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Hi - advice is much appreciated on the below.

When I took up the floor in the bathroom. Underneath where the shower tray was, the floor joist had water damage and sections of the joist formed cuboid shapes pieces of wood that had broken off.

When getting an independent damp survey done on another part of the house I asked my surveyor how to remedy this and they advised cutting out a section where joist had the cuboid pieces and replaces that section with a joist piece.

Ive seen other advice online to sister the joist instead.

I’m attaching the image for you to see. Ive just cut out the sections that had the cuboid pieces crumbling off.

As you can see on the other side of the joist it’s very close to the wall so wont be able to tighten any bolts/gain access to that side of the joist.

What would be the best thing to do that is structurally safe and cost effective?

There will be a porcelain tiling going on so keen to get this right without having to take up the floor in the future.

I assume this is first floor and that is the topside of a ceiling we are looking at ? And I also assume you would rather go down the least invasive route. Removing and replacing the joist is possible but that would require the ceiling down and the pipes moving in all likelihood. The piecing in joist sections seems to be possible as the joist still seems to have a decent amount of strength left . It would require a section or sections cutting to fit into these “cubes” and gluing and screwing into place to bring the top surface up to level .
To me sistering would seem the better idea as it will be quicker and easier . I’d do it with something like 2” or 50mm material as close to the depth of the joists as possible . To fix it I’d use coach screws . These can be augmented with timber connectors or glue for that matter . It could be possible to drill through into the wall and use resin and all thread but that is probably overkill .

I agree with G, there are many ways to do this, it depends if weight loading is an issue. Sistering is definitely the easiest option as is a metal plate coach screwed on. In the past, the cutout section would have been repaired with a wedged scarf joint but that is a faff and you need to know what you are doing cutting templates etc. I would sister.

DWD

Thanks both - I’m going to go with the sistering option using m12 coach screws that are 100mm long. In terms of securing to the joist ends to the wall. What do you recommend I could use? I cant use a joist hanger as it can only secure on one side of the newly sistered joist. I’m a very basic DIYer so as much instruction / specific products to use would be really appreciated.

The joist end looks okay from my view of the picture so a sistered joist with the end butted tight to the wall should give the support you need. As I understand it, it will be a shower tray over so it is not like a bath full of water weight.

DWD

Probably belt and braces but if you are concerned about the wall/sister junction you could use a thick section piece of steel angle screws to sister and wall or perhaps a couple of resin anchors into the wall . You may need to source the section at a local engineering place as it’s unlikely somewhere like B&Q would stock such an item . You’d probably need something like 75-100mm wide by 10-12mm thick.