You need the advice of a solicitor.
You appear to have at least three options; outright gift to you, purchase at market value, or you pay for the roof and your Dad becomes your debtor. Each will have there merits and disadvantages which will depend upon your individual circumstances.
Assuming your Mum is no longer living with your Dad, as she has not been mentioned, the basic IHT threshold is £325,000. However, as property is involved, (if he were to still own it at the time of death), there is a residence nil band of £175,000. Thus making the exemption £500,000.
If your Mum died and at the time was living in the UK with your Dad, then at death, your Dad will also benefit from any unused IHT allowance from your Mum's estate.