Plywood subfloor : choice of screws ?

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I am laying down some insulation tape on top of all my joists which will compress to approx 5mm once it's sandwiched under the ply I intend to screw down.

In view of this, what's the best screw selection for securing 18mm ply to joists ?

I've got 4 x 30mm, 4 x 40mm and 4 x 50 mm to choose from.

Also what centres are best 200, 300 , 400 mm ?
Reynard - the basic formula for screws is 1/3 = thickness of material to be fixed & 2/3 into the support. So for your 18mm ply, screw length is 18 x 3 = 54mm ... the newest screw to this is 50mm; that's the size you use.
200mm apart is fine. Make sure any joins between the sheets of ply are supported below, ie. mid-point over joists or support with noggins (at least 75mm x 50mm) for joins at right angles to the joists. Failure to do this will allow the ply to flex at the join and will cause damage to floor covering above (cracked tiles).

Why are you laying "some insulation tape on top of all my joists"?
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Why are you laying "some insulation tape on top of all my joists"?

Thank you for your guidance. I'll go for the 50mm length. What's the best screw size as I notice size 4 seems to be the most popular choice in the shops. Is it worth making pilot holes first or is this overkill ?

The tape on joists is optional for me. I'm not sure if I can be bothered. Basically I've bought acoustic tape for soundproofing and more acoustic tape for the perimeter soundproofing. We'll see.

Thanks again.
Reynard - Spax single thread woodscrew (or similar) are fine for this application although you could use Turbogold (from Screwfix). No need to pilot drill as these screws are 'easy starters' (sort of self-drilling). Tip - 'cos you've got lots to do first lightly tap a row of them into position with a hammer (this is only to hold them in position for electric screwdriving - you don't need to hammer them in very far, just enough for them to stand to attention so you don't have to hold them). Failure to do this WILL result in damaged finger-tips from the driver-point slipping into your screwholding hand.

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