Every roofer has an opinion on this and most of them are financially interested in the answer. Here is a method you can apply yourself, with the figures we actually quote in the Mansfield area.
Work out your cost per year
Take the quoted cost of a full re-roof and divide it by the realistic remaining life of a new one. A £7,500 concrete tile re-roof over a conservative 45 years is about £165 a year. Now take what you have spent on repairs over the last five years and divide by five. If your repair figure is approaching your re-roof figure, you already have your answer — and the re-roof comes with a roof that does not leak.
Factor in what the leaks are costing you elsewhere
Roof repairs are rarely the whole bill. Water that has been getting in for two years means replastering, redecorating, and sometimes replacing insulation that has been wet and has stopped insulating. Those costs sit on the repair side of the ledger, not the replacement side, and people almost always forget them.
Be honest about the underlay
This is the single fact that decides most cases. If the underlay is sound, repairs are worth doing more or less indefinitely. If it has perished, you are relying on the covering being perfect forever, and repairs are buying time rather than fixing anything. You cannot replace underlay from outside without stripping the roof, so its condition effectively makes the decision for you. Get into the loft and look.
Consider how long you are staying
If you are selling within two years, a well-documented repair with a written guarantee is a reasonable commercial decision, provided you are not concealing a known structural problem. If this is a forever home, spending £2,000 on repairs to a roof that needs replacing in three years is £2,000 you will never see again.
When repair is clearly right
Isolated storm damage. A single failed valley or chimney flashing. A mortar ridge that has broken down on a covering that is otherwise sound and has decades left. A flat roof on an extension where the main pitched roof is fine. In all of these, replacing the whole roof would be selling you something you do not need, and any roofer suggesting it should be asked why.
When replacement is clearly right
Perished underlay across the roof. Widespread nail fatigue on slate. Concrete tiles that have lost their surface everywhere. Structural sag. Or a roof where three separate contractors have each fixed something and it is still leaking — at that point you are not buying repairs, you are buying guesses.
Get it in writing either way
Whichever you decide, get the reasoning in writing with photographs. It protects you if the work does not perform, it is what an insurer will ask for, and it is what a buyer's surveyor will want to see. If nobody has offered you that, ask — and if they will not, that tells you something too. Our roof repair cost guide sets out the figures behind all of this.