This is just a shed, but want it solid.

The shed is having a flat roof which will be felted by a flat roofing company.

The shed is 4m long, 1.7m wide.  Looking to run timbers 1.7m long left to right, not front to back at 4m long.

What spacing would you go for these?  400 or 600mm?

What size timbers to span the 1.7m?
Wickes do some 88x36mm planed treated (would prefer planed as left exposed).  Left it too late to get any from the merchants for the weekend.  Will this do, or shall I wait until next week?

Would you nogg down the middle?

The roof deck will likely be ply or OSB - not sure yet, depends what the roofing company would prefer.

all my roof timbers are 3x2 and are quite smooth

ask for 4x2 and go for 610 centers or cls 38mmx 89mm[1 1/2"x 3 1/2"] at 407 centers

and make sure you roof is thick enough or the nails will stick through [at least16mm for 1/2 inch nails]

most sheet material is 1220x2440 imperial mesurements so 16" or 24" centers
some are metric as in 2.4 x600 so 400 0r 600
so choose your sheet material then your spacings

toe/skew nailing is more for holding wood in place rather than transferring loads in shear[vertical load]
can you not fix them on top and reduce the sidewall frame accordingly
if you extend the joist outside say 3" less than the roof overhang you can secure the guttering brackets to them

if you use joist hangers the roof will try and go flat when screwed to the side frames as the frames are flat
of course if you suss out the slope and work out the natural level [say 6mm you can have the bottom off the joist proud by 6mm allowing the roof slope to the far edge at the same angle

That makes sense, I have never had a flat roof done from scratch, only re-roofs, but guess the first layer needs fixing.

The wall one side is the garage wall, and can fix a bearer to it, with the joists sitting on it, and then nogg between.  On the other side, the wall is built up from dungboard, and I can bolt a bearer to it at any height I guess.

The way the fall of the roof needs to go, and the way the joists are us awkward, meaning the bearer on each side will run at a fall, the joists all 'leaning' a tad.  The fall is front to back, and the joists left to right.  Had to do it this way as the neighbour is considering extending the flat roof their side to do a similar thing to me, otherwise, fall would be the other way.

Joist hangers a real no in that direction?

Rough pic from ipad!

   

added a pic as you replied!

Sealing the roof to the garage wall might be a pain. Not sure how best to do that if the wall is rendered.

Those extensions are a bit agricultural, you won't get into the chippy academy with those  .

From the looks of it you could remove them and nail joists, as sort of wall plates, onto the top of the posts. You could add packers onto the top of the posts to give you the required fall. Then you could just run your ceiling joists on top of them, skew nailing them into the wall plates. I'd add a few noggins to make sure it didn't twist too.