How to replace a wooden subfloor without taking the walls down?!

[TCX]

Confused! I'm about to rip up an existing timber (sub-)floor and replace with yellowtongue particleboard, in preparation for tiling. However - given that the current floorboards run N-S, and the joists run (obviously) E-W, then the new boards are going to run N-S. Not a problem in most of the room, however, it is bounded by a northern and southern wall, which sit on (i.e. parallel to) the joists, so the ends of the boards won't be able to sit on those joists, since the walls are already in place, and in the way. (Is this making any sense?!)

I'm assuming the way to go is to attach a "duplicate" joist to the existing one, thereby providing a "shelf" for the ends of the board to sit on. However, how should I attach this to the existing joist so that it is structurally sound? Or will I need to put new supports (i.e. stumps) in under the "duplicate" joist? I assume there is an accepted practice for replacing floors when the walls are already in place (i.e. in existing homes) but would just like some advice as to what it is!<O</O
<O

Cheers,
Tania

[TermiMonster]

Depends on the construction, but you may find that the wall is sitting on 2 joists, with each half off-set, see pic, or else do what you say, scab a joist to the side of the existing. It will be picked up by the bearers occasionally, anyway, and no extra stumps would be required.

[SilentButDeadly]

I'm assuming the way to go is to attach a "duplicate" joist to the existing one, thereby providing a "shelf" for the ends of the board to sit on. However, how should I attach this to the existing joist so that it is structurally sound?

You made perfect sense. Yes. Just whack on a duplicate joist. Use either nails 65mm x 2.8mm bullets heads or Type 17 50mm bugle head batten screws.

Use the nails in a staggered formation about a half meter apart along the joist. The screws can be placed a half metre apart through the centre of the joist - usually best to pre drill the duplicate joist first though.