Garden Room Roof Structure

Shish63

I am self-building a raised timber frame garden room and looking for advise on how to design a flat roof structure. Been struggling with it for weeks now.

The problem it is not a rectungular frame but a hex kindof shape like in the sketchup image below. The reason for the odd shape is due to my garden being 'odd shape' and I wanted to fit this timber room in that shape. Dim. are 6m x 4m plus a triangle at the back 6m x2.55m x5.6m.

The front needs to be under 3.0m high and the back 2.5m for premitted development. Also, will be looking to insulate (thermal and sound insulation)..but that's later.

I want a flat roof which is level inside and tappered on top (for rain water). But can't work out a structure to support a downwards slanting roof. I have bought 6" x 2" timber - 4.8m in length and planning to use them..somehow...Appreciate any advise, ideas..

Doozergirl

At over 30 square metres, your building is subject to building control approval.   You know that?   Thinking about foundations and insulation isn't something you do later when you have to make it compliant.  

You've got that far with sketchup, what do you want the roof to look like from outside, exactly?   A flat roof only needs a fall of 5 degrees or so.  You're going to need at least one larger joist to build the roof.  

To get the slope on that, I'd build the roof flat and use furrings strips on top to get the fall.  

I don't know the shape of your garden, but even if the room is irregular, I'd plan it to be four sides, still.  The cost of that specific 160 degree angle between the two left-forming walls (that could be one) outweighs the very limited floor space it provides.  Not just the awkwardness of yet another angle, but it's that side taking you into Building Control territory.  That said, I'd build to regs at that size anyway because it's a bit insignificant asset to the house.  

Shish63

Thanks for your response. Yes, of course it is 30m2 and 2 meters away from the fence and under 50% of the total build area. I have partly made the base frame. I would like roof to be flat with the front facia like in this image below.
 

Shish63

I see what you mean. My garden shape is like in the image below. The back is never used so I wanted to use that space, hence this odd bit at the end. I am trying to make it above building regs. I already bought Rockwool RW3 100mm to go in the studs walls and also using aluminum sheets for thermal bridging, HD palsterboards etc.....
So is this "large joist" you mentioned be the 6 m span across the rectangular bit? Is it better to lay joists parrellel to the front or at 90 degs. Can you please expand on this because that is where I have a mental block....:-)

Doozergirl

On plan, I think one 6m joist that runs across the back of the front 'box' with the other joists hanging from it, front and behind. 

The shorter joists at the back triangular section run parallel with the 2.55 length.  

A 5 degree fall is plenty, I really wouldn't complicate it more than you need to.  Furring strips would just run parallel with each other from front to back, ignoring the funny shapes, just cutting appropriately at the edges.  

That structure is just under 32 square metres, not under 30 though... 

Shish63

With regards to the area, I have spoken to BRs and they inform me that the area they look at is internal, i.e. minus the walls. So my walls are made of 6"x2" joists, so the internal area comes to under 30. The front is actuall less than 6m. Thanks for your advise, sound good.