Your new joists want to sit on top of the wall plate just like the attic truss does there is no need to remove the old ceiling as the truss and joist will sit together flush and nailed together down into the wall plate and to get over the angle creating weakness fix the top of the joist to the rafter use a block of wood the same thickness as the truss.
How to fit floor joists in a loft.
Take measurements in metres this makes it easier when you are buying boards.
Trimmer joists form the periphery of the new floor.
A basic floor system is installed in about 10 stages from preparation to boarding the floor.
The strong timbers approx 225mm x 75mm timber are secured to the wall plates brickwork and bolted to the walls.
A new floor system is installed using a frame trimmer joists to support the floor joists that span the width of the loft.
With the supporting ashlering in place the internal struts and braces can now safely be removed.
In smaller lofts it is often the case that the floor joists themselves will be used to support the sloping rafters.