Hi,
From our understanding of your question, you let out part of your property to flatmates and are looking to determine if any or all of the expenses incurred replacing a worn out roof may be claimed against future rental income. In this situation, it is important to work out what type of rental business you are trading under. If you are trading using the rent-a-room scheme, then any expenses incurred cannot be claimed for.
However, as you have mentioned an RTM company, we would advise to contact our Corporation Tax team, who may be able to offer further advice surrounding this expense.
Corporation Tax: general enquiries
Thanks