Closing an opening in a first floor - joists

Following-on from the earlier question about OSB for flooring ...

The hole that I want to close-up was cut many years ago to install a back staircase that had (apparently) been forgotten about in the original 1930s build. They cut through three 8x2 joists and put an 8x3 trimmer across, one end is half-housed in a joist and the other end is (might be) resting on a wall. There aren't any worrying signs, although it seems odd to hack half way through a joist at mid-span to insert the end of a trimmer! I'm planning to fix a 1200mm 8x2 wall plate opposite the trimmer using lightning bolts, then to fit joist hangars to the wall plate and the trimmer to carry the 1600mm 8x2 joists. My slight worry is the additional floor load that will be going on the trimmer. A nagging doubt keeps telling me I should get some acrows, remove the trimmer and bolt the new joists onto the old ones ... but that's a lot more disruption and work. Is anyone here knowledgeable enough to comment?

Have a look at the backside of the joist to which the trimmer is attached i f you can see a tongue with a wedge inserted then it is a joint used in the past to stop the joists spreading. The only other reason I can think of us ing a housing is to counter any twisting of the trimmer but you would only need a small housing rather than cut halfway through.

I would leave the trimmer in place and use joist hangers to remount the int ervening joists. You can also use joist hangers direct on the wall side and save yourself the extra work of installing another timber. The last thing I would consider is bolting together even with star washers between joists. You may find yourself having to rip up a lot of flooring to get a decent o verlap.

Richard