I would use 1/4" stainless flat head machine screws. Drill countersunk pilot holes right through the Ipe. Fasten the boards from the top, with nuts below, utilizing the welded-on washers.

Tony,

I've done that job a couple of times, but not by the method that you are suggesting. IMHO, your method isn't going to work in the long run. When you screw into Ipe, there is a good chance of splitting it, or breaking the screws.
Besides for that, 1/2" of thread into the Ipe isn't enough to keep it from moving, and it's going to want to move. It'll want to cup and bow, and it will have it's way. Another problem is that some of your Ipe will be bowed to begin with, and your contractor will have to straighten it out before screwing it. Those screws will not be able to hold it from springing back to being bowed, leaving uneven gaps.
I could go on, but you already know that I don't approve of your contractor's method.
Good luck with your deck.

steve
www.urbanexteriors.biz

Thanks Steve. i already have this as part of my contract with my GC but just plugged you on another forum

Hey Steve !! back from the Ricka ?

Good to see Ya. John

Hey J-mon.
Yeah...got back sunday...shoulda stayed longer. It's freakin cold here in Brooklyn.

steve

Tony,

What does that mean...what forum?

If you let your GC do it that way, I think that you'll regret it. If it's not too late, I have a better solution.

steve

ok i will email you