responsibility for joists in apartment building

Dry rot was discovered in the floor below the bath in one of my flats.

It's also in the joists which are my bathroom floor joists and are the ceiling joists of the flat below.

Is the remedial work (and cost) the joint responsibility of the two flats to deal with as the joists are common to both apartments?

First suggestion - read your deeds, conveyancing documents, contract etc. Because general principles may not apply if you have agreed to something else.
(My version of RTFM)

Deeds are silent on the matter.

I'm pretty sure costs of joists between apartments are split 50/50 as the joists are jointly owned. Just not sure.

Not if the rot was caused by a leak in your bathroom.

It may not explictly say, but it will be covered somewhere almost certainly. Look for wording such as "internal party walls....." which in the absence of anything more specific would cover the ceiling/floor. Its very likely such party walls you own half way thru, so would depend on where the joists are, but more than likely they would be closer to the floor of the flat above, than your ceiling. Regardless from a responsibility point of view, its infinately more likely they have or have had a leak which has caused them to rot, than the issue being caused by the flat below.

Try asking the management company who's responsbile, it may fall under common parts and be their liability too. If all else fails, offer to go 50/50 as you say. At the end of the day you just need it resolving really.

If the leases are silent as to who is responsible for the fabric of the building then I would have thought freeholder would be responsible.

Having said that if the bath is responsible for the rot then I'd have hoped the bath owner would pay.

This - except the freeholder will almost certainly have put a mgmt company in place to pay for such like, so the cost will come back out of the service charge which is ultimately the leaseholders cumulatively.

As said though, if its down to a leaking bath or shower then the leaseholder above is clearly liable and they or their home insurance policy should pay for the repair.

Full disclosure 1): This is in Scotland where virtually all property is freehold. My (technically my wife's) apartment is one of 12 in the building which are a mixture of privately owned and HA owned. The HA are also the factors (managers) of the building and their management includes organising the comprehensive common buildings insurance policy which covers risks to both the individual dwellings and the common areas.

Full disclosure 2): I've already sorted it, accepting that the insurers may or may not pay out on the claim. I've also contacted the HA whose maintenance manager says it's above his pay grade to decide whether or not they can pay half the costs of the joist remedy and has referred me to their Director of Assets.

Next week I'll be sending her the full bhoona paperwork/pics/vids etc and then no doubt wait a few weeks for their committee to meet and decide on it.

Personally, I think the insurers will repudiate the claim.

More confident that the HA will share the costs (eventually).

O and here's a tiny extract from a Housing Tribunal hearing:

"...The Tribunal’s Decision:

The Tribunal have already determined that the joists are the property of the flats concerned and they not a common part of the tenement...."

Not necessarily - if the bath had overflowed & damaged the flat beneath then it's up to the owners of the flat beneath to claim on their insurance, not the owners of the bath. If the freeholder/management company isn't going to take on the responsibilty then it might be the two flats are jointly responsible for half the joist each.

Not sure that's the case.

An overflowing bath is clearly negligent and the responsibility of the bath owner

A leak in the bath or pipes may not involve negligence or a specific event so may be down the flat below.