How Long Does a Felt Roof Last?

The short answer: it depends, and anyone who gives one number without seeing the roof is guessing. Manufacturers publish design lives for modern reinforced bituminous membrane (RBM) systems that commonly sit in the range of 20 to 40 years, while the lightweight felts of past decades often managed nearer 10 to 15. Your roof's answer lies in its system, detailing and care.

Why is there no single number?

Because "felt" describes a family, not a product. A three-layer polyester-reinforced membrane torched over sound insulation, with generous upstands and clean drainage, is a different machine from a single skin of decades-old mineral felt nailed to garage boards. Add the other variables, quality of installation, standard of detailing, exposure to sun and standing water, and whether anyone has ever cleared the outlets, and the honest range stretches from a decade to several.

That is why the useful question is rarely "how long does felt last?" and nearly always "how much life is left in this felt, on this roof?"

What do manufacturers actually publish?

Membrane manufacturers publish design lives and guarantee terms for their specific systems, and for current polyester-reinforced RBM build-ups those published figures commonly fall between 20 and 40 years, tied to the exact system, the build-up specified and installation to their standards. Guarantees typically run shorter than design lives and, as their documents set out, remain conditional on documented maintenance.

Older-generation felts, the thin rag-based and early glass-fibre sheets found on garages and extensions from the 1970s and 1980s, were never expected to serve much past 10 to 15 years, which is why so many of them, still in place, are now living entirely on borrowed time.

What shortens a felt roof's life?

Four accelerants do most of the damage:

• Water that stays. Standing water accelerates ageing wherever it sits.
• Sun that bakes. Ultraviolet slowly embrittles any exposed bitumen surface, which is what the mineral finish exists to resist.
• Details that were never right. Skimpy upstands, unsupported edges and laps facing the weather fail long before the field of the membrane does.
• Blocked drainage. It compounds all three, and it is the one factor entirely within an owner's control.

What stretches it?

Boring, inexpensive attention. Outlets cleared before autumn, an annual look at the laps and upstands, small faults repaired while they are small, and reflective or mineral surfaces kept intact. A roof treated this way does not just fail later; it fails politely, with warning, instead of over a bank holiday weekend.

Paperwork stretches lifespan too, indirectly but measurably. A roof with a known specification and an inspection history gets the right repair first time, keeps its guarantee enforceable, and never loses years to a wrong diagnosis; the undocumented roof pays for its mystery at every intervention.

Overlay or renew: how is the call made?

By evidence, not by birthday. If a condition survey finds the deck dry and the existing covering well bonded, a new RBM system can often be laid over the old as an overlay, renewing the waterproofing without the cost and waste of a strip. If moisture has reached the deck or insulation, overlaying would seal the problem in, and the honest recommendation becomes refurbishment down to a sound base. Either way, the number that matters is not the roof's age; it is the moisture reading under the membrane.

Can you tell a roof's age just by looking?

Roughly, and the clues are worth knowing. A cap sheet shedding its mineral surface, showing black bitumen in patches, has been losing its ultraviolet protection for years. Edges tell their own story: several generations of covering visible at a drip edge mean the roof has been overlaid before, which matters, because each overlay is usually the last sensible one. A patchwork of repairs in different materials speaks of a roof that has been failing in instalments.

None of this replaces a moisture reading, and that is the point. Looks date the surface; only investigation dates the roof underneath. Two roofs can photograph identically while one hides a soaked deck, which is why the visual clues should send you towards a survey rather than a verdict.

So what should an owner actually do?

Stop guessing the age and start recording the condition. One proper survey establishes where the roof genuinely is; from there, yearly checks keep the answer current and catch the cheap faults early. When the end does approach, you will see it coming seasons ahead, with options priced calmly instead of decided by a bucket in the hallway.