Repair to terrace - waterproof membrane

I'm not sure there is too much you can do in practice. The contract will between the block of flats and the builder, not between you and the builder. The amministratore is the key figure here. It is his role to price up projects like this, employ builders etc. Any worries you have, you should raise with him. He is after all employed and paid by you (and the other flat owners) to look after your collective interests and can probably be voted out every year if you are unsatisfied with him as a group. Always good to be on friendly terms with your amministratore. Good ones can help you a lot.

Your individual circumstances could be different for some reason. (I've tried to edit my post by inserting "normally" but can't do for some reason) so for your peace of mind its probably better to check out again with ammin. especially where the conract and liablity rests. Its quite, common, for bills to be shared according to floor space. For example, someone with 100m2 will pay double compared to someone with 50m2. You seem to imply that your share of the bill is significantly higher, however,  than others? Is there a reason for this?

Hi Capo Boi - we have got Allan interested now.  I asked him to read our postings and then comment.  This is what he said:"The bit about the share of costs being proportional to floor area does seem to be the general outline, but there are any number of complications.  I believe that it is all laid down by law and the administrator carries around a big handbook which tells him all the percentages that apply to all the different parts of the building.  In the case of this particular terrace, it does form part of our garden so we pay the largest share.  But on the other hand it is the roof of the communal garage so everyone pays their bit as well.  The phrase "lastrico solare" keeps coming up, and broadly this seems to be the top covering of the building, whether it is roof, terrace or whatever.  A quick Google search for the Lastrico solare seems to show that it is the source of endless discussion and hair-splitting."Looking our garden terrace, you would not guess that there was any problem underneath in the garages.  I just want to make sure we get value for our 5000 Euros when we pay it, and that the problem is rectified.