very good question, first of all, did you buy your massaranduba air dried of artificially dried? Your supplier should know. I don't know where exactly you live, if it is dry or humid the climate, but if your massaranduba is humid and your climate is dry, you may expect some 'workout', massaranduba is know for cupping, cracking twisting and shrinking... Please go to your supplier first. If you need the details on shrinkage, please check the link given below. If you dry your boards yourself, space each and every board so that air can travel thru the entire pallet

Here is a link that might be useful: shrinkage figures

Thanks Roquevalente, very informative site.

The boards were kiln dried to 15% but have been sitting in my climate (near Philadelphia) for 5 weeks and possibly for several months before that in an unconditioned warehouse. I suspect they are now in equilibrium with the air humidity. The air tends to be on the humid side where I live most of the year. I am tending to spacing them wider than suggested, 3/16th in or even 1/4 in, unless shrinkage is a real possibility. Am I being paranoid for going so wide?

Say the spacing does become unacceptable. Any reason why I couldn't unscrew and reattach with corrected spacing?

Dear Jaansu, so your boards are 15% and have been in some pallet (with stickers between the boards so that air can flow thru?) and Philadelphia is I guess pretty humid (as far as I know from my travels one day to Washington DC), I think that 3/16" is perfect. Here we recommend 5mm spacing which would be your 3/16". KD also should not work so much. Don't forget to use stainless steel screws and if you like to keep your color then use UV filtering vernish (at least once a year).

Thanks Roquevalente, 3/16ths or 5 mm it is then. Yes, I have used spacers, old pieces of thin molding recovered from my shop, while sitting in the garage. Efloors sold me the SS screws as well, although my contractor hates them (says strip too easily), Anchorseal for any cut ends, and a UV protective stain they recommend.

I'm sorry, what does KD refer to??

Your deck boards are going to shrink. I'm in Brooklyn, NY, so our climates are similar. If your deck is in the sun, your acclimatized KD (kiln dried) boards will shrink, and leave you with a big gap...+1/4"...if you set them at 3/16"
I'd set the gap at 3/32nds.
If it's a shady site, then disregard my advice.

steve

Thanks Brooklyn, the deck is about 1/3rd in shade, 2/3rds in sun but is all in shade by early afternoon. I guess there is no easy answer to this question.

KD means kiln dried. If your contractor hates SS screws what is he proposing to use? I thought you were doing this yourself?