Front and Back Dormer - how to support a ridge beam where main roof apex meets hip roof

[Handra]

I'm having a think about how to create more space in our upstairs attic space.  It is currently just a large 4x4m room with coomed/sloped walls.

 

Thinking is to do a standard flat roof dormer to the front, and a flat roof dormer style finish to the rear.  The rear "dormer" will continue on after the eaves with a 1.7m protrusion that will sit on the top of a previously completed single storey rear extension.  We intentionally used 10" x 2" (47 x 245) roof joists in the extension (over a 3m span) thinking about one day building on top of it.  The foundations are >200mm thick and 600-700mm wide.

 

The roof joists in the dormers will span ~3.4m to the front and potentially ~5.4m to the rear.  Looking at timber span tables for a flat roof the longer span (5.4m) can be achieved by using 10" x 2" [potentially even sistering where required].

 

For the flat roof, we would have to replace the ridge board with a ridge beam, still need to determine whether to use RSJ or a 10x2 flitch beam.  On one side of the ridge beam, we could support it using the shared chimney at the party wall.  The problem is the other side of the ridge beam where the roof apex meets a connected hipped roof.  There would be no place to put a column underneath the point where the ridge beam intersects with the hip rafters as the column would land at the top of the existing staircase, which we want to retain.  Also don't want to do anything with the hip roof end, but clearly it will need supported.

 

In that arrangement what is the typical design to support the flat roof and the adjacent hip roof?

 

I was thinking about 2# of 6x2 flitch beams to support the last two common rafters before the hip rafters, either with bolted plates at the apex, or even a continuous welded flitch steel plate.  These two could be supported with a ceiling/floor joist acting as a rafter tie, which catches a load bearing wall below.

 

Thanks

[Handra]

Here's a rough sketch of my thinking

[markc]

Dependant on which way your roosts run, you could support the ridge with tied rafters (the tie being between or part of the joists). Or you could use steel rafters with a suitable connection at the ridge (this wouldn’t need ties). I have just done a similar thing on my new man shed - stone and block walls and didn’t want portal frame columns so the rafters work like a portal but with foot plates that land on top of the walls.

[Handra]

Been having an outside the box think about things.  What about this possibility @markc?

 

Rather than trying to run the flat roof joists off a structural ridge beam, we leave the ridge beam as is, as a ridge board. Then we run the flat roof joists parallel to the ridge board (in the back dormer that’s the shorter run anyway) and they bear onto the dormer cheeks that are structurally supported underneath, by I-beams in the single storey extension roof and by flitch beam doubled up rafters where the dormer cheek meets the roof pitch.

 

The front and back dormer flat roofs connect underneath the ridge with noggins. The firrings are placed perpendicular to the roof joists above the bottom line of the ridge with osb above but with a 10-15mm expansion gap where it meets the ridge with appropriate dressing…

[markc]

@Handra, sorry only just seen this.

nothing wrong with thinking outside the box, but I am struggling to visualise your suggestion at the moment. Any chance of a sketch or doodle?