The short answer
Not properly. The underlay, often still called felt, is laid over the rafters and below the battens, so reaching it to replace it means taking the tiles off the slope. Some partial measures exist for small areas, but a worn underlay is cured by stripping the slope, renewing the underlay and battens as needed, and relaying the tiles.
The layer that has failed is the one you cannot see. Here is why the tiles have to come off and what limited fixes exist.
Where the felt sits and why that matters
On a tiled roof, the rafters carry the underlay, a sheet membrane that backs up the tiles when rain or snow gets past them. Over the underlay come the battens, the horizontal timber strips, and over the battens come the tiles. The underlay is tucked beneath all of it.
Why can't a new sheet just be slid in?
Because the old one is pinned down by battens nailed through it into the rafters, with the tiles hanging from those. To lay a continuous new sheet, the roofer takes off the tiles and the battens, rolls out the underlay from eaves to ridge with proper laps, and then refixes battens and tiles on top.
Underlay on older roofs is often bitumen felt, which dries, cracks and tears with age. Modern sheets are synthetic. Either way, the sheet only works as a continuous layer, which is why patching through the tiles gives you a patch, not a new underlay, and the original problem remains around it.
If tiles must come off anyway, it is the moment to inspect the battens too, and the answer on whether roof battens need replacing covers the signs.
What partial fixes exist
There are limited options, and each has a cost in what it cannot solve. They suit a small, known fault better than a roof that is simply worn out.
Can a section be done by lifting some tiles?
Yes. For a leak in one area, a roofer can strip a local patch, lay a new piece of underlay lapped under the course above and over the course below, then relay the same tiles. That is a genuine repair. It still means removing tiles, only fewer of them.
What about sealing from the loft?
Sprays, foam and sealed patches applied from underneath can stop a drip for a while, but they treat the symptom. The Energy Saving Trust is blunt about foam sprayed onto a leaking roof: it does not recommend the practice. Foam stuck to the underside can also hide a fault, trap moisture and complicate later work, and our page on spray foam removal covers the aftermath.
For a whole slope, a partial fix is rarely the better buy. Going slope by slope is the subject of the answer on re-tiling one side of a roof. A leak that keeps coming back after a patch usually means the sheet around it has aged too, and chasing it patch by patch ends up costing more than the larger job would have.
Which underlay goes back, and the loft ventilation
When the underlay is renewed, the type chosen affects the loft. HR underlay, the high-resistance kind, is non-breathable, so it needs ventilation at the eaves. Marley sets an eaves opening of 25 mm for pitches of 15 degrees or less and 10 mm for anything steeper. A breather underlay of the low-resistance type, with a well-sealed ceiling below, can lean on 3 mm of incidental eaves ventilation where third-party certification backs it.
Does a new underlay mean new ventilation?
It can. Moving from a breathable sheet to a non-breathable one, or the reverse, changes how the roof space handles moisture, and BS 5250 is the code of practice on that subject. The answer on roof underlay types compares the choices.
This is another reason to do the job properly while the roof is open. A roofer who sees the whole slope can check eaves vents, insulation at the wall plate and the state of the rafters in one visit, which is hard to do through a few lifted tiles. Cold, damp lofts after a re-roof are often traced to a ventilation path that was missed when the sheet changed.
Re-roofing, cost and next step
Replacing underlay across a whole slope usually means re-roofing that slope, because the battens and tiles go with it. If the tiles are sound they can often go back on, with new ones in for any that break in handling.
What will it cost?
It depends on the size of the slope, the access and what the strip-off reveals. For context, MyJobQuote gives £3,000 to £8,000 for a full re-tile of 80 m² as a typical UK range (Oct 2026), and HomeBuilding gives £10,000 to £15,000 for a complete new roof on a three-bed semi (Sep 2025). A local repair costs far less, and which of these you face is for the survey to say.
The thing to avoid is a quote for a cheap fix that never says what it will actually do. Ask what is lifted, what is relaid and how the underlay is lapped. Other roof work is on the pitched roofing page. Our survey covers the loft and the slope, you see photographs of the underlay, and one written price covers the repair that fits what we find.
More on roof felt
How do I know if my roof felt has failed?
Take a torch into the loft. Torn or sagging underlay, daylight at the laps, damp patches on rafters or insulation, and mould or staining on the underside of the sheet all point to a problem. Keep to the joists and off the plasterboard. A roofer can confirm what you find.
Can I patch the felt from inside the loft?
A small tear can sometimes be taped as a stopgap, but a taped patch from below is not a repair, because it does not lap under the course above and water still runs behind it. For a small tear on an otherwise sound roof, ask a roofer to lift the tiles locally and lap a new piece in properly.
The full picture: Pitched roofing in Taunton · More short answers