Underlay is the sheet material laid over the rafters (the timbers running up the slope) and beneath the battens (the timber laths the tiles hang from). Its first task is to catch the rain and snow that blow under tiles, since no tile roof is watertight at every joint, and carry it down into the gutter. Its second, on modern roofs, is to let water vapour escape from the loft.
There are two broad families: traditional bitumen-based felt, and breathable membranes of different grades. Which you have changes how the roof should be ventilated, what a repair can achieve and what happens when the roof is next renewed. We explain each below, using the ventilation figures a manufacturer publishes from BS 5250. If you have spotted a tear and are worried about it, start with torn felt in the loft. The wider service is pitched roofing. For ventilation itself, see our ventilation and condensation guide.
What underlay is for, and what it is not
Think of the tiles as the first barrier and the underlay as a second, quieter one. Tiles shed most of the rain. Underlay deals with what gets past: wind-driven rain pushed up beneath the laps, snow that powders through gaps, and condensation that forms on the underside of the covering. It lets that water run down the slope to the eaves, where it drops into the gutter, in a controlled way.
It is not a waterproof roof in its own right. Underlay is laid with overlaps rather than welded seams, it is pierced by every batten nail, and it is designed to work with a covering above it. A roof stripped of its tiles and left under bare underlay is a temporary measure only.
Why the batten line matters
Rain that passes under the tiles reaches the underlay, travels down it and runs along the underside of the battens. The membrane is usually laid with a slight drape between rafters so that water follows the slope rather than collecting. At the eaves, a strip of eaves tray or a dressed overhang carries the water over the fascia board and into the gutter. If that detail is wrong, a perfectly sound membrane can still let water into the wall head.
What it does for the loft
In a cold roof, with insulation laid across the loft floor and the void above left unheated, the underlay is part of the ventilation story. The type decides how much air movement the roof needs, and that is where the main types differ most.
Bitumen felt, the traditional underlay
For decades the standard underlay was bitumen-impregnated felt, often called sarking felt, in a reinforced roll. It is also described as non-breathable, because it does not let water vapour through. Manufacturers' ventilation guidance, drawn from BS 5250, calls this type HR (high resistance) underlay.
Felt is cheap and familiar, and a sound layer in good condition does its job. Its weaknesses show with age. The bitumen dries out and the felt becomes brittle, especially where it is stretched taut at the eaves or the ridge. Batten nails pierce it, and where a nail has missed the batten, the hole stays open. Laps at the sides can open as the roof timbers move. A torn or split felt can often be seen from the loft as daylight or as a drooping flap of material.
Ventilation with felt
Because felt does not pass vapour, the loft needs deliberate ventilation to carry moist air away. Marley's reading of BS 5250 asks for an opening of 25 mm running the full eaves length when the pitch is no more than 15 degrees, and 10 mm on steeper roofs. A further 5 mm slot at high level applies when the pitch exceeds 35 degrees, the span exceeds 10 m, or the roof is a single-slope lean-to. Those figures are a manufacturer's reading of the standard, so check the product's own data sheet and your building control body.
If a felt roof has no ventilation at all, condensation drips from the underside of the felt and the timbers stay damp. The cure is ventilation, not a new membrane; our ventilation guide goes through the options.
Breathable membranes and the LR grade
A breathable membrane (sometimes called a breather membrane) lets water vapour pass through while keeping liquid water out. The second family in the BS 5250 guidance is LR, or low resistance, underlay. These are the modern membranes used on most re-roofs, and they are manufactured in several grades and weights.
Breathability brings a practical benefit. Moist air from the house can pass through the membrane and escape through the tiles, which relieves some of the condensation load on the loft. Marley's guidance, again citing BS 5250, says that where an LR underlay is combined with a tightly sealed ceiling and a covering that lets air through, the incidental gap left by ordinary eaves construction, about 3 mm, is enough, or else 5 mm at the ridge or high level.
Conditions that apply
The wording carries conditions. The ceiling must be well sealed, meaning no gaps around loft hatches, light fittings or pipes where warm air leaks upward. The covering must allow air to move. With a normal ceiling, the roof void is ventilated at the eaves, and the batten space too if the covering is air-impermeable. A breathable membrane is not a licence to block every vent.
Not all breathables are equal
Grades differ in how much vapour they pass and in their strength and handling. A membrane suited to a fully supported roof is not necessarily right for a roof where it will run over open rafters. We choose the product against the roof, the covering and the pitch, and name it in the written quote. How long a breathable membrane lasts depends on the grade, how it was handled and what it sits under, so we judge it on the survey and not from a number.
Reading your own underlay from the loft
A torch and a stepladder tell you plenty, safely, from the loft hatch. Do not walk on ceiling plasterboard between the joists. Only put weight on boards or joists.
Look at the colour and surface first. A black, slightly glossy or papery roll with printed bitumen lines is usually felt. A grey or white fabric-like sheet that feels like a woven plastic is more likely to be a breathable membrane. A manufacturer's logo printed along the roll often names it and sometimes gives the grade.
Signs of trouble
Look for torn flaps, sagging between rafters, dark trails of staining that mark where water has been running, and daylight at the eaves or ridge. Frayed edges at the eaves often show where the membrane was never lapped into the gutter properly. Rusted or missing clouts (the large-headed nails in the batten line) point to wet conditions over time.
Look for damp patches on the sarking boards or rafters where condensation has formed rather than rain. A grey bloom of mould and a musty smell point the same way. How to tell the two apart is covered in our guide to why tiled roofs leak.
What we record
On a survey we photograph the underlay at the eaves, the middle and the ridge, because it ages differently in each place. The prints from your own roof are added to the survey report, so what we say about its condition can be checked against them.
When underlay needs replacing
There is no fixed age at which underlay must be replaced. The question is condition. A felt that is still supple, unbroken at the laps and free of staining may carry on for a long time under sound tiles. One that has cracked, perished or been punctured in many places is no longer doing its job.
We recommend renewal when the membrane is broken across areas rather than in spots, when the battens beneath it are showing rot, when condensation is persistent and the existing felt is part of the cause, or when the tiles themselves are being renewed. Re-tiling over an old underlay saves very little and leaves the weakest layer in the roof.
Why replacing it means stripping tiles
This is the point that surprises most people. The underlay is trapped between the rafters and the battens, and the battens are covered by the tiles. To get at it, the tiles must come off, then the battens, then the old membrane is removed, the new one laid, and the battens fixed back before the tiles go on again. Sound tiles can be reused; cracked ones are replaced to match.
That is why the job is quoted as a strip and re-lay, not as a repair. A tear spotted from the loft can sometimes be patched locally, but the patch leaves the remainder of the sheet untouched. Our torn felt guide explains when a patch is reasonable and when it is only delaying the bigger job.
Underlay on older and protected roofs
Many older clay-tiled and stone-slated roofs in the Stroud valleys were laid without any underlay at all. Torching, a lime mortar pointed beneath the tiles from inside the loft, filled the gaps instead, and daylight could be seen through the roof. A building with a roof like that works differently from a modern one, and it often depends on that air movement to keep its timbers dry.
Fitting a modern membrane to a roof like this is not a neutral decision. It changes the airflow, it may need a different batten arrangement, and on a protected building it can need consent. For a listed home, or any house in a conservation area, ask the council's conservation team which membrane is acceptable. Our guide to clay-tiled roofs on listed and conservation-area homes covers the consent side.
Like for like
Like stays with like on an older roof. Where a roof has survived for generations without a sheet beneath it, we do not add one by reflex; we discuss the options with you and with the conservation officer where one applies, and we write the decision into the quote.
Breathability matters most on these roofs because their timbers are often old, and trapped moisture rots them. A traditional tiled roof should not simply be sealed up in a plastic skin.
Cost, regulation and what the quote should show
Underlay itself is a minor part of the cost of a re-roof; the labour of stripping and re-laying is the main part. For a typical house, MyBuilder's 2026 figures for a full re-roof run from about £5,000 to £12,000, and Checkatrade's 2026 UK average is about £8,000. Both are third-party national guides. Any strip needs scaffold, which MyBuilder prices at about £1,125 and Checkatrade at £700 to £1,300.
After the survey, the quote names the membrane and its grade, the battens, tiles reused or replaced, scaffold, labour and waste.
Regulations
By the Planning Portal's thresholds, building control approval is not usually required while under 25 per cent of the building envelope and under 50 per cent of the roof is touched. Going beyond that with a re-roof, or raising the covering's weight by 15 per cent or more, needs approval. A larger job can also bring in a thermal upgrade under Part L. We talk through consent, and notifiable work gets its Building Regulations notification filed for you.
The finished job comes with the waste gone, the site tidy, and our 10-year workmanship guarantee in writing. Underlay is one of the things we photograph before it disappears under the battens, which leaves a visual record of what sits under your tiles.