IMHO, you have a bigger issue here, that being, deck boards are a poor choice for this application. The material has a radius edge, and will provide hundreds of lineal feet of cracks to catch dirt. There will be gaps, at least on a seasonal basis, and there will always be dirt stuck in the radiuses, and gaps. Traditionally covered porch floors were done with tongue and groove flooring, with 3/4" thick Fir being a popular choice. Deck boards are really a bad idea for what is, essentially, an indoor application.