On Sunday, June 5, 2016 at 6:03:35 PM UTC+1, Phil L wrote:
DICEGEORGE wrote:
top corner is where the orange saw is, water will flow out diagonally
opposite

That's as clear as mud.
Is the water going the way of the blue arrow or from the saw handle to the
bottom of the saw?
If it's running from the saw, then you don't need blocks of wood under the
purlin as it's running down already and if it's running the way of the
arrow, the joists are in the wrong way, they need to run from left to right.

The purlin is not straight or level.
Highest point will be corner where saw is.
Lowest point the diagonally opposite corner.
Water will flow towards the middle of the bottom edge of the photo.
You can see a block under the second joist,
a scrap of OSB under the third,
and the fourth just sitting on the corner of the purlin,
I'm musing on how to improve this.

Wood with the grain parallel to the purlin would probably split over time.

But as Cursitor Doom noted cutting them to exactly fit will be difficult,
especially as the purlin is not straight or square!

Maybe do it as well as I can with cutting and chiseling and sanding
and then put car body filler in between to fill the gaps,
or glue,
or lead from old flashing,
or flexible frame filler,
or chisel off the top of the purlin so the joist will lay on it?

(The 5x2 joists are not big enough for the 3.4 meter span,
I'm having 7x2s delivered next week)
[george]