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What would be the structurally best solution when levelling the floor joist marked red in the middle?

The timber is about 95mm deep. I have a piece of timber that is 100mm deep and another that is 170mm deep.

What would be the best structurally sound solution?

The other floor joists are 170mm

The easiest way is to sister a joist section to each and draw your level by aligning them. You can mess around with planing and shims but sistering is the easier way. It depends on potential loading in the area if 4 by 2 sistered is sufficient or a full joist section.

DWD