Roofing felt covers three quite different products that share a name. There is the thin mineral sheet sold in rolls for sheds. There is the underlay laid beneath tiles and slates on a pitched roof. And there is built up roofing, a bonded multi layer system that is a serious flat roof covering. Being sold one when you are pricing another is the most common confusion in this part of the trade.
Shed felt is a consumable
The green or grey mineral roll from a builders merchant is designed for a garden building and a handful of years of service. It is nailed down, it has no meaningful resistance to standing water, and it embrittles with ultraviolet exposure. Used on a shed with a decent pitch and replaced when it hardens, it is entirely fit for its purpose. Used on a house extension it will fail, and quickly.
Underlay does a different job
Beneath tiles and slates sits an underlay, which is a secondary barrier rather than the waterproof layer. It catches wind driven rain and any water that gets past the covering and carries it to the gutter. Modern breathable underlays also affect how the roof space ventilates, which is why swapping one type for another during a re roof is a decision rather than a detail. Approved Document F is where the ventilation expectations sit.
Built up roofing is the real system
A proper felt flat roof is two or three layers of polymer modified bitumen sheet, bonded together and to the deck, with a mineral or reflective cap sheet. Modern SBS and APP membranes are elastic, durable and repairable, and a well laid three layer system is a long life covering rather than a cheap option. It is the layering and the detailing at upstands that makes it work.
Torch on, and why the flame matters
Torch on membranes are bonded by melting the underside with a gas torch as the roll is laid. It is quick and it makes an excellent bond, and it is also hot work on a timber building, which is why competent contractors keep the flame away from eaves and abutments, use self adhesive or cold bonded membranes at vulnerable details, and carry out a fire watch after finishing. If a contractor cannot tell you how they manage hot work near a timber eaves, that is a real answer about how they work.
Questions people ask about roofing felt
How long does a felt roof last?
A shed felt is a few years. A modern three layer built up system, laid to falls with good detailing, is a long life flat roof covering. The spread between those two is why the single word felt is unhelpful when comparing quotes.
Is felt still used on new flat roofs?
Yes. High performance built up bitumen membranes remain a mainstream flat roof specification, particularly where durability and repairability matter. They compete with single ply and liquid systems rather than being superseded by them.
Can felt be repaired?
Readily, which is one of its advantages. A split or a lifted lap can be cut out and patched in with a matching membrane. A roof that needs repeated patching in different places is usually telling you about the deck or the falls.
What is the felt under my roof tiles for?
It is a secondary barrier. Tiles and slates shed most water, but wind driven rain gets past them, and the underlay catches it and drains it to the gutter. If yours is a brittle old bitumen sheet with tears in it, that is a normal finding on a roof of a certain age.