on end of the joists where they go over the wall / wall plate.

 

[cdbe]

If you don't mind the extra gutter you could fall it both ways from the centre to reduce the height by half (and not that it will matter but that keeps the depth of the joists thicker at mid span). Similarly, if you do cut them down falling one way by 50-70mm as suggested, at mid span you're only reducing the joist depth by half that.

 

[tony1851]

7”??

Eh??

 

[Selfbuild2345]

My only concern bout cutting the joists at a slope are how perfect do they have to be as i would be doing it in situe and would it loose the strength? Span is 3.8m wall to wall and i used 9 x 2 c24 i think i should have the strength still if cut down on a slope.