What can I do to fill the cracks between pine floor boards on my covered veranda?

Although pine is not a very rot resistant wood, a four-year life span for deck boards does seem remarkably short. I suspect there may be very little ventilation underneath your veranda floor, and this could be a larger problem than you realize. If floorboards are rotting in just four years, what about the joists and beams that are underneath them? Your first step is to ensure that lots of fresh air gets under the veranda floor, if that’s not happening already. Second, you need to use either pressure treated wood or cedar lumber for replacing floorboards. Neither of these woods will last a long time without ventilation, but you will enjoy several decades of deck life if they’re installed over a properly ventilated framework. Another thing you need to realize is that all solid wood expands and contracts seasonally with changes in humidity. This is why it’s impossible to fill the spaces between deck boards and have that filler last for more than a season or two. Traditional verandas were often built with narrow boards that fit together with tongue and groove edges. Narrow board width minimizes board to board gaps developing, and interlocking edges conceal those small gaps that do appear. Composite deck boards are another option you may consider. Most brands are completely rot proof, and because they’re made with a high proportion of plastic they don’t expand and contract seasonally.