1. You've Had the Same Section Repaired More Than Twice
If the same area of the roof has been repaired multiple times and continues to fail, the problem is systemic rather than isolated. On Derby's older estate stock, this usually means the underlay beneath the tiles has failed — it's no longer providing the secondary weathering barrier that the tile is designed to work with. Individual tile repairs don't address failed underlay; only a full strip and relay does.
2. Widespread Mortar Failure Across Multiple Elements
If the ridge mortar is failing, the verge pointing is crumbling, and the hip mortar is loose — all on the same property — the roof is at end-of-mortar-life across its entirety. Repointing all three elements is a legitimate repair that will last another 15–20 years. But if the batten nails are also rusting through (a separate issue on very old roofs) and the tiles themselves are beginning to spall at the surface, the economics shift toward full replacement.
3. Visible Daylight in the Loft
Daylight visible through the roof structure — not through the ventilation gaps at the eaves, but through the tile or felt layer — means the secondary weathering barrier has failed. Water follows. Any amount of daylight visible through a roof structure is a replacement indicator rather than a repair.
4. Widespread Spalling or Delaminating Tiles
Concrete tiles that have lost their surface weathering layer (spalling) or are beginning to delaminate will continue to deteriorate regardless of repair. Individual cracked tiles are a repair; a roof where 30%+ of tiles show surface degradation is a replacement. On Derby's 1960s estate stock, tiles from this era have now passed the end of their expected surface life on many properties.
5. The Underlay is Visible and Brittle
In the loft, look at the underside of the roof between the battens. The roofing felt (or modern breathable underlay) should be intact and either flat or very slightly sagging between battens. If the felt is visible from outside through gaps, if it's torn in multiple places, or if it crumbles when touched in the loft, the secondary barrier has failed and the tile work above is the only thing between you and a wet loft.
6. Multiple Leaks From Different Locations
A single leak from a single failure point is a repair. Multiple leaks appearing in different locations after the same rainfall event indicates system-wide failure — the roof is finding multiple routes for water ingress simultaneously. This pattern points toward full replacement.
7. The Roof Is Over 50 Years Old and Has Never Been Replaced
Derby's 1960s estate properties are now over 60 years old. The original concrete interlocking tile will often still look reasonable from the ground. But the underlay, the battens, the mortar and the nail fixings are all at or past end of expected lifespan. A full survey is warranted on any Derby property over 50 years old without a documented replacement — not because repair is impossible, but because informed decisions require knowing the full picture.
8. Repair Cost Approaching 50% of Replacement
When multiple repairs quoted across all failing elements approach 50% of the cost of full replacement, the replacement gives significantly better value: new underlay, new battens, full tile warranty, 40+ year expected life. The repair leaves old underlay, old battens, and the certainty of further repair costs within 10 years. We make this calculation explicitly in every survey and present it honestly.
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A well-maintained concrete interlocking tile roof should last 40–60 years. Welsh slate correctly installed lasts 80–100 years. GRP flat roofs have a 25-year material warranty. The main variable is maintenance — a roof that has never been inspected or had minor repairs attended to will need replacing sooner than one that has had routine maintenance.
It depends on the condition of the underlay, battens and tile surface — not just the age. A 60-year-old roof that has had periodic maintenance, has sound underlay, and shows only isolated tile or mortar failure is worth repairing. The same age roof with failed underlay, rusting nail fixings and widespread tile degradation is better replaced.
A standard 3-bed semi in Derby costs £4,000–£8,000 to re-roof in concrete interlocking tile. Welsh slate re-roofs run £6,000–£12,000. Larger detached properties £8,000–£15,000. See our full cost guide for detailed breakdowns.
Yes — we offer free surveys across Derby with a written condition report. Call 0133 240 5131 to arrange.