Rot behind wall in bathroom. Floor has caved in

[Erene]

I've just purchased a house... Without a structural survey! I feel so stupid as I've just finally realised why the floor at the very top of the stairs and the small room just to the front/right side of the stairs has caved in... The bathroom which is to the right at the top of the staircase had probably been leaking for years. There are large gaps between the tiles in the bathroom wall in a large section just above the bathtub (which is abutting the wall of the adjacent small room where the floor has sloped down). It looks like a very bad tiling job however the rest of the bathroom tiles are placed perfectly. My guess is that when trying to repair the bathroom the previous owners realised that they had a massive rotting problem with a fallen floor beneath it and so covered it back up. There's another new bathroom downstairs. So the floor around the wall where the long edge of the bathtub is has caved in (both the floor of the adjacent room and the top of the staircase). The bathroom tub and floor seems fine though - perhaps it was a cover up job.

My questions:
1. How much does such a large issue cost to repair (I ideally want the floor in the area around the bath flat again!)?

2. Will my insurance cover me or may they tell me that it's my fault since I didn't get a surveyor or, because it was an issue caused before I got the property?

3. Is it essential to fix? I can live with a sloped floor but don't wish to do that if it may collapse more?

I'm in a real rut and need some advice/knowledge from you kind and knowledgeable people, please?

Edit: I've now added a photo as requested.

 

[Mr Chibs]

Post some pictures, as it’s impossible for us to guess how bad it is.

Replacing joists/floorboards is generally not that difficult, you might be able to do it yourself, so you are just paying materials.

 

[Lower]

I sincerely hope this is an april fools joke.

 

[Erene]

?? It's not a joke. The only thing that pictures will show is the tiles coming off the wall slightly but someone has recently gone over the gaps with silicone. The floor dip is too slight to show on pictures. I'll attach a photo...

 

[Lower]

Caved in is significantly different to a slight floor dip that is too slight to show on photos......

Its unlikely that your insurance will cover a defect that was pre-existing in a property that you've just bought.

The only way to tell what the cause of the dip is by getting under the bath and having a look at the joists. It may be an old leak that's been repaired but they didn't get the floor flat, or it may be rotten joists that need replacing. It needs investigating by the sound of things.

You may be able to get the bath panel off and have a look underneath the bath. It'll be easy to spot if there has been significant leakage to cause a joist to rot.

 

[Mr Chibs]

It was the floor I wanted to see, you’re right the tiling picture tells us not a lot.

 

[m0t]

That’s a really poor tiling job plus the sealant in the corner and round the bath looks terrible. I wouldn’t be surprised to find water had been getting through. The sloping floor could just be because of age/movement rather than rot. When mine failed the whole floor bounced as you walked across it.

I’m not a builder but I had to do this job myself.

You need to take the side panel of the bath off to see what the floor looks like underneath. Take some photos (ideally both close and further out) and post them here.

If the floor does have wet rot underneath then the fix will depend on the extent of the problem and where on the joists it is. If it’s the ends then you can cut them off and replace them with bower beams, they cost around £20 per end for materials. You can also sister the joists with new ones.

Levelling the floor may require you to pack the tops of the joists before putting the boards back down.