Fixing SW t&g flooring

I just used spax 45 mm screws diagonally through the tongue of my Wickes 'solid hardwood' flooring, but I predrilled every hole with one of those devices that drills and countersinks at the same time to avoid splitting the tongue.

tongue-tite or lost-tite screws, I used them into oak rather than softwood

any of those work. no need for pairs. You're only stopping it moving horizontally, the nails take no weight, and they're already constrained by their neighbours.

likely to cause a fit problem

If you want, but I get the impression you're overthinking it. I wouldn't contemplate hiring.

NT

Yup, that's what I used on engineered ok boards. I pre-drilled to save splitting any. Quite quick since you don't need that many per board.

I started using those but managed to snap a couple so I reverted to conventional spax wood screws, predrilled and countersunk.

The tongue-tite screws are very slender so if a lot of expansion and contraction is likely over the course of a year, or many, will they fail ?.

I didn't know 'flooring screws' existed, so thanks, yes have used these. Working well on the whole. A couple of splits on where screwing near the board ends.

TW

They are just countersunk screws made by Spax for any job. Box says 3.5 * 50 Barcode number 4 003530 006760, but having a normal countersunk head, you need to predrill and countersink the hole. Takes more time but DIY time is free !

For your 1 inch battens, Wickes do 40mm ones for half the price

If you have a second drill to hold the countersink bit, you save a lot of time. DIY time on one job takes time off some other project.

I have a selection of drill bits that have a counsink collar that clamps onto a special flat section of the bit. Drill and countersink in one go.