Repair laundry floor - screw size, board type, noggin length?

I’m replacing a damaged section of 60 year old tassie oak floor in my laundry. I have cut the boards flush with the joist and plan to sister (glue & screw) some 90 x 45mm treated pine along the existing 100x 45mm joist to take the edges of the new boards.

Query 1: Will 8-10 75mm treated pine screws be sufficient to hold the pine - or should I use heavier screws?

Query 2: which flooring option? I have the choice of yellow tongue or solid old tassie oak floorboards for the repair. Will be covered by fiber cement tile underlay. (It’s an old ‘knock down’ so doesn’t need to be perfect.)

Query 3: I’ll add a noggin/block to pick up the ends of the other two removed boards. Planning to use a 400mm length - which will extend a board on each side of the gap. Too much/ not enough length?

Here is the pic of the main hole - 460mm wide and 1400mm long (and the other two boards).

Attachment 538135

Cut boards to mid joist, no need to sister another joist.
YT will do the job but if its going to be a wet area I personally would use solid timber of your choice.

if its going to be covered, and with something that is eventually going to be tiled i'd just use the 17mm or 19mm ecoply (structural) rather then yellow tongue, especially exposed to the ground/air underneath. for $100 at least it wont compromise under the tile underlay causing issues later.

screws, get 12g or 14g bugles and send them in




the other cheapest option is to just sister the new joists level with the exsiting floor boards and just put the FC sheets on top. holes don't look that big that the FC sheet should fail (assuming your using a 19mm sheet or similar)