I have just started laying the decking on my new project, and am having some problems with the butt joins splitting slightly when driving the screws home. I am using ironbark decking and Macsim 10g 50mm type 17 stainless decking screws. My joists are 90x45mm, so I have positioned the screws 12mm out from the butt join as a compromise between distance away from join and still having some meat in the joist to bite into (about 10mm). I知 using the Festool drill/countersink depth stop bit to drill a 3.5mm pilot hole, but have enlarged the hole to 4.5mm at butt joins to try and stop the splitting. Using double or wider joists would have been better, but too late now.

I think the main problem is the depth of countersink versus the thickness of the head on the macsim screw. I have set the depth stop so that the diameter of the countersink is just larger than the diameter of the screw head so that it looks neat. But when the screw bottoms out in the countersink, the head still sticks up. In the middle of the boards the self reaming head can be driven in until flush without many problems, but this causes splitting at the ends of the boards.

Does anyone have any good ideas? I was trying to avoid countersinking to a depth to allow the screw to sit flush without the head self-sinking because the diameter of the countersink is then a fair bit larger than the screw and it痴 just more chance for water to pool and looks a bit messy. Am I just being too fussy? The splits are only small now, but I知 assuming they値l open up as it ages.

Is a bigger diameter deeper countersink my only option?

Cheers,
Gibbo.

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