I am trying to repair some water damage below an ensuite shower on the upper floor of a house. The room is only 120cm wide and the wet rot damage is isolated to the joist directly below. The damage to the wood at the end of the plank which is resting on a concrete wall below. The joists are 225mm high and as of right now are not showing signs that they will collapse.
I have removed all of the loose wood shards and I have treated the damaged areas with a wood hardener. The joists live in the floor cavity which is the same height as the joists. Because the bathroom is only 120cm, there is no way I can get a 3 meter long beam under the shower without ripping up a huge portion of the floor in the room next to the bathroom and dropping the new joist down to sister it to the damaged joist. It is possible for me to get a 75mm plank under the floor without having to damage any walls or floors. Here is my proposed solution using 75mmx35mm planks that are sistered to the damaged joist. Would this be enough to reinforce the damaged joist?