Under every tiled or slated roof there is a second line of defence. Wind can drive rain past a tile, and snow can blow in through the gaps between them. The underlay catches that water and sends it down to the gutter. Until the 1990s that layer was almost always bitumen felt. Now a roofer can lay either bitumen or a synthetic sheet, and the two behave differently in the loft, so the choice is a real decision rather than a line on an invoice.
The difference is whether moisture vapour from the house can pass through the sheet. That one property changes how the loft needs to be ventilated, how condensation builds up and how long an old underlay stays dry. This guide sets out the two types in plain terms, what BS 5250 says about each, when it makes sense to renew underlay on its own, and what to ask before you agree a price. For the wider picture of tiled and slated work, see our pitched roofing service.
What underlay does and how felt differs from membrane
Underlay is a flexible sheet laid across the rafters, under the battens and tiles. It is a back-up, not the waterproof layer itself: the tiles do the main work, and the underlay deals with whatever gets past them. Without it, driven rain or fine snow would land on the loft insulation and the ceiling below.
What is roofing felt?
Traditional roofing felt is a fabric base soaked in bitumen. It sheds water well but it does not let moisture vapour through, so a roofer describes it as non-breathable. BS 5250 calls this class of underlay HR, for high resistance to vapour. The warm, damp air that a household produces while cooking, washing and breathing rises into the loft. Meeting a felt underlay, that vapour cannot escape through the roof, so the loft has to be ventilated to carry it away.
What is breathable membrane?
A breathable membrane, sometimes called a vapour-permeable or breather underlay, is a synthetic sheet made to block liquid water but let vapour pass. BS 5250 uses the label LR, for low resistance. The same moisture can pass up and out of the loft through the sheet, which reduces the amount that settles as condensation on cold timber. Neither type makes a leaking tile watertight. Both need to be laid with correct overlaps and supported properly, and fixed to the manufacturer's instructions.
Breathable does not mean the loft can be left unventilated. As the next section shows, the standard only relaxes ventilation in a narrow set of circumstances.
How BS 5250 sets ventilation for each type
BS 5250:2021 is the British Standard on managing moisture in buildings, and it is listed by BSI as current but under review. Marley's guidance on cold roofs, where the insulation lies at ceiling level and the roof space above stays cold, sets out what it asks for at the eaves. Those are the gaps at the bottom of each slope where air enters.
What does a felt roof need?
With an HR underlay, Marley says the eaves ventilation should run along the whole length of the eaves: 25mm for pitches of 15° or less, and 10mm for pitches over 15°. These gaps are filled by vents in the soffit board, or by tile or eaves ventilators. Many lofts built before such guidance have no such gap at all, or have it blocked by insulation pushed hard into the corner, which is a common reason for damp timber.
What does a breathable roof need?
For an LR membrane the standard's route is narrower than people expect. Marley explains that ventilation can then come from the ordinary eaves construction, a gap of about 3mm, provided the ceiling is airtight, the covering is air-permeable and the product has independent third-party certification. If a product does not carry that certification, the felt rules above apply, even though the sheet is breathable. The practical question for any quote is therefore not just which sheet is going on, but also what ventilation the roofer plans and which product certificate backs it up.
Our guide on loft condensation and roof ventilation explains how to tell whether your own loft has the air it needs.
Choosing between felt and membrane for your roof
Neither material wins everywhere. The better choice depends on the roof you have, what you are building around it and how much you want to rely on ventilation alone.
When does felt make sense?
Felt remains a sound choice where the loft is generously ventilated, the roof is a simple cold roof and the ventilation gaps at the eaves are in place or can easily be made. It is a familiar material, widely used and easy for any roofer to lay well. The point is to treat it as part of a system: felt plus real eaves gaps, and, where the design calls for it, ridge or tile vents high up as well.
When does membrane make sense?
Membrane earns its place where ventilation is hard to arrange. A loft that has been boarded and insulated tightly, a low-pitched slope with little room at the eaves, and a roof with a complicated shape that leaves pockets of still air are the usual cases. It also suits houses where the household makes a lot of moisture. In each case the sheet gives vapour a way out that felt does not.
Does one last longer?
Neither is a promise. Underlay ages from sunlight at the eaves, from contact with wet timber and from the heat cycles of the loft, and the length of a maker's warranty tells you little about how long the sheet will actually last. Ask for the manufacturer's data sheet for the product named in your quote. It should state the intended use and the roof pitch, the lap sizes and the ventilation it assumes. A roofer who cannot say which product is going on has told you something already.
When underlay alone justifies the work
Underlay sits beneath the battens, so it cannot be renewed without lifting the tiles and battens above it. That makes underlay alone an expensive job to do in isolation. It only makes sense when the evidence is strong, or when the tiles are coming up for some other reason.
What signs point to a failed underlay?
Pick a dry afternoon and go up with a torch. Torn or sagging sheet between the rafters, daylight at the eaves, dark marks along the battens and drips landing on the insulation in rain all suggest the sheet has failed. A perished underlay can also crumble when touched. Damp at the eaves, where the sheet should lap into the gutter, is a frequent place to start; our page on rotten felt at the eaves covers it.
Can it be done without taking the tiles off?
Sometimes a local fix is possible, such as sliding a new section under the tiles at the eaves or around a repaired patch. Renewing a whole slope's underlay is different, because the full run has to be laid under the battens. We have written separately about whether you can replace roof felt without removing tiles.
When is it worth doing on its own?
The case is strongest where the tiles are sound and reusable but the underlay has clearly failed and is letting water in. Lifting good tiles, renewing the sheet and battens beneath and laying the same tiles back costs less than buying a new covering. The case is weaker when the tiles are tired themselves, because you would meet the same scaffold and labour bill a second time before long.
Rules, bats and what the work triggers
Underlay work looks minor, yet it can touch a few formal requirements. None of them should stop a job, but each is easier to deal with before the scaffold goes up.
Do Building Regulations apply?
Renewing the underlay across a slope normally means re-roofing it. The Planning Portal's test for when approval is usually unnecessary is that under 25 per cent of the building envelope and under 50 per cent of the roof is touched, and one slope of a small house can cross that line. In Crawley, the council's building control team lists replacement of more than a quarter of a roof among the jobs it must approve, so a bigger job there is worth raising with them first. Elsewhere, ask your own building control body. A heavier covering matters too: the Planning Portal asks for approval when it adds 15 per cent or more in weight.
Is BS 5534 the law?
No. Marley points out that fitting a roof to BS 5534 is a matter of good practice and not a legal duty. The standard deals with how slates, tiles and battens are fixed over the underlay, and a careful roofer follows it anyway.
What about bats?
The law shields every bat species and the spots where they roost, and GOV.UK's bat guidance puts re-roofing and roof repairs on its list of risky jobs. Gaps under tiles suit roosting bats, so lifting a covering is the moment they are most likely to be disturbed. If you notice droppings or see one emerge at dusk, tell your roofer before work begins so the proper steps are taken.
Typical prices and what to ask in a quote
Underlay is only one part of a re-roof or a stripped slope, so it is rarely priced alone. What you pay is driven by access, the area of the slope, how much of the covering can be reused and what the survey finds beneath it.
What does a re-roof cost nationally?
Checkatrade's roof installation guide, in its October 2026 table, puts pitched tiling at £120 to £275 per m², with plain tiles at £120 to £230 and clay at £160 to £275. The same company's roof replacement guide quotes £700 to £1,300 for the scaffold on a new roof. Treat these as a typical UK range to orient yourself rather than an estimate for your house, and bear in mind VAT at 20% may sit on top.
What should the written quote tell you?
Whoever you ask, the quote should name the underlay by manufacturer and product, so you can look up its data sheet. It should state if the total covers VAT and the scaffold, what ventilation goes in at the eaves and ridge, and what happens if the rafters or battens are rotten once the tiles are off. Ask whether the old tiles will be reused and who takes the waste away.
Each of our jobs begins with a survey, and you are sent pictures from your own rafters and eaves, so the state of the sheet is clear before anything is lifted. The price then follows as one written total. If you already have a survey report or a photo from the loft, send it with your enquiry and we will go through it first.
Part of our Pitched Roofing Services in Crawley
Underlay queries
Can I put breathable membrane over old felt?
That is not good practice. Old felt is non-breathable, so it would stop vapour passing through and defeat the point of the new sheet. Underlay is also laid under the battens, so the old one has to come out with the tiles. Ask your roofer to explain exactly what is being removed and what goes back.
Do I still need roof vents if I have breathable membrane?
Often yes. The relaxation described in BS 5250 is limited to certified products where the ceiling is airtight and the tiles or slates let some air pass. Outside that, normal ventilation is still expected. Ask which product is going on, what certificate backs it and where the air will enter.
Will new underlay stop condensation in my loft?
It can help, but it is rarely the whole answer. Condensation starts with damp air in the house, so extractor fans, a sealed loft hatch and clear eaves all matter. Insulation stuffed into the eaves can undo the effect of any underlay. A survey of the loft should come before a decision on the sheet.
