Rotten floorboards - joist under wall

Should have myself clearer, nut & bolt with a round washer, & plate washers on each side.

Others on here might have other suggestions.

 

A few spiders ain't going to cause any problems, unless you suffer from Arachnopbia.

 

Get the joist in and wedge it up tight to the floor boards off the walls with some slate, you could bolt it or just nail it then, it's not going anywhere once the boards are fixed back down. I would personally have staggered the boards so that the the ends did not finish on the same joist, should be ok though, and not that easy to do with T and G floorboards.