A bit of progress to report
I've decided on the roof design. At one end I will have rafter ties at wall plate level, raised ties at the other end and a section in the middle with no ties.
The ties at wall plate level will be on three rafter pairs (covering ~1.2m). They will be above where I plan to have a work bench etc (extra head room not needed). Once boarded this will give me a decent storage area above benches etc and also somewhere to mount lighting for the bench.
The area with no ties (~3.6m/four rafter pairs) will give me a work area with unrestricted head room, enough to raise one end of a tall vehicle substantially.
The area of raised ties (~1.7m/five rafter pairs) will still give decent headroom at ~2.5m and allow storage of long/awkward stuff.
I'm planning on using 2x8 for the ties. Which for a loading of no more than .25kN/M² the max span is a touch under 4m. I will be careful (weight-wise) with what I store in the storage offered by the ties so I'm happy with ties of this size... Unless I'm told otherwise!
As for actual physical progress, it's been slow. Only having weekends (with short days and awful weather!) and with other commitments time has been limited.
However, I have put together three of the six 'frames' - the plan is to make six frames, the ends of the building being single frames and the long side spans being made in two frames each.
The three frames I've made each have a window or door so have been a little more complex, The remaining three frames are straight forward and shouldn't take too long.
The first frame I made unassisted, it was slow and once made I couldn't move it (timber was wet)! So I decided to cut all the studs and base/top plates.
I remember Mike G saying that the cut ends of timber are the most vulnerable, with this in mind all cuts have been treated. A bit over-kill perhaps but I got a great deal on the preservative and I think for the short time involved in applying the treatment, it is worthwhile.
Incidentally, I was surprised at how little the pressure treatment penetrated some of the timber. Looking at the photo, is this normal? Or is my timber supplier just supplying a poor product?
Still thinking about the roof, when I get there I plan to use the 'step out method' for marking the rafter cuts.
It seems straight forward enough and to keep things simple I will use a 5/12 pitch which at 22.62º is marginally more than the planned 22.5º.
Or, can someone recommend an easier way or point me to a useful guide or 'how to'?
While I'm talking about rafters I've read that the plumb cut of the birds mouth should be no more than 1/3 of the rafter depth, this doesn't worry me as I'm using 2x8 rafters so there's plenty of material. However for my application I'm thinking the seat cut can dictate the plumb cut. But, what should the seat cut be? Perhaps around the same as the wall plate - 96mm?