Typically from my past experience the thickness of a 8d nail is all that is required, although most people do not install decking with nails any longer. Make sure the ends are gapped properly and sealed as well as these types of wood tend to move more length wise than width wise.

We usually recommend 3/32" gap spacing. 1/4" is big, but I suspect your contractor is just being cautious due to the less-than-ideal clearance over the concrete slab.

Thanks for your help. Do you think the wood will shrink, leaving a wider gap? It's been very dry here recently. Today is 70 ° and 3% humidity.

As millworkman said 8p nail which is 1/8" in diameter... 1/4 is way too big for IPE.

If the wood was properly acclimated prior to installation, I don't see it shrinking much if at all. Wood shrinks when it's dry, and 3% humidity is already almost as dry as you can get. I'd guess it's more likely to expand when the humidity goes up, which may be another reason why your contractor made the spacing so wide.

That said, I'm on the opposite end of the country from you, so I'm not really familiar with the Los Angeles climate and what kind of weather/humidity fluctuations you get there.

I prefer a larger gap between boards. This will keep leaf matter and other detritus from collecting in the gap.

AdvantageLumber is right there is not much shrinkage in IPE lumber 4" board typically shrinks about 1/8" and a 6" board about 1/4" so if you gap boards 1/4" + you will end up with a big space.

With that said, being worried something getting stuck in the gap, you cannot go by that and think that excessive gapping will prevent something from collecting there... you can leave a 1" gap and something will still get stuck there.

The only thing I’m worried about getting stuck, is my toes.

@caddi63 You don't want that to happen so try to keep reveal per manufacturer guidelines.