home made softshell

I'd be tempted to use an edge-locked plain stitch* first (with an overlocking machine, ideally, or edge-locking stitch on a conventional machine), and then sew this down afterwards; then you have three layers of fabric, rather than four with a flat-lock* seam. It's also easier to make.

This construction method is commonly used in commercial softshells.

And no, I see no point trying to seal the seams, or use waterproof zips, when the fabric itself isn't waterproof (manufacturers, please note…)

* must have been away with the fairies last night: I meant plain seam and lap fell seam.

I agree. I'll probably just use a coil zipper placed "inside out" to hide the teeth. some companies definitely go a bit too far with the "tech zipper" look and end up with a jacket that looks like a costume from Tron.

for example:

http://www.backcountry.com/outdoorgear/Stoic-Bombshell-Jacket-Mens/SIC0021M.html

http://www.habosarcade.com/images/DoT/Tron-Fighting.gif

Better off with a pattern. Reason is that dissecting an old bit of clothing leaves you with all the stretched bits, and it is hard to sort them out.

Nah, seam sealing is not worth while, or even suitable, with this sort of fabric. As to what sort of seam – depends on your machine.

Cheers