Nobody wants to be told they need a new roof. It is a five-figure conversation on most houses, and plenty of people will happily sell you a re-roof you did not need — just as plenty of others will sell you a repair on a roof that is past saving. Here is how to tell the difference before anyone gives you a price.
1. You can see daylight through the underlay
Go into the loft on a bright day with the light off and let your eyes adjust. Pinpricks of daylight at the eaves are normal ventilation. Visible light through the felt across the slope means the underlay has torn or perished, and underlay is the roof's second line of defence. Once it has gone, every tile has to be perfect forever, which no roof manages.
2. The underlay crumbles when you touch it
Old bitumen felt goes brittle and then powdery. Press it gently at the eaves — if it cracks and flakes away in your fingers, it has failed everywhere, not just where you touched it. You cannot replace underlay without taking the covering off, which means this one finding effectively decides the question.
3. Slates are dropping faster than you can replace them
This is nail fatigue. The nails holding a slate roof corrode from the head down, and once a few start failing the rest are the same age and the same distance through the same process. One slate a year is a repair. Three or four a winter is the roof telling you what comes next.
4. The ridge line has gone wavy
Sight along the ridge from the ground. It should be dead straight. A dip in the middle or a visible sag between the rafters points to structural movement — a failed purlin, spread rafters, or timber that has taken water for years. That is a structural job, not a covering job, and it is worth getting looked at soon rather than eventually.
5. You are calling a roofer every winter
Add up what you have spent on repairs over the last five years. When that figure starts approaching a third of the cost of a re-roof, and the roof is still leaking, the repairs have stopped being repairs and become an annual subscription with no end date.
6. The mortar is breaking down everywhere at once
Ridge and hip tiles bedded in mortar, verges pointed in mortar, and mortar fillets at abutments all have a life of roughly 20 to 30 years in this climate. When all of it is cracking and dropping out at the same time, it is not a coincidence — it is the whole roof reaching the same age simultaneously.
7. Concrete tiles have lost their surface
Concrete interlocking tiles are made with a coloured, sealed surface layer. When that weathers off, the tile beneath is porous — it absorbs water, grows moss readily and delaminates in frost. A roof where the tiles have gone uniformly pale and rough has reached the end of its material life regardless of whether any of them have actually broken yet.
8. It is simply old enough
Concrete tile: 40 to 60 years. Clay plain tile: 60 plus. Natural slate: 80 to 100. If your roof is original and the house is of a certain age, the arithmetic does the work for you. That said, age alone is not a reason to replace a roof that is performing — plenty of Victorian slate is still doing its job perfectly well.
The honest summary
One or two of these signs generally means a targeted repair. Four or five together, particularly if perished underlay is among them, means you are spending money on a roof that has already told you what it needs. If you want a second opinion in writing before you commit to anything, that is exactly what a roof survey is for.