How to extend joist horizontally

BarraOG

I cut away a ply floor and the joist is flush to the wall on one side. What dimension timbre do you recommend I attach to the joist so I can screw down a new floor(9mm ply) into it? Will be putting cement board on top of that(6mm). Then tiles. What screws do you recommend and what screws are typically used when screwing down ply in 200mm joists?

10-10-20

I'd just screw in 44x18's, then angle the plywood screws slightly towards the joist.

https://www.goodwins.ie/products/Planed-Timber-44-x-18mm-2.4-Metre-2x1-8Ft-.html?filter_set%5B%5D=20,286,1401

Rows Grower

I'd use the same size as the joist that's there for the sake of a few euro, you only get one chance to do it. 38mm screws into the plywood and plenty of them.

The Continental Op

https://www.boards.ie/discussion/comment/120566114#Comment_120566114

Agree on the joist size but 38mm screws seem a bit on the small size 4 x 50mm part threaded would be my choice but then I sometimes use thousands of them so like 4 x 50mm. Part threaded so they pull down.

I'd also ad noggins to support the joints in the ply 9mm seems very thin and will need support at the edges.

Dr_Colossus

Agree on 9mm ply being too light for flooring, even with cement board on top. For tiles you want to the floor as rigid as possible so would have plenty of noggins, 18mm OSB, 6mm magnesium board (more rigid than cement board) and then your tiles. Would also have the magnesium board fixed (flexi tile adhesive) and screwed to the OSB every 15cm or so.

I also like 4 x 50mm screws for flooring, have also used 4 x 60mm.

Have you tried cutting a piece out of the bottom of the plasterboard? Presume the house is dry lined so if like mine there's a 3"x3" batton running along the bottom of the wall a couple of inches up off the floor to which the plasterboard is screwed and fiberglass or similar in behind as the insulation. If you have a similar void you'll be able to get that old flooring up and the new section onto the existing joist.