This comes under the heading of ‘latent defects’ for which there is no contractual procedure as the contract has now been delivered and reached a natural conclusion.
So, (as non-lawyer so might not use exactly the right legal terminology), you need to come to an agreement that :
A. effectively these are breaches of contract in that the works are not ‘fit for purpose’ (under the Sale of Goods Act) for the part which they provided and
B. agree a course of action to correct that breach e.g. they fix it or pay the Employer’s / Client’s costs to fix it.
If you can’t do this, then you need to be prepared to take legal action.
However, you do not state whether these are elements are designed by the Contractor or whether X15 was specified. The reason I say this is that if the Contractor can demonstrate that it is due to your design that, for instance, the roof leaks, then it would not be in breach of their contract. And if the Contractor did the design and X15 was specified then its liability would be reduced to reasonable skill and care for the design only, with materials and workmanship being fit for purpose.