Ask an Property Solicitor. Get an Answer ASAP.
The structure of the building and normally the roof is the responsibility of the freeholder and the buildings insurance that you actually pay for through the service charge should normally cover this. Without seeing your Lease Deed and examining the covenants therein I cannot say why you are being asked to contribute. Reviewing the Title Deed would attract an additional small charge as it is often an extensive and not simple document to review, if you want that you can advise and I will ask customer support to send you their offer.
Any follow up please do not hesitate to send your message. Always glad to assist and to clarify anything