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Airhead's suggestion is a pretty sound one.
Out of curiosity are you sure there's nothing more substantial behind it? I would expect an external door to be sitting on some kind of brickwork or masonary at the very least. I'd be tempted to try and put a cheap drill bit through one of the more rotten places to see how deep it goes before it hits something solid. It may be that you don't have to worry.
We had an extractor fan fitted in our bathroom earlier in the year to combat the steam and condensation build up when having a shower. When it was first fitted you really noticed the difference, since then we've re-decorated the bathroom and I swapped the shaver socket for one of those wall mounted toothbrush chargers. This week I've noticed that there seems to be a lot more condensation building up when having a shower, water droplets running down the wall etc.
The fan looks to be working fine and is pulling air through so I'm at a bit of a loss as to why there's been a change in its effectiveness.
Anyone got any ideas? Could switching the shaver socket be effecting the power supply to the fan?