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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.

Current rot damage to joist

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?

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You've asked whether it's possible to get the repair joists in place. We can't really know that. The plan is reasonable, though. The key is to make the sistered joists long enough (or long as possible with adequate fastening). The shorter they are the more torque (shear) is applied to your fasteners and the wood fibers. Ideally you have a 4:1 lap-to-span ratio or better.

The block in the center isn't really necessary. It won't do anything of value. Install the other two and secure them to the original joist with construction (not black drywall) screws, or through-bolts, and heavy-duty construction adhesive.

Since it's just one joist that needs repair, it's not critical that you meet sky-high standards for load bearing. If you approximate the strength of the original joist the repair will be adequate. Good luck.

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