Underlay life
How long does roofing felt last under tiles?
Underlay under tiles has no fixed lifespan, so it is judged by condition. Below: the figures people confuse it with and how we assess it.
The short answer
Felt or underlay beneath roof tiles has no fixed lifespan, so it is judged by its condition and not by its age. The figures you may have seen belong to torch-on felt on flat roofs, where HomeBuilding gives 10-15 years and Less says 10 to 20 years depending on quality and installation. That is a different product with a different exposure. We look at the underlay from the loft before deciding whether a roof needs a repair or a retile.
5 min read · Part of Pitched roofing
Why a flat roof figure is the wrong guide
Searches for felt lifespans land on flat roof pages, because torch-on felt is the covering of many flat roofs and it is the part that sees the weather. The published ranges are worth knowing so you can tell what they describe, and so you can spot them when a website quotes one for the wrong roof. Both are ranges rather than promises, and both depend on how well the work was done.
| Published by | Figure |
|---|---|
| HomeBuilding | 10-15 years for felt |
| Less | Torch-on felt lasts 10 to 20 years depending on quality and installation |
Under tiles the felt plays a different part. It is a second layer that catches what gets past the covering, so it is shielded from sun and weather and it never carries the roof's waterproofing by itself. Borrowing a flat roof number for it would be wrong in both directions, and Marley's tile figures say nothing about what lies beneath. Neither guide above is describing a pitched roof, so neither should be read as one. If a quote or an article gives you a number of years for felt, ask which roof it was measured on.
How the underlay is read from the loft
Without a figure to lean on, the sound method is to look. The loft gives the clearest view of the underside of the underlay, so a loft visit comes early in any survey of a tiled roof. We are not guessing an age. We are asking whether the layer is still doing its job, and on a loft visit that comes down to a short list of things we can see with a torch and a steady hand:
- tears, splits or holes in the felt, particularly around the eaves and the ridge
- sagging between the rafters where the felt has slumped or the battens have moved
- daylight showing through, or stains and darkened patches on the timbers
- felt that has gone brittle, or crumbles when touched
- laps and joints that have opened
A sound loft with dry timbers and an intact underlay points towards repair. Staining, torn felt and a covering that is failing across the slope point the other way, and in that case we explain why before any price is mentioned. Our slipped tiles guide sets out where we draw that line for tiles.
Ventilation and the type of underlay
The kind of underlay changes how the roof must breathe. A high-resistance underlay, often called HR felt, does not let vapour through, so Marley gives eaves ventilation figures for a cold roof under BS 5250 (BSI lists the 2021 edition as Current, Under Review). A breathable (LR) type, laid above a ceiling that is properly sealed and under a covering that lets air through, needs far less.
| Underlay | Pitch | Eaves opening |
|---|---|---|
| High resistance | 15° or less | 25 mm along the eaves |
| High resistance | More than 15° | 10 mm along the eaves |
| LR type, sealed ceiling, air-permeable covering | Any | 3 mm from normal eaves construction |
For the 3 mm case Marley adds that UKAS third-party certification must be met in full. If we find an HR felt in a loft with blocked eaves, the question changes from how old it is to whether the roof can dry. That is a ventilation problem, and a new covering alone would not cure it. We note the eaves detail in the survey whichever underlay is found, because it decides what can safely be laid back.
When the underlay is renewed
Underlay is replaced when the tiles above it come off, since the old layer can only be changed with the covering lifted. That makes the felt a deciding factor in repair or retile. A slope that is mended tile by tile keeps its underlay, so a repair is only sound where the felt beneath is sound. A slope that is stripped gets a fresh underlay as a matter of course, and the battens are checked at the same time, since both are exposed together and neither costs much to inspect once the tiles are off.
Marley cites a BRE minimum life expectancy of 60 years for clay tiles, which shows how far a good covering can outlast what sits beneath it, and why we look under the tiles rather than only at them. If the felt is the only failing part, we tell you that plainly and price only the work that fits. The cost to retile page gives published whole-roof ranges, and the pitched roofing page shows the rest of the job.
More questions
Can I tell how old my roofing felt is from the loft?
Not reliably. Underlay beneath tiles has no fixed lifespan to read an age against, so we judge it by what we can see: tears, sagging, brittleness, open laps, and stains on the timbers. The roof is surveyed from the loft and from outside, and you receive a plain report of what the felt is doing before any work is proposed.
Does HR underlay need the roof ventilated at the eaves?
Yes. A high-resistance underlay does not let vapour through, and Marley gives 25 mm along the eaves for pitches of 15° or less and 10 mm for steeper pitches in a cold roof. A breathable (LR) type can need less, depending on the ceiling and the covering, so the underlay fitted matters whenever a roof is re-laid.
Tell us what's wrong with the roof.
You'll get one written total price.