Is there a way to fix drainage on a deck with zero board spacing?

I’ve run into this specific issue on several older builds where the boards were basically tongue-and-groove tight. If the water is pooling where a hot tub used to be, there’s a good chance that section of the deck has **settled** or bowed under the weight over the years, creating a low point that the lack of gaps only makes worse. Drilling holes might seem like an easy fix, but it’s ussually a bad idea. You’ll end up with localized rot around every hole because the end grain is exposed to constant moisture, and you can’t easily seal the inside of a small drill hole. Plus, if the deck is sitting directly on the slab, that water is just going to sit in the dark under the wood anyway, which is a recipe for mold and structural failure of the joists. The better approach for widening those gaps without a full teardown is using a circular saw with a guide. You can set the depth just slightly deeper than the board thickness and run it right down the existing seams. This creates a uniform **1/8-inch kerf** that allows water to pass through. You have to be extremely careful about hitting existing nails or screws, though. If they used galvanized nails, you'll probly spark a bit and dull the blade, but if they used deck screws, you'll snap the teeth off your saw blade instantly.

As for the slab, you definitly don't want water trapped between the wood and the concrete. If you widen the gaps, you need to make sure the concrete underneath is actually sloped away from the house. If it’s flat or sloping toward the foundation, any water you let through the deck is going to end up against your sill plate. If that's the case, your only real fix is pulling the boards and potentially adding a sleeper system or a drainage membrane to direct that water out.

That saw trick sounds kind of terrifying for a newbie. If I hit a hidden screw or nail with a circular saw, is it gonna kick back or fly out of my hands? I’m also wondering how you even find the seams to follow if the boards are as tight as the OP says. Mine look like one solid piece of weathered wood at this point. woud using a smaller tool like an oscillating saw be safer, even if it takes way longer? I’d hate to ruin my only saw or get hurt trying to fix a puddle.

Skip the oscillating tool. It’s way too slow and you'll never get a clean, straight kerf. If you’re scared of the circular saw catching a screw, grab a carbide-tipped 'demo' blade specifically rated for wood with nails. They handle the impact way better than a standard finishing blade. To find your lines, try a stiff wire brush along the suspected seams to clear out the gunk. But honestly, if that section where the tub sat has settled, cutting gaps might just let water sit on the slab in a deeper puddle. I'd pry up a single board in that low spot first. If the joists are mushy from decades of moisture, cutting gaps is just putting a band-aid on a corpse.