The short answer
A tiled roof with no felt is not automatically a problem. Plenty of older roofs have no underlay and rely on tight laps, or on lime torching, a mortar fillet pressed under the tiles from inside. Work is needed when torching has crumbled, tiles have slipped, or wind-driven rain and dust are reaching the loft. Adding underlay then means lifting the covering.
Why older roofs have no felt
Underlay is a sheet that lies across the rafters beneath the battens, there to catch water slipping past the covering. Bitumen felt came into general use during the twentieth century. Before that, builders relied on the covering itself, laid with enough overlap to shed rain, and on the careful fitting of each tile or slate.
So when you climb into the loft of an older Bath house and see the underside of the tiles, with battens and daylight in the gaps and no sheet in between, that is probably the original arrangement. Clay plain tiles, stone tiles and slates all shed water by overlapping, and in normal rain they do their job unaided.
The roof was designed as a system with its own ways of coping. Clay tiles are porous, so they take on damp and release it again. The loft, often draughty, dried everything out. Adding a sealed sheet to a roof that was built to breathe changes how it behaves, which is a reason to take care when deciding what to add.
Torching: the mortar you can see from the loft
Some roofs look as if they have a pale grey layer pressed up between the tiles from below. That is torching, a fillet of lime mortar, often mixed with hair, spread by hand on the underside of the tiles where they rest on the battens. Its purpose is to stop wind-driven rain, snow and dust being blown up between the tiles and into the loft.
Torching was widely used on clay and stone roofs. It is not waterproofing in the way a felt is. It slows the entry of fine water and stops draughts. Over time it can dry, shrink and fall away in lumps, and you may find grey crumbs on the loft floor. That is the main sign it has done its work.
Traditional roofs need care with materials, and modern cement mixes can trap damp against old timber. Where torching is replaced, lime mortar is the usual choice, and we match what was there. It is detailed work, and it is done from inside the loft against the tiles, not from the roof.
When no felt is fine, and when it is not
A roof without felt is fine when the tiles are sound, the laps are tight, the torching is intact and no water is reaching the timbers. Many such roofs run for generations with only occasional repairs, replacing slipped tiles or patching lead.
Problems show up in a few ways. Powdery grey debris on the loft floor points to failing torching. Damp or dark patches on rafters and battens, or white salt and rot, show that water is getting in. Daylight through the covering at more places than a few is another warning, and so is a draught that makes the loft icy in winter. Fine snow blown in during a storm is a clear sign that the roof no longer excludes the weather.
The cause is often small: a few slipped or cracked tiles, a failed valley, or lead flashing that has lifted. Those are repairs, and our page on pitched roof maintenance sets out the checks that keep a roof like this sound. Where the covering has broadly failed, the question changes from repair to re-covering.
Adding underlay: what it involves
You cannot slide a new underlay under an existing covering. To fit one, the tiles or slates on that slope are taken off and set aside, the battens are lifted or renewed, the underlay is laid over the rafters, new battens are fixed over it, and the covering is relaid. Sound old tiles are reused, with matching tiles making up any shortfall.
That makes it a job of the scale of a re-roof, with the cost and the scaffold that follow. It is usually done when the covering is due for renewal anyway, not as a stand-alone upgrade. Planning matters on listed houses and in the conservation area, where the designation reaches roughly half the city, so speak to B&NES before changing the character of a roof.
There is also a ventilation question. Breathable membranes and non-breathable ones need different amounts of air movement, as Marley's guidance based on BS 5250 sets out. Re-roofing with a membrane needs eaves and ridge ventilation to match, otherwise condensation can form in the loft. Re-roofing can also be notifiable. Building Regulations approval is not normally needed where the work touches less than 25 per cent of the building envelope and less than 50 per cent of the roof, but beyond that point an insulation upgrade can be required.
Checking your own roof and deciding what to do
You can learn a lot from the loft with a torch on a bright day. Look for light through the covering, damp stains on timber, grey mortar crumbs on the floor, and tiles that look out of line from below. Mark any wet patches and note whether they appear after heavy rain or all the time. Take photographs.
From outside, check for slipped or missing tiles from the ground, using binoculars if you have them. Avoid climbing up: tiles are easily broken, which our guide on whether it is safe to walk on roof tiles explains.
The covering also makes a difference. Concrete tiles, which are often found on later roofs, are usually laid over felt, and a missing underlay under them is a more serious omission than under old clay. Our page on concrete roof tiles explains the difference.
If you find signs of water, a leak is made safe before anything else. The survey costs nothing and covers the loft and the outside of your own roof, followed by a written fixed price. Where repair works we repair, and a roof without felt that is otherwise sound is often left as it is. Our pitched roofing page sets out the wider range of work.
