Wet Rot on Rim Joist

[NeilK]

As a preliminary to getting a new kitchen installed, I had a plasterer come to deal with a ceiling that had some staining issues. He discovered that above the ceiling was an area which was obviously getting very wet. I have found that the cause was that when a bathroom was installed above, the installed removed a toilet overflow pipe that passed through the wall but failed to block up the hole, which couldn't be easily seen from outside the house.

The result is a rim joist that is very damp, and has an area of white wet rot about 1m long. Obviously, I've blocked up the hole to prevent further water ingress, and have fans going to try and dry things out. The plan was to dry the joist etc, scrape off the fungus and any rotten wood, apply hardener and fungus treatment. However, taking up a panel in the bathroom shows that this joist goes up into there a little way, and there is some white fungus there as well. I think the floorboards must sit in a groove in the joist.

The wood doesn't seem badly rotten, though there is a lot of fungus, but I am concerned that I won't be able to treat all of the joist (since the floorboards are in the way). The most complete approach would of course to be to take out the floorboards and replace the rotten section of joist, but if I can avoid having to strip out my new bathroom it would be good!!!

So my question is, would scraping the fungal growth off where I can get at it above and below the floorboards, drying everything out thoroughly, and applying hardener and anti-fungal treatment, be sufficient so long as the wood isn't deeply decayed?

I can get a photo uploaded later today.

[welsh brickie]

if you try and stab the joist with a screwdriver and it goes in more than 10mm then the joist needs replacing.
To make sure the treatment enteres the joist then drill the top of it pour the solution in.