Working on the theory of one third of screw length = thickness of wood and two thirds to screw in to attach, ie 9mm wood then use 27mm screws (or nearest to) Just to confirm for me if ply is 9mm and screwing to floorboards that are 19.5mm (3/4 inch) I should adjust this to 9 + 19.5 mm = 28.5 mm Less 4 mm for countersinking = screws 24.5 (or nearest)
I am thinking that I should not use screws that will puncture the underside of the floorboards due to possible central heating pipes etc.
Correct, I would think 1 inch screws would be OK, perhaps even a bit less.
I know people often say you don't need to with modern screws, but if the floor is a bit uneven I would generally drill pilot holes through the ply to make sure it is pulled down firmly. Don't need to explicitly countersink, though, a decent drill will easily pull them flush.
What screws were you planning to use? I mostly use quicksilver or equivalent, but the experts often go for turbo gold.
You do have a nice cordless drill, I trust? Invest in a box of new driver bits, too.
I've never bothered to drill and I've not had a problem with it pulling down tight.
One of my favourite screws is Screw-tite from toolstation. They have a thinner, unthreaded shank towards the head which helps with avoiding this
If you use turbo gold or turbo silver screws from SF they self drill and countersink at the same time. I used 25 x 4 screws for exactly the same job rock solid job no movement.
Richard
I'd take a slightly more specific variation:
or Floor-Tite (TM)
The latter has a plain shank to allow pulling tight and the former has a secondary thread to pull tight and lock the ply sheet.
Indeed I would concur... DAMHIK ;-)
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