Is it normal for a deck to have zero gaps between the boards?

Running a circular saw down the seams is the fastest way to fix this without making your dad pull up every single board. You'll want to set the blade depth to exactly the thickness of the decking boards—usally around 25mm—and use a guide or a steady hand to cut right through the joints where the boards meet. This will leave you with a kerf width of about 3mm, which isn't the 5mm you initially wanted, but it's enought to prevent the boards from **buckling** when they inevitably swell this winter.

Since the boards were put down during a dry spell, they are at thier absolute minimum width. If you leave them flush, they will expand outward as they soak up moisture, and because they have nowhere to go, they'll either pop the screws or arch up in the middle. The bigger concern is acutally that plastic sheeting underneath. Without gaps, any water that gets through the tiny cracks will sit on that plastic and won't be able to evaporate. You'll end up with a damp, rotting environment for the joists much faster than you’d expect.

And I hate to be the bearer of more work, but those boards are likely upside down if they have the deep grooves facing up. Those channels are actually designed to sit face-down to allow **airflow** over the joists and prevent moisture trap. When they're face-up, they just hold water and dirt, which makes the wood slippery and prone to rot.

If he's really against flipping them, at least do the saw trick. Just make sure he uses a string line so the cuts stay relatively straight, otherwise the deck will look a bit wonky. It's a much better compromise than letting the whole thing warp itself apart by next March. Getting some air moving under there is the only way to save those joists from the plastic sheeting mistake.

I've seen this mess before. If he keeps those boards tight, they're gonna pop the heads right off the screws when they swell. It's a nightmare to fix once the wood starts splitting. Since he's already used plastic on the ground, he's basically made a swamp under there. Even with the saw trick, tell him to drill a few larger drainage holes in the plastic thru the gaps if he can reach 'em. That moisture's gotta go somewhere or it'll rot the joists from the bottom up in no time.

Spot on with the circular saw advice, it's defintely the least painful way to save the structure. Just keep in mind that once those cuts are made, you're leaving raw, untreated wood edges exposed right where the moisture collects. Since he's in the UK, he realy needs to run a bit of end-grain preservative or wood sealer down those new gaps with a small brush. Without sealing those fresh kerf lines, the expansion might be fixed, but the rot will start from the inside of the boards much faster than the rest of the deck.