Flying Rafters

[Rick Hughes]

I am creating a small cut roof, all 4 sides slope at 35 pitch to a short ridge board (rather than to an apex) on 3 side rafters will be birdsmouth cut to rest on a 150 x 50mm ring beam . At the front I want to fly the rafter ends out ... i.e extend roof ... same 35 degree pitch but extend out by around 300mm .... as if I were extending a soffit (but no soffits in this case)

i.e like this side ways view

Assume I would create a horizontal extension, and vertical upstand (x & y) I would run a tie brace 'z' from one side to the other Are there any guidelines for ratio of length x ... was thinking of having 300mm extension i.e to left of ring beam, and using a 1/3 2/3 have 2/3 to the right of the beam, then joining the vertical ..... does

1/3 to 2/3 seem right, or is there a recommended ratio.

Roof is to be plain tile clad.

[harry]

That is a very weak structure. The thing would be better triangulated. Bolt the joints using timber connectors or use gang nails. If you haven't done this work before, hipped roofs (which is what you effectively have ) are difficult.

[Andrew Gabriel]

Your diagram isn't showing what structural element is preventing the roof from spreading. Conventionally this is done with the ceiling joists in tension tied to the lower rafter ends, and any vertical supports hanging from the rafters (more often triangulated) are just to prevent the ceiling joists sagging, as being primarily in tension, they are usually relatively thin timbers. Can you extend your ceiling joists (I presume that's the ones marked 50x150) to both outer rafter ends? (Then X is no longer needed.) Your Y upstand needs to be moved to the left so it's transfering the roof weight directly onto the outer supporting wall. Ceiling joists and rafter ends need to be tied. Y upstands need to be tied to ceiling joists and rafters. You will also need diagonal bracing between rafters (or some other method) to prevent them all falling sideways in parallel, and also between the adjacent Y upstands for the same reason. You might want to put a purlin/roofplate across the top of the Y upstands. Alternativey, instead of the Y upstands, you could continue the outer wall up above the ceiling joists to meet the rafters. You'll probably want wallplates along the tops of both walls (not shown, but maybe implied).

PS, I'm not a structural surveyor...

[Rick Hughes]

Have done a hipped roof before .... but these came as trussed items, and I just had to set the horizontal section back to the first truss ..... in this instance this is only a roof to a deck area ... no trusses involved.

I will use bolts at all intersections

[Rick Hughes]

as it slopes on all 4 sides ... I could extend joists out to be horizontals .... good idea that man.

For some reason I had it in my head to run joists left to right ... when I can run them front to back.

I'm using 100 x 50 for rafters & joists. (C24 grade treated) I know 100 x 32 is big enough .... (truss thickness) but none of local builders merchants stock it ...so using 100 x 50 at 400c/s

There is no outer wall just a 150 x 50 ring beam sitting - thru bolted onto 100 x100 corner posts .... open sided roof to a deck area.

i.e like in this sketch:

The posts go through deck are bolted to concrete pads on floor and thru bolted to deck frame on both levels.