Second storey on top of existing garage. Roof height/shape?

[Deluks]

I'm planning on reroofing my garage to give a more secure and soundproof roof, with increased head height in the garage itself. I'll be using steel uprights in conjunction with a single block wall where the door is, plus steel frame perimeter, to hold the roof up.

I've been toying with the idea of making a loft room up inside the roofspace, for additional storage/hobby space. Much in the same way as 'posh attic' non-regs loft conversions type usage.

The trouble is knowing how to make the roof sit on top without looking odd. the steel 'goal-post' internal frame will sit at 2.5m high (8'2"). I was simply gonna sit new roof trusses on this. But as the clear span between the steels will be about 13' - 14', and the overall width of the roof at the eaves will be no more than 17' How can I get a sensible head height in the loft room without having it as steep as a witches hat??

Existing walls are not up to supporting any decent loading so everything has to sit on the steel.

 

[noseall]

are you aware of the foundation sizes for the steel columns? i'm sure they will be pretty hefty as the building's weight will be concentrated in this area.

more headroom can be gained in a roof by using dormers.

 

[Deluks]

The foundation pads will be something in the region of 30" deep.

I've thought of sticking a dormer on the side, but the main issue is head height at the ridge, I'm guessing a minimum height for the roof would be around 2.2m, on top of the 2.4, obviously this takes me into planning territory which I'm not overly concerned about as it's 80ft down the garden and there's no other houses within about 60ft of it.

So a 2.2m high roof that's only about 5m wide would be very steep and might look daft.

I'm not overly concerned about usable width, 6' wide should be usable, and its nearly 20' long.

I know there's a formula for working out the pitch but I don't know what it is.

I'll probably go into the garden tomorrow and mock up a truss of these dimensions out of 4x2 and see how bad it will actually be, and use an angle finder to measure pitch.

 

[noseall]

based on the dimensions given, you will end up with a reasonably steep 41 degrees. this isn't swiss chalet by any standards and would look ok.

30" deep for the steel foundations? seems a little bit on the lean side to me.